Surface Water: Rivers and Lakes

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At any one time, more than 97% of all the water in the hydrologic cycle is contained in the earth's oceans. By comparison, the rivers contain only 0.0001% of the earth's water at any one time and its lakes 0.007%. Nonetheless, this tiny fraction of the total water supply has shaped the course of human development. Throughout human history, societies have depended on these surface water resources for food, drinking water, transportation, commerce, power, and recreation.

In the United States, water from streams, rivers, and lakes accounts for 76% of the total freshwater consumption; the rest comes from groundwater. Of the total available supply—about 1,380 billion gallons per day (bgd)—only about 262 bgd is actually used daily. Public utilities depend on surface water for about 63% of their water needs; industries consume surface water for 80% of their requirements; and crop irrigation uses surface water for about 58% of its water needs.

The withdrawal of surface water varies greatly by location. In New England, for example, where rainfall is plentiful, less than 1% of the annual renewable water supply is used. By contrast, almost the entire annual supply is consumed in the area of the arid Colorado River Basin and the Rio Grande Valley.


Rivers and Streams

The great rivers of the world have been very influential in human history. Settlement locations on rivers have thrived since earliest recorded history, with most of the world's great civilizations growing up along rivers. Flowing rivers provided water to drink, fish and shellfish to eat, dispersion and removal of wastes, and transport for goods. The bountiful supply of freshwater in flowing rivers is one of the primary reasons for the rapid growth of settlement, industry, and agriculture in the United States during both colonial and modern times.

Rivers and streams, unlike lakes, consist of flowing water. Perennial rivers and streams flow continuously, year-round, although the volume may vary with runoff conditions. Intermittent, or ephemeral, rivers and streams stop flowing for some period, usually because of dry conditions. Both large and small rivers and streams are an important part of the hydrologic cycle.

Rivers receive water from rain and melting snow, springs from underground aquifers and lakes. A large river is usually fed by tributaries (smaller rivers and streams), and so increases in size as it travels from its source, or origin. Its final destination may be an ocean, a lake, or sometimes open land, where the water simply evaporates. This phenomenon usually happens only with small rivers or streams.

As water flows down a river, it carries with it grains of soil, sand, and, where there is a very strong current, small stones and other debris. These objects are important in two ways. First, as they are pulled along by the river's current, they grind against the bottom and sides of the riverbank and slowly cut the riverbed deeper and deeper into the earth (for example, the Grand Canyon), thereby changing the contour of the land. Second, when the river reaches its destination (an ocean or lake), the flow is slowed and then stopped where the bodies of water meet, and the soil that has been carried along is deposited as silt (sediment).

Eventually, these deposits build into substantial accumulations. Over long periods, they form deltas, which often provide a rich base for agriculture. The great civilizations of Egypt, for example, depended on the delta of the Nile River for their food supply. On the other hand, silt can also be a nuisance, filling lakes and harbors and smothering aquatic life. Many ports and harbors in the United States must be dredged regularly to remove deposits that would otherwise obstruct navigation.

The two longest rivers in the world are the Nile (4,157 miles) and the Amazon (3,915 miles). The Mississippi-Missouri river system is the third longest in the world. The Mississippi River has a watershed of 1,150,000 square miles, or about 40% of the total land area of the lower forty-eight states. A watershed, or drainage area, is the land from which a river receives runoff water from rainfall or snowmelt. The Mississippi River discharges water into the Gulf of Mexico at an average rate of 620,000 cubic feet per second. This amounts to 133 cubic miles per year, or approximately 34% of the total discharge from all the rivers in the United States. (A cubic mile contains about one billion gallons of water.)

By comparison, the Columbia River discharges less than seventy-five cubic miles of water per year. The Colorado River, which carved the Grand Canyon, discharges only about five cubic miles each year. Table 3.1 shows the largest rivers in the United States.


Unlike rivers and streams, lakes and ponds are depressions in the earth that hold water for extended periods of time. Reservoirs are man-made lakes that are generally used for recreation or to provide drinking water. Some ponds are also man-made, for purposes including livestock watering, fire control, stormwater management, duck and fish habitat, and recreation. The source of the water in lakes, reservoirs, and ponds may be rivers, streams, groundwater, rainfall, melting snow runoff, or a combination of these. Any of these sources may carry contaminants. Because water exits from these water bodies at a slow rate, pollutants can become trapped. For this reason, lakes, ponds, and reservoirs are particularly vulnerable to the deposit of pollutants from the air and to pollution from human activity.

Many of the world's lake beds were formed during the Ice Age, when advancing and retreating glaciers gouged holes in the soft bedrock and spread dirt and debris in uneven patterns. Some lakes fill the craters of extinct volcanoes, and others have formed in the shallow basins of ocean bottoms uplifted by geological activity to become part of the Earth's solid surface.

As soon as a lake or pond is formed, it is destined to die. "Death" occurs over a long time, particularly in the case of large lakes. Soil and debris carried by in-flowing rivers and streams slowly build up the basin floor. At the same time, water is removed by out-flowing rivers and streams, whose channels become ever wider and deeper, allowing them to carry more water away. Even lakes that have no river inlets or outlets eventually fill with soil eroded from the surrounding land.


The large freshwater lakes of the world contain nearly 30,000 cubic

RiverLocation at mouthAverage volume at mouth (cfs)Lengtha(mi.)
St. LawrenceCanada348,0001,900
OhioIL, KY281,0001,310
cfs=cubic feet per second
aIncluding headwaters and sections in Canada.
bBelow Mississippi diversion, without headwaters.

miles of water and cover a combined surface area of about 330,000 square miles. About 26% of the world's freshwater stored in lakes is found in North America. By surface area, Lake Superior, located on the U.S.–Canada border, is the largest freshwater lake in the world. Lake Baikal in Asiatic Russia, however, is so deep that it could hold all the water in Lake Superior and the other four Great Lakes, and an additional 300 cubic miles of water. Lake Superior has more than two-and-a-half times the surface area (26,418 square miles) of Lake Baikal, but at its maximum depth Lake Baikal is more than four times deeper (5,715 feet).

The large lakes of Africa contain nearly 29% of the water of all freshwater lakes in the world, followed by the lakes of North America with 26%, and those of Asia with 21% (almost all of which is in Lake Baikal). Large lakes in Europe, South America, and Australia account for only 2% of the world's freshwater.

The saline (saltwater) lakes of the world contain almost as much water as the freshwater lakes (25,000 cubic miles) and cover almost as many square miles (270,000). Of that water, however, 75% is in the Caspian Sea, which borders Russia and Iran (19,240 cubic miles), and most of the remainder is in lakes in Asia. North America's shallow Great Salt Lake is comparatively insignificant, with seven cubic miles of water.


People have always congregated on the shores of lakes and rivers. They established permanent homes, then towns, cities, and industries, benefiting from the many advantages of nearby water sources. One of these advantages has been that lakes or rivers were convenient places to dispose of waste. As industrial societies developed, the amount of waste became enormous. Frequently, the waste contained synthetic and toxic materials that could not be assimilated by the waters' ecosystems. Millions of tons of sewage, pesticides, chemicals, and garbage were dumped into waterways worldwide until there were few that were not contaminated to some extent. Some were—and some still are—contaminated to the point of ecological "death," unable to sustain a balanced aquatic-life system.

Clean Water Act

On October 18, 2002, President George W. Bush proclaimed the beginning of the Year of Clean Water in commemoration of the thirtieth anniversary of the signing of the Clean Water Act (CWA), the full name of which is the Federal Water Pollution Control Act (PL 92–500).

The CWA was enacted by Congress in 1972 in response to growing public concern over the nation's polluted waters. The problem of environmental pollution was thrust into the public consciousness when the Cuyahoga River in Cleveland, Ohio, burst into flames on June 22, 1969, the result of oil and debris that had accumulated on the river's surface.

The objective of the CWA was to "restore and maintain the chemical, physical, and biological integrity of the Nation's waters." The law is jointly enforced by the Environmental Protection Agency (EPA) and the U.S. Army Corps of Engineers.

The Act requires that, where attainable, water quality be such that it "provides for the protection and propagation of fish, shellfish, and wildlife and provides for recreation in and on the water." This requirement is referred to as the act's "fishable/swimmable" goal. Many people credit the Clean Water Act with reversing, in a single generation, what had been a decline in the health of the nation's water since the mid-nineteenth century.

Water-quality standards are the driving force of the CWA. A water-quality standard has three components:

  • Designated uses—The CWA envisions that all waters, at a minimum, be able to be used for recreation and the protection and propagation of aquatic life (fish and shellfish, and the plants, insects, and other organisms that are required to support them). Examples of additional uses are drinking water and water where the fish are safe to eat. Water bodies frequently have more than one designated use.
  • Criteria—The numerical or narrative limits assigned to protect each use. Examples are chemical-specific levels (numerical) that protect humans or fish from exposure to levels that may cause harm, or descriptions (narrative) of the best-possible biological condition of aquatic communities.
  • Antidegradation policy—A statement of intent to prevent waters that meet their standards from deteriorating from their current condition.

Each state is required under the CWA to adopt water-quality standards for each of its water bodies. The EPA is required to approve each state's standards. Each state must specifically designate a use for every surface-water body in the state. The state then establishes water-quality numeric and narrative criteria to protect each use. More than one designated use is frequently assigned to a water body. Most water bodies are designated for recreation, drinking-water use, and protection of aquatic life. For water bodies with more than one designated use, the states consolidate the individual use support information into a summary use. Figure 3.1 lists the summary uses.

The states collect data and other information that allow them to assess whether the quality of their water meets the designated uses expressed in the water-quality standards that each state sets. Under section 305(b) of the CWA, the states are required to submit assessments of their water quality to the EPA every two years. The EPA is required to summarize this information in a biennial report to Congress. As of the spring of 2005, the most recent EPA report to have been published was the 2000 National Water Quality Inventory, though some state-level data reported to the EPA is available online at

The CWA also established the National Pollutant Discharge Elimination System (NPDES). This program requires anyone who discharges pollutants to get a permit. Congress intended that, after the EPA had established regulations for obtaining a NPDES permit, each state would be granted "primacy." Primacy means that the states would have the primary responsibility for issuing permits and enforcing the requirements of the NPDES program. To be given primacy by the EPA, a state must adopt NPDES regulations and conduct monitoring and enforcement programs at least as stringent as those established by the EPA.

Agriculture Takes Up the Challenge

According to 2002 Census of Agriculture (Washington, DC: U.S. Census Bureau, June 2004), about 938 million acres, or roughly half of the continental United States, is used for agricultural production. Cropland accounts for 46% of the acreage, while pasture and range land make up another 40%. Agricultural land use is recognized in many jurisdictions and localities throughout the United States as the most desirable land use for economic, environmental, and social reasons. At the same time, the public and the agricultural community recognize that agricultural practices are a source of nonpoint pollution nationwide. (Nonpoint pollution is caused by runoff that has dispersed to areas outside of its origin.) This situation presents a challenge to water-quality management efforts.

The growing national concern over water-quality degradation has also permeated the agricultural community. There has been a steady increase in the use of best management practices and implementation of farm water-quality plans to protect wetlands and water bodies. The success of this effort can be seen in the decrease since 1982 in soil erosion reported in the National Resources Inventory—2001 Annual NRI (Washington, DC: U.S. Department of Agriculture, July 2003). (See Figure 3.2.) (Note that the amount of soil erosion remained at a steady level from 1997 to 2001.) The USDA, together with state and local agencies, is providing technical assistance and financial incentives through numerous programs to help farmers balance good stewardship of natural resources with market demands. Technical assistance through these programs has had success in getting farmers to voluntarily adopt more environmentally sensitive practices.


Grant money for use in supporting environmental projects is made available under the Clean Water Act. These funds have been instrumental in the development of many programs that have been successful in restoring and repairing impaired watersheds across the United States. In 1994 the EPA published the first in a series of reports highlighting successful programs developed under section 319 of the CWA. As of 2005 the most recent report is Section 319 Success Stories, Volume III: The Successful Implementation of the Clean Water Act's Section 319 Nonpoint Source Pollution Program (Washington, DC: EPA, February 2002).

One of the many stories highlighted in this publication is that of the Little Rabbit River Watershed Project, which was designed to improve the water quality in the Little Rabbit River Watershed in southwest Michigan. The dominant land use in the watershed is agriculture. The primary means identified for accomplishing the goal was to reduce the amount of sediment and nutrients entering the surface water. A section 319 CWA watershed grant in the amount of $380,936 was awarded to the Allegan Conservation District for use in implementing the Little Rabbit River Watershed Project.

As reported in the EPA's Section 319 Success Stories, Volume III, the success of this project was measurable within the first three years. A total of 19,852 tons of sediment was prevented from entering the Little Rabbit River during the first three years of the project. Nutrients entering the system were also reduced. A total of 19,706 pounds of phosphorus and 39,321 pounds of nitrogen was kept from entering the watershed due to the project.

The implementation of best management practices (BMPs) by landowners in the area was credited for the pollution abatement success. The project required a partnership between government agencies, local governing bodies, and landowners in the area. Together these stakeholders took the following actions, among others, that led to the success of the Little Rabbit River Watershed Project.

  • Installed eighteen acres of filter strips
  • Restored more than nine acres of wetlands
  • Installed four stream crossings and a watering facility
  • Stabilized 190 linear feet of stream bank
  • Built five animal waste storage facilities
  • Implemented mulch-till and no-till practices on 3,000 acres

Although the section 319 portion of the project was completed in 2000, water quality improvements and protection efforts continue in the area. Awareness of water quality issues in the community increased during the project, and the BMPs put into place during the project itself continue to reduce the entry of silt and nutrients into the watershed.


Defining water quality is a little like trying to determine "how clean is clean?" Currently, the best measure is the degree to which a water body is capable of supporting its designated uses.

State 305(b) Reports

Because of funding limitations, most states assess only a portion of their total water resources during each CWA-required two-year reporting cycle. The goal is to rotate the sites that are assessed in each cycle so that over a five-year period all waters are assessed. In the 2000 National Water Quality Inventory, the states evaluated about 19% of the nation's river and stream miles and about 43% of lake, pond, and reservoir acres. State data for 2002 has been reported to the EPA, but this information had not yet been consolidated into a national report as of mid-2005.

The states use chemical and biological monitoring results and other types of data, such as water-quality models, surveys of fisheries, and information from citizens, to evaluate their water quality. The data are compared with the water-quality criteria adopted to protect each use designated for a particular water body, and water bodies are rated as to how they meet their uses. Every two years the results of these evaluations are reported to the EPA in each state's 305(b) report.

Because of sparse reporting by the states and differences in criteria and measurement techniques between states, a completely accurate assessment of the quality of the nation's surface waters is not yet possible. The reports, however, are valuable as a measure of estimated overall water quality and as a means of identifying the major sources and causes of pollution.

States use two categories of data to assess water quality. The first and best category is monitored data. Monitored data are field measurements of biological, habitat, toxicity, physical, and chemical conditions in water, sediment, and fish tissue. These data are gathered at least every five years. The second category, used to fill information gaps, is evaluated data, which include field measurements more than five years old and estimates that are generated using land-use and pollution-source information, predictive models, and surveys of fish and game. These data can provide an indicator of water quality, but because they vary in quality and confidence, their use is limited.

Index of Watershed Indicators

Reporting the health of the nation's aquatic resources is more difficult than reporting water quality. To meet this challenge, the EPA, the states, and their many public and private partners have developed the Index of Watershed Indicators (IWI). IWI looks at a variety of "indicators" that point to whether rivers, lakes, streams, wetlands, and coastal areas are "well" or "ailing," and whether the activities taking place on surrounding lands are placing them at risk. The objective is to establish a national baseline on the condition and vulnerability of aquatic resources—a baseline that can be used, over time, to help measure progress toward the goal that all water-sheds be healthy and productive places. In Index of Watershed Indicators: An Overview (EPA, August 2002), the following watershed conditions were reported:

  • 15% of the nation's watersheds had relatively good water quality.
  • 36% had moderate problems.
  • 22% of the watersheds had more serious water-quality problems.
  • 27% did not have enough information to be characterized.

The report also revealed that one in fifteen water-sheds nationally was highly vulnerable to further degradation.

Point and Nonpoint Sources of Pollution

Pollutants enter a body of water in any number of ways. Pollutant sources can be divided into two major types: point and nonpoint. Point sources are those that disperse pollutants from a specific source or area, such as a wastewater treatment plant discharge. Pollutants that are commonly discharged from point sources include bacteria, toxic and nontoxic chemicals, nutrients, and heavy metals from industrial plants.

Nonpoint sources are those that are spread out over a large area and have no specific outlet or discharge point. For example, agricultural and urban runoff; runoff from mining and construction sites; and accidental spills, as when a train or truck carrying toxic chemicals derails or overturns, releasing its contents. Nonpoint source pollutants can include bacteria from cat and dog wastes, pesticides, fertilizers, toxic chemicals, and salts from road construction. (Figure 3.3 shows activities that can contribute to nonpoint pollution.)

The designated use of a water body is "impaired" when the amount of pollutant in that water body reaches the level where the water cannot meet the water-quality criteria for its designated use or uses. This does not necessarily mean that the water body is badly degraded. For example, a stream may exceed the water-quality criteria for temperature established to protect a cold-water fish such as trout and still support an active trout fishery. The "impairment" may be only slightly in excess of water-quality criteria. A water body can be impaired for one designated use and still fully support other designated uses. For example, a lake may have a thriving recreational fishery but be unsafe for swimming because of high bacteria levels in the water, thereby meeting the "fishable" designation but not the "swimmable" designation. In other cases, impairment may be so bad that one or more uses are lost. An example would be a lake that has been choked by noxious weeds from excessive nutrients, resulting in fishkills and severely reduced fish populations, leading to the loss of an active recreational fishery.

Where impaired conditions are deemed irreversible, the water quality is determined to be unattainable. To reach this determination, the states perform a use-attainability analysis to show that one or more designated uses cannot be supported because of biological, physical, chemical, or economic/social conditions. Examples of conditions that might result in a determination that a use is not attainable include low flow, naturally occurring high levels of pollutants, or the presence of dams or other hydrologic modifications that permanently alter water body characteristics.

Managing Water Quality

Once states have determined that a specific water body does not meet the water-quality criteria needed to protect its designated uses, they begin the process of determining what actions are necessary to restore water quality. The first step is to identify the total maximum daily load (TMDL) for each pollutant. The TMDL is the maximum amount of a pollutant that a water body can receive on a periodic basis and still support its intended uses. TMDLs are generally developed by identifying the pollutants and the source of the pollutants causing a water-quality problem, and determining how much the pollutants need to be reduced in order to enable the water body to meet the water-quality standards. Reductions in pollutants are then achieved through limits in discharge permits, requirements for best management practices, and other regulatory or voluntary practices.


The EPA's 2000 National Water Quality Inventory showed that 61% of the rivers and streams evaluated by the states were found to be fully supporting their designated uses. However, an estimated 8% of those same waters were identified as threatened, meaning that they might become impaired if pollution control action was not taken. Of the rivers and streams that were assessed, 39% were impaired for one or more uses.

Causes of Pollution in Rivers and Streams

The states reported that pathogens (bacteria), siltation (the smothering of river and streambeds by sediment, usually from soil erosion), habitat alterations, and oxygen-depleting substances were the four most common causes of pollution in our nation's rivers and streams. (See Figure 3.4.) In 2000 pathogens from point and nonpoint sources affected about 34% of all polluted river miles. Silt was the second most common cause of pollution in the assessed rivers and streams, affecting 31% of those considered impaired. Habitat alterations were listed as the cause for impairment of 8% of the assessed river and stream miles, or 22% of the impaired miles. Oxygen-depleting substances were responsible for approximately 20% of the polluted miles. (Note that these percentages total more than 100%, as pollution in some areas came from more than one source.)


In the 2000 National Water Quality Inventory, pathogens (bacteria) were identified as the leading cause of water-quality impairment, responsible for polluting 93,431 river and stream miles. Bacteria provide evidence of possible fecal contamination that may cause illness. States use bacterial indicators to determine if rivers are safe for drinking or swimming. The most common sources of bacteria are urban runoff, inadequately treated human sewage, and runoff from pastures and feedlots.


Silt was the second most important source of pollutants to rivers and streams. In an earlier report, National Water Quality Inventory—1998 Report to Congress (June 2000), silt had ranked as the top cause of pollution entering the waterways of the United States. In the later report, siltation had dropped to second place. Silt, composed of tiny soil particles, impaired 12% of the assessed rivers and streams, which is 31% of the impaired river and stream miles reported in the 2000 EPA Inventory. (See Figure 3.4.)

Silt alters aquatic habitats, suffocates bottom-dwelling organisms and fish eggs, interferes with light transmission to underwater plants, and clogs the gills of fish. The habitat of aquatic insects that live in the spaces between pebbles and rocks is destroyed when these spaces are filled with silt. Loss of aquatic organisms can radically affect the health of certain fish species and other wildlife that eat them. (See Figure 3.5.) Excessive silt can also interfere with recreational use and drinking-water treatment. The primary sources of silt are agriculture, urban runoff, forestry, logging, and construction.


Oxygen is vital for the animal life in waterways. Lack of oxygen occurs as a result of the oxygen-consuming processes by which organic matter decays. The risk of oxygen depletion is therefore greatest in waters affected by high discharge of organic matter, and also where substantial production of algae and other plants occur. Oxygen-depleting substances were the third leading pollutant type listed in the 2000 National Water Quality Inventory, affecting 55,398 impaired river miles. Although habitat alteration is the third item listed in Figure 3.4, it is considered a stressor to the system rather than a pollutant.

Sources of Pollution in Rivers and Streams

The three leading sources of pathogens, siltation, and oxygen-depletion in U.S. rivers and streams in 2000 were agriculture, hydrologic modifications, and habitat modifications. (See Figure 3.6.)


Agriculture was the leading source of pollution, responsible for about 48% of the reported water-quality problems in impaired rivers and streams. (See Figure 3.6.) The term "agriculture" captures a number of activities, from large-scale factory farming to landscape plant nurseries and fish farming. The agricultural uses that are most frequently responsible for contributing pollutants to water were:

  • Nonirrigated crop production (Rain is the sole water source.)
  • Irrigated crop production
  • Range grazing
  • Pasture grazing (land where a specific crop is grown to feed animals either by grazing animals among the crops or harvesting the crops)
  • Animal-feeding operations

The EPA's 2000 report shows that the three agricultural activities that had the most degrading impact on rivers and streams together were responsible for 53.7% of the impaired river and stream miles reported; nonirrigated crop production (26,830 miles); animal feeding operations (24,616 miles), and irrigated crop production (17,667 miles).


Hydrologic modifications were responsible for 20% of the impaired miles of rivers and streams (53,850 miles). (See Figure 3.6.) These modifications include such things as flow regulation, channelization, dredging, and the construction of dams. Modifications of these kinds alter the habitats of rivers and in so doing can cause them to become far less suitable for aquatic life.


The modification of river and stream habitats can have a similar and equally destabilizing impact on aquatic life. The EPA report defines habitat modifications as all those changes to habitat that do not directly affect water flow. That would include such things as the removal of woody debris, logging activities, and/or land-clearing practices. Habitat modifications were ranked third in terms of sources of pollutants impairing rivers and streams in the 2000 National Water Quality Inventory. A reported 37,654 river and stream miles were degraded due to habitat modifications, accounting for 14% of the impaired river and stream miles. (See Figure 3.6.)

Municipal Sewerage Systems

Large cities, suburban areas, and many small towns are served by municipal sewerage systems. Wastewater treatment facilities vary in capacity from treating as little as 10,000 gallons per day (gpd) to treating 400 million gpd. The degree of treatment provided to the sewage also varies and may be primary, secondary, or tertiary treatment. Primary sewage treatment uses screens to remove debris, a grit chamber to settle out grit and sand, and solids settling. Following this process, the liquid waste is generally disinfected with chlorine and discharged to a water body. The settled solids are transported to a sludge digester for microbial digestion and conversion to bio-solids. Primary treatment achieves a 40% to 60% reduction in bacteria and total solids (nutrients and oxygen-demanding substances).

Secondary sewage treatment uses primary treatment, adds another settling step, and aerates the effluent to accelerate microbial digestion, resulting in a 70% to 90% reduction in bacteria and suspended solids. The liquid waste is disinfected before discharge. Tertiary treatment removes nutrients and additional suspended solids by adding chemical or microbial treatments, or filtration to remove additional suspended solids and reduce nutrients. Sometimes wetlands are also used to provide additional treatment. The effluents are generally disinfected before discharge. Tertiary treatment is generally used in nutrient reduction strategies to reduce algal blooms, trophic levels, and hypoxia (oxygen deficiency). The success of each category of wastewater treatment is directly dependent on the treatment method selected and on whether the treatment plant is properly sized for the volume of wastes handled. If the plant is overloaded, treatment success declines. Since 1974 progress has been made in upgrading sewage treatment plants, but much remains to be done.

Sewage sludge is the solid, semisolid, or liquid untreated residue generated during the treatment of domestic sewage in a treatment facility. The method of biosolids disposal depends on the level of treatment the sludge receives and on its contaminants. The most common methods are land application as fertilizer or soil conditioner, transport to landfills, incineration, and composting for sale as a lawn and garden additive. Overboard disposal of sludge in oceans and estuaries is no longer permitted. When properly treated and processed, sewage sludge becomes biosolids, a nutrient-rich material that can be safely recycled and applied as fertilizer. Biosolids production is strictly controlled by federal and state regulations.

Many older towns and cities have combined sewer systems; that is, sewer systems that transport sewage in dry weather and sewage and rainwater in wet weather. Combined sewers during heavy rainfall events can quickly overload a wastewater treatment plant, resulting in the need to "bypass" or divert portions of the combined flow overboard. Bypass releases a mixture of rainwater, raw sewage, oil, and gasoline from runoff, along with fertilizers and pesticides from lawns and gardens, into waterways.

Combined sewer overflows are one of the major sources of bacteria, nutrients, and silt in waterways. The solution to combined sewer overflow is separation of the sewer and storm water systems. This is prohibitively expensive, as it frequently requires tearing up urban streets and sometimes buildings to replace the existing combined sewer with separate lines for sewage and storm water runoff. Alternatives include expanding the waste-water treatment plant capacity, building separate treatment facilities for the storm water flow, capturing and storing storm water flow and feeding it into the treatment plant during periods of low sewage flow, and constructing special wetlands to treat the effluent. Each of these alternatives is land-intensive and costly. One estimate has placed the cost of correcting the combined sewer problem as greater than the annual gross national product of the United States.


The United States is second only to the Republic of China in the use of dams. Some 100,000 dams regulate America's rivers and creeks; 5,550 are more than fifty feet high. Nationwide, reservoirs created by dams encompass an area equivalent to New Hampshire and Vermont combined. Of all the rivers more than 600 miles in length in the lower forty-eight states, only the Yellowstone River still flows freely.

Being a world leader in dams was a point of pride during the golden age of dam building, a fifty-year flurry of construction that ended about 1980. Dam construction was also a way to employ many people who were out of work during the Great Depression of the 1930s and to foster national pride. Dams epitomized progress, Yankee ingenuity, and humanity's mastery of nature. After 1980 dam construction fell into disfavor. Three factors accounted for most of the decline: public resistance to the enormous costs; a growing belief that politicians were foolishly spending taxpayers' money on "pork barrel" projects, such as dams, that only benefited their local economies; and a developing public awareness of the environmental degradation that dams can cause.

Where Have All the Rivers Gone?

Dams provide a source of energy generation; flood control; drinking water; irrigation; recreation for pleasure boaters, skiers, and anglers; and locks for the passage of barges and commercial shipping vessels. However, they also alter rivers and streams, the land abutting them, the water bodies they join, and the aquatic life throughout, resulting in significant changes in the river system.

Recognizing the need to protect and maintain the aquatic environments and biological diversity of river systems, more dam operators are now required to maintain a minimum flow in the river below the dam. In addition, many dams are being retrofitted with fish ladders (a series of pools arranged like steps alongside a river or stream) and other means of access to permit fish to reach spawning areas above the dam. Fish ladders are also helpful in allowing juvenile fish to reach the river below the dam. Some states have programs that remove abandoned or obsolete dams, such as those that once served to power flour mills, or to remove other obstructions to fish passage, such as road culverts.

Snake River Dams

Correcting the environmental damage done by dams can be a costly and time-consuming undertaking. One example of a project designed to remedy damage caused by dams is the U.S. Army Corps of Engineers' project on the 1,040-mile Snake River, a major tributary of the Columbia River that runs from Yellowstone National Park in Wyoming through southern Idaho to join the Columbia River in Washington State. The goal for this ongoing project is to improve salmon and steelhead migration cycles through the dams on the Lower Snake River. Wild salmon and steelhead use the waters of the Columbia River Basin as their spawning grounds.

The U.S. Army Corps of Engineers (often referred to simply as the corps) operates nine out of the ten major federal projects in the Columbia and Snake River Basin. Of these, four dams generate hydroelectric power on the Lower Snake River. These Snake River dams are used (1) to generate 5% of the hydroelectric power for the Pacific Northwest, (2) for irrigation in the region, and (3) to enable navigation on the Snake River. Each of the dams is about 100 feet high and 2,655 to 3,791 feet wide.

Salmon stocks have declined in the Pacific Northwest during the twentieth century. In 1991 the National Marine Fisheries Service officially declared Snake River sockeye salmon an endangered species. The following year Snake River Chinook salmon were designated as a threatened species. These declarations triggered a series of actions required by law under the federal Endangered Species Act of 1973. One of the actions required was the development of a recovery plan. The Columbia River Fish Migration program grew out of this recovery plan.

According to a report by the Pacific Salmon Coordination Office of the U.S. Army Corps of Engineers (Columbia River Basin—Dams and Salmon,, there are many factors that account for the sharp decline of salmon stock in the Columbia and Snake River Basin. Among those factors are dams. The Columbia River Basin report maintains that

Dams clearly have had a significant impact, particularly those that eliminated access to fresh water habitat (preventing adult fish from returning to spawn), and those through which fish passage is provided but at reduced levels from natural conditions.

The dams impede juvenile and adult migrations to and from the ocean by their physical presence and by creating reservoirs. The reservoirs behind the dams slow water velocities, alter river temperatures, and increase predation potential.

Studies have been going on for many years to determine how best to alter the Snake River dams to reduce their negative impact on salmon and steelhead populations. All four of the dams on the Snake River have fish ladders for upriver migration of salmon returning to spawn and a bypass system for the downriver migration of juvenile salmon. As part of its Columbia River Fish Mitigation program, the corps is focusing on improving the passage of adult and juvenile salmon around these dams.

The corps has been evaluating four fish passage alternatives, the most controversial of which is breaching the dams; that is, removing the earthen portions of the dams and allowing the river to course around the remaining concrete structure. Breaching the dams would help the salmon but it would eliminate a source of hydro-electric power, water for irrigation, and a waterway for barge transport to ports 140 miles upstream. The other alternatives are to:

  • Maintain current operations
  • Increase the transportation of juvenile salmon around the dams
  • Make improvements to the dams' systems for collecting the juvenile salmon and barging or trucking them past the dams

Because changing the dams' operation can have significant environmental consequences, the corps prepared a draft environmental impact statement (EIS), which was published in February 2002. The corps' recommendation in the EIS was to move forward with the last alternative mentioned—improving the dams for the collection of juvenile salmon. However, no direct action on this plan had been taken as of May 2003, when a legal ruling in a related case led to a reconsideration of the dam-breaching alternative.

On May 7, 2003, U.S. District Judge James A. Redden ruled in a case brought against the National Marine Fisheries Service (NMFS) by seventeen conservation and fishing organizations (Ruling against NMFS BiOp Revives Dam Breaching Debate, Columbia River Inter-Tribal Fish Commission, The ruling found that the NMFS' 2000 biological opinion, upon which the corps' Columbia River Fish Mitigation program was based, fell short of the requirements of the Endangered Species Act. As a result, the corps was required to prepare a revised EIS based on a more stringent NMFS biological opinion. In March 2005, however, Judge Redden again ruled against the NMFS, Army Corps of Engineers, and Bureau of Reclamation, stating that the 2004 "Updated Proposed Action" of the three agencies did "even less to protect salmon survival" than the 2000 version (, March 21, 2005).


In the 2000 National Water Quality Inventory report, which assessed 43% of the nation's 40.6 million acres of lakes, ponds, and reservoirs, 47% were fully supporting their designated uses. However, about 8% of the lake acres were threatened. Of the lakes assessed, 45% could only partially support their designated uses.

Leading Pollutants in Lakes, Ponds, and Reservoirs

A lake's water quality reflects the condition and management of its watershed, that is, the land area that drains to the lake. Elevated levels of nutrients were identified as the most common pollutants, contributing to 50% of the impaired water quality in lakes. Figure 3.7 shows the top pollutants and the percentage of impaired lake acres affected by each type.


The leading pollutant of lakes, ponds, and reservoirs in 2000 was an excess of nutrients, which was reported in 22% of the assessed lake acres and 50% of the impaired lake acres. Nutrients in small quantities are found in healthy lake ecosystems. The presence of excess nutrients disrupts the balance of a lake's ecosystem by creating an environment in which algae and aquatic weeds become too abundant. As these plants die they sink to the bottom of the lake, pond, or reservoir and decompose. In the process of decomposing, dissolved oxygen is used, leaving oxygen levels lower than they would be in a more balanced environment. Two things that typically occur when a lake environment is low on dissolved oxygen is that fish die off and the lake emits foul odors.


Metals were the second most prevalent pollutant, affecting 42% of the impaired lake acres. This finding was caused mostly by the widespread detection of mercury in fish tissue. Because it is difficult to measure mercury in water, and because mercury readily accumulates in tissue (bioaccumulates), most states measure mercury contamination using fish tissue samples. Evaluating the extent of the mercury problem is complex because it involves atmospheric transport from power-generating facilities, waste incinerators, and other sources.


The third most common pollutant of lakes reported in the EPA's 2000 inventory was siltation or sedimentation. Nine percent of the lakes assessed in the report were shown to have been impaired by siltation, making this pollutant responsible for 21% of the lake acres designated as impaired. (See Figure 3.7.)

Sources of Pollutants in Lakes


As in the case of rivers and streams, agricultural runoff was the most extensive source of pollution for lakes, affecting 41% of impaired lake acres. (See Figure 3.7.) Pasture grazing and both irrigated and nonirrigated crop production were the leading sources of agricultural impairments to lake water quality.


The second most commonly found cause of lake impairment was hydrologic modifications. These modifications, resulting from flow regulation, dredging, and construction of dams, degraded 8% of the assessed lake, pond, and reservoir acres and 18% of the impaired acres.


A nearly equal percentage of lake acres were degraded by urban runoff and storm sewers as were degraded by hydrologic modifications.

Trophic Status of U.S. Lakes

Lakes naturally change over the years, filling with silt and organic material that alter many of the basic characteristics, such as lake depth, biological life, oxygen levels, and the inherent clearness of the water. This natural aging process is called eutrophication. Human activities often speed up eutrophication by increasing nutrient levels. Figure 3.5 compares a healthy lake system with a system impaired by excessive nutrients.

Naturally occurring eutrophication progression includes several stages. Oligotrophic lakes are clear waters with little organic matter or silt; mesotrophic waters contain more organic material, and the oxygen level is being depleted; eutrophic waters are extremely high in nutrients, and the water is murky and shallow with lots of algae and a depleted oxygen level. Under natural conditions, eutrophication of a large lake can take thousands of years. Human activity can speed up this process, reducing the time to a few decades.


The 2000 EPA Inventory of the Great Lakes

The Great Lakes basin, which is shared with Canada, is home to more than thirty-three million people, thirty million of whom rely on it for drinking water. The five lakes are the largest system of fresh surface water in the world, containing about 20% of the world's freshwater. The water in the Great Lakes accounts for 95% of all the freshwater in the United States. The total shoreline of the Great Lakes in the United States and Canada, a "fourth seacoast," is more than 10,000 miles long and equal to about one-quarter of the Earth's circumference. The region generates more than 50% of the total U.S. manufacturing output. The eight Great Lakes states (Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania, and Wisconsin) account for 30% of U.S. agricultural sales. International shipping on the Lakes annually transports fifty million tons of cargo while sport and commercial fishing contribute $4.5 billion to the economy.

This prosperity has taxed the ecological health of the Great Lakes system. Urban and industrial discharges, agricultural and forestry activity, development of recreation facilities, poor waste disposal practices, invasive species, and habitat degradation have all contributed to ecosystem decline. Despite these problems, however, the watershed still contains many ecologically rich areas. More than thirty of the basin's biological communities, and more than one hundred species, are found only in the Great Lakes basin or are globally rare.

In 2000 the states assessed 92% of the Great Lakes shoreline. The states reported that 22% of the assessed shoreline supported designated uses and 78% was impaired. Only four of the eight Great Lakes states reported specific pollutants and sources of those pollutants for use in the EPA's 2000 report. As the report states, "limited conclusions can be drawn from this fraction of the nation's Great Lakes shoreline miles."

The data presented in Figure 3.8 show the percentage of Great Lakes shoreline miles that either fully support or only partially support (or fail to support) each of six use categories. In all but one case, the percentage of assessed miles that support their use category well exceed the percentage that only partially support or fail to support a use category. The one use category in which partial failure to support or total failure to support was reported for all shoreline miles was fish consumption. Not being able to eat Great Lakes fish was the greatest use impairment. All of the states bordering the Great Lakes have issued advisories to restrict the eating of fish caught in the lakes.

Great Lakes Water Quality Agreement

Since the 1960s and 1970s, when Lake Erie was so degraded that it was considered by many to be "dead," much time and effort has been invested in trying to restore the Great Lakes system. In 1972 the United States and Canada entered into the Great Lakes Water Quality Agreement (GLWQA), which is a worldwide model for cooperative environmental protection and natural resource management.

There have been many successes since then, and the ecosystem is in recovery. For example, excess phosphorous and nitrogen loads that smothered the Great Lakes with nuisance algae have been successfully stabilized and in some cases reduced through strict nutrient goals. The trend in phosphorous concentration has been mostly downward and stable on all lakes other than Lake Erie, where readings from 2001 and 2002 show an upward spike. The endangered double-crested cormorant, a large fish-eating bird that was near extinction in the 1970s, is now more numerous on Lake Ontario than at any time in its previous recorded history.

State of the Lakes Ecosystem Conferences

The GLWQA imposes reporting requirements on both of its member countries and in an attempt to meet these requirements a conference series was established. The conferences, which are held every two years, are called the State of the Lakes Ecosystem Conference (SOLEC). The first such conference was convened in 1994.

The SOLEC meetings are designed as a venue for scientists and policy makers to share information about the state of the Great Lakes ecosystem. The focus is on assessing and sharing information about the results of Great Lakes programs and studies. In the year following each conference, the United States and Canada prepare a report that presents the findings accumulated at the SOLEC.

According to the EPA's Web site (, after the 1996 SOLEC those involved recognized a need for a standard set of basin-wide indicators. When many parties are involved in studying a subject and then comparing the results of their studies, it is helpful to work with a standard set of indicators so that progress can be measured reliably across both time and geography.

Since the 1996 SOLEC, work has been done to establish a formalized set of indicators with which to assess the state of the Great Lakes at each consecutive SOLEC. The indicators are used sort of like a doctor might use a patient's weight and blood pressure to gauge his or her general health. Over time if an adult patient's weight and blood pressure are rising it is a sign of troubled health. Similarly, a rising phosphate level in a lake is a sign that the lake's ecosystem is ailing.

The report that came out after SOLEC 2002, State of the Great Lakes 2003, presented the following mixed news about the chemical, physical, and biological integrity of the waters of the Great Lakes Basin ecosystem:

Recreational waters have become contaminated with animal and human feces from sources such as combined sewer overflows that occur in certain areas after heavy rains, agricultural runoff, and poorly treated sewage.

Overall the quality of the drinking water in the Great Lakes basin is good. This is in large part due to our current technologies.

Since the 1970s, there have been declines in many persistent bioaccumulative toxic (PBT) chemicals in the Great Lakes basin. However, PBT chemicals, because of their ability to bioaccumulate and persist in the environment, continue to be a significant concern.

Wetlands continue to be lost and degraded, yet the ability to track and determine the extent and rate of this loss in a standardized way is not yet feasible.

These findings were based on the assessments of nineteen of the forty-five indicators reported on at the SOLEC 2002 meeting.

Pollutants and Sources of Pollution in the Great Lakes

The U.S. strategy for meeting the goals of the GLWQA is discussed in Great Lakes 2001—A Plan for the New Millennium. The draft strategy was a partnership among the eight Great Lakes states, Great Lakes tribal governments, the Great Lakes Fishery Commission, and nine federal agencies. These groups were working together to implement the actions described in the strategy by coordinating and enhancing their environmental protection and natural resource management efforts.

Priority toxic organic chemicals, nutrients, and pathogens are the three most common pollutants affecting the waters of the Great Lakes. Toxic substances such as mercury, heavy metals, dichlorodiphenyltrichloroethane (DDT), and polychlorinated biphenyls (PCBs) have been responsible for many of the problems. There has been some improvement, however. The levels of DDT, a banned pesticide, have steadily decreased since the mid-1970s, and there are currently no DDT-based advisories against eating fish from the Great Lakes. Total PCB levels have shown the same decline as DDT. PCB concentrations in lake trout, walleye, and salmon are one-tenth of the concentrations reported in the mid-1970s. The PCB levels, however, are still high enough to keep advisories against eating these fish from places in all five lakes. Contaminated sediments and urban runoff are the primary sources of the pollutants impairing the Great Lakes.


A mixing zone is an area in a river or lake where pollutants are mixed with cleaner waters to dilute pollutant concentrations in the water. Inside a mixing zone, discharges are allowed to exceed water-quality criteria by a fixed amount determined on a case-by-case basis. Outside the mixing zone, pollutant levels must meet water-quality standards.

Certain organic pollutants such as DDT, PCBs, and methyl mercury, even though their concentrations are so low that they cannot be measured in surrounding waters, bioaccumulate. The EPA has identified twenty-two bioaccumulative chemicals of concern. In November 2000 the EPA adopted a new regulation for the Great Lakes, prohibiting the use of mixing zones with new discharges of bioaccumulative chemicals. The rule also phased out over a ten-year period the use of existing Great Lakes mixing zones for these chemicals. The regulation was aimed at reducing, by up to 700,000 pounds, the annual discharge into the Great Lakes of chemicals that had the potential to accumulate in fish and wildlife. The EPA estimated that this new regulation would affect about 300 of the 600 major Great Lakes dischargers.


Some organic pollutants, including PCBs and DDT, have two properties that lead to high bioaccumulation rates. These pollutants do not have an affinity to water (they are hydrophobic) and therefore readily attach to particles such as clay and small aquatic plants called phytoplankton. The pollutants have an affinity for lipids or fatty tissues (they are lipophilic) and are therefore stored readily in the fatty tissues of plants and animals. Because of these properties, these organic pollutants bio-accumulate in phytoplankton, sediment, and fat tissue at concentrations that exceed the pollutant concentrations in surrounding waters. Frequently, the concentration in surrounding waters is so low that it cannot be measured even with very sensitive instruments and methods.

Zooplankton (microscopic plant-eaters) and fish consume vast quantities of phytoplankton. As a result, any organic chemicals accumulated by the phytoplankton are further concentrated in the fish, particularly in their fatty tissues. These concentrations are increased at each level of the food chain. (See Figure 3.9.) The process of increasing pollutant concentration through the food chain is called biomagnification.

The top predators in a food chain, such as lake trout and Chinook salmon, and fish-eating gulls, hawks, and eagles, may accumulate concentrations of these organic chemicals high enough to cause serious deformities or death, or impair their ability to reproduce. DDT, for example, causes the eggs of eagles, osprey, and other fish-eating birds to easily break, and also causes deformities in chicks, impairing the birds' ability to successfully reproduce.

Biomagnification of pollutants in the food chain can also be a significant concern for human health. To protect their residents from these risks, states issue fish consumption advisories or warnings about eating certain types of fish or shellfish.

The Threat of Alien Invasive Species

The introduction of a nonnative species into an ecosystem can have serious consequences. In fact, according to the State of the Great Lakes Ecosystem 2001, jointly published by the U.S. Environmental Protection Agency and Environment Canada, "invasive, non-native aquatic species are the greatest biological threat to the Great Lakes aquatic ecosystem."

Within an ecosystem there exists a balance. A new nonnative species brought into a system causes an imbalance and can cause great damage as native species are not capable of resisting infection, infestation, predation, or competition from the alien species. Two examples of such invasive nonnative species are the zebra mussel and purple loosestrife, an aquatic plant.

According to the International Joint Commission's 2002 report, researchers widely believe that the costs of biological pollution from alien invasive species are both massive and rising. The costs to native ecosystems, natural resources, fisheries, and agriculture are estimated in one study to reach $137 billion per year in the United States alone. By comparison, the costs associated with losses from Hurricane Andrew in 1992 totaled $16 billion.

Invasive alien species are introduced into the Great Lakes in various ways, including aquaculture, canals, baitfish disposal, recreational boating, and ship fouling. The most significant source of introduction of such species is through ballast water on ships, the water that ships take in at sea to equalize their loads. This ballast water, which may contain nonnative species, is often discharged in the Great Lakes.

Guidelines for ballast water were introduced as a part of the Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 as amended by the National Invasive Species Act of 1996. Nonetheless, according to the International Joint Commission's 2002 report, "despite increased awareness of the risks, the 1990s saw no discernable improvement" in the introduction of nonnative species into the Great Lakes.

In its Twelfth Biennial Report on Great Lakes Water Quality, released in September 2004 (, the International Joint Commission estimated that the number of alien nonnative species had risen from approximately 162 in 2001 to more than 170 by late 2004. Scientists on the Commission predicted that one new invasive non-native species will be discovered in the Great Lakes every eight months. As of 2005, the International Maritime Organization Convention, which would enact a series of standards for ships entering the Great Lakes region to slow the number of nonnative species introduced to the area on the ballasts of ships, still had not been ratified. In the interim between its proposal and its eventual ratification, scientists estimate that as many as twelve new alien species could be introduced to the lakes and waterways of the region.



Meeting the "fishable" goal of the Clean Water Act means providing a level of water quality that protects and promotes successful populations of fish, shellfish, and wildlife. The "fish consumption" use—the ability for humans to safely eat the fish—is a higher use than most states assign to their waters. When fish or shellfish in particular locations contain harmful levels of pollutants, the state issues advisories against eating the fish to recreational fishermen. Commercial fishing is usually banned.

Figure 3.10 shows the number of advisories against eating fish or wildlife reported by the states to the EPA as of 2003. These advisories are specific as to location, species, and pollutant. Some advisories caution against eating any fish from a particular location; while others caution against eating a particular species of fish only because it is more likely to bioaccumulate the chemical of concern. Advisories from the EPA and the U.S. Food and Drug Administration (FDA) in 2003 included warnings that women who are pregnant or nursing and young children avoid eating certain kinds of fish. Consuming mercury can damage the developing nervous systems of babies and children.

In 2003, forty-eight states, the District of Columbia, and American Samoa reported 3,094 fish and wildlife consumption advisories. The bioaccumulative chemicals—mercury, PCBs, chlordane, dioxins, and DDT—caused most of the advisories. (See Table 3.2 and Table 3.3.) Coal-fired utilities were the most common cause of air-borne mercury contamination in 2003. The use of PCBs, chlordane, and DDT has been banned for more than twenty years, yet these compounds persist in the sediments and are taken in through the food chain and biomagnified.

According to "Fish Warnings Up Due to Mercury Pollution—EPA," (Reuters News Service, August 26, 2004), U.S. coal-burning utility plants are the largest unregulated source of mercury in the United States, releasing about forty-eight tons of the toxin annually. In early 2004 the administration of President George W. Bush proposed a standard that would require these facilities to reduce their mercury emissions by 70% by 2018.


Meeting the "swimmable" goal is defined by the EPA as providing water quality that allows recreational activities both in and on the water. In the 2000 National Water Quality Inventory, four states reported that they had no record of recreation restrictions reported to them by their respective health departments; thirteen states and tribes identified 233 sites where recreation was restricted at least once during the reporting cycle. Local health departments closed many of those sites more than once. Pathogens (bacteria) caused most of the restrictions. State reporting on recreational restrictions, such as beach closures, is often incomplete because agencies rely on local health departments to voluntarily monitor and report beach closures.

The Centers for Disease Control and Prevention (CDC) report "Surveillance for Waterborne-Disease Outbreaks—United States, 2001–2002" (Morbidity and Mortality Weekly Report, October 22, 2004) listed the incidence of disease outbreaks caused by recreational water contact. During this two-year period, twenty-three states reported sixty-five outbreaks, affecting 2,536 persons. Of the sixty-five recreational waterborne disease outbreaks reported, thirty involved gastroenteritis. There were fifteen such outbreaks in 1999 and twenty-one in 2000. Figure 3.11 shows the number of waterborne disease outbreaks due to recreational water use annually from 1978 to 2002, with a breakdown by illness. The year 2000 saw the highest number of outbreaks for the entire period.

As part of the Beaches Act of 2000, the U.S. Congress has directed the EPA to develop a new set of guidelines for recreational water based on new water-quality indicators. Beginning in 2003 the EPA was required to conduct a series of epidemiologic studies at recreational freshwater and marine beaches. These studies will be used to help in the development of the new guidelines for recreational water.

YearPercent national river miles underPercent national lake acres under advisoryNational number of river under miles advisoryNational number of lake acres under advisory
Although current advisories in the United States have been issued for 40 different pollutants, most advisories involve five primary bioaccumulative contaminants:
• Mercury: 2,362 advisories active in 2003 (up 10% from 2002)
• PCBs: 884 advisories active in 2003 (up 9% from 2002)
• Chlordane: 89 advisories active in 2003 (down from 97 advisories in 2002)
• Dioxins: 90 advisories active in 2003 (up from 74 advisories in 2002)
• DDT and metabolites: 52 advisories active in 2003 (up from 48 in 2002)
The increase in advisories issued by the states generally reflects an increase in the number of assessments of contaminants in fish and wildlife tissues.