Surface Water: Rivers, Streams, and Lakes

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Chapter 3
Surface Water: Rivers, Streams, and Lakes

Most of the earth's water, about 97%, is the saltwater of the oceans. (See Figure 1.2 in Chapter 1.) By comparison, freshwater comprises only 3% of the earth's water, and surface water is only 0.3% of that 3%. Furthermore, rivers and lakes comprise 2% and 87%, respectively, of surface water. Thus, rivers contain only 0.00018% and lakes only 0.0078% of the earth's water. 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 2000, out of a total of 345 billion gallons per day (Bgal/d) of the total freshwater consumption in the United States, water from streams, rivers, and lakes accounted for 262 Bgal/d (76%). (See Table 2.1 in Chapter 2.) The remaining 83.3 Bgal/d (24%) came from groundwater. Public utilities (public and private water suppliers) used 27.3 Bgal/d (63%) of surface water for their operations. (See Table 2.3 in Chapter 2.) Industries consumed 14.9 Bgal/d (76%) of surface freshwater for their requirements. (See Table 2.7 in Chapter 2.) Meanwhile, crop irrigation used 80 Bgal/d (58%) of surface water to water crops. (See Table 2.5 in Chapter 2.)

The withdrawal of surface water varies greatly throughout the United States. Figure 2.2 (see Chapter 2) shows that in 2000 California, Texas, and Florida used the most water per day, whereas the Dakotas used the least.


Rivers and Streams

The great rivers of the world have influenced human history. Settlements 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, 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. (See Figure 1.3 in Chapter 1.)

Rivers receive water from rain and melting snow, from underground springs and aquifers, and from 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 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, thereby changing the contour of the land. (The Grand Canyon is an example of how a river can carve 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. These deposits are called sediment. Finely grained sediment is called silt.

Over long periods, sediment deposited at the mouths of rivers forms triangular-shaped areas called deltas. During flood conditions some of the sediment of the delta flood-waters is deposited on the low-lying land (floodplain) surrounding the river. Enriched with this sediment, the delta floodplain often provides a rich base for agriculture. The ancient civilizations of Egypt, for example, depended on the land surrounding the delta of the Nile River to grow their food supply. On the contrary, deposited sediment can become a nuisance by 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 U.S. Geological Survey reports in "Lengths of the Major Rivers" (August 28, 2006, that the two longest rivers in the world are the Nile (4,132 miles) and the Amazon (4,000 miles). The Mississippi-Missouri river system is the third longest in the world. Taken separately, the Missouri River (2,540 miles) is the longest in the United States and the Mississippi (2,340 miles) the second longest. (See Table 3.1.)

The Mississippi River has an enormous watershed, the land from which it receives runoff water from rainfall or snowmelt. According to the Mississippi National River and Recreation Area (April 12, 2004,, this watershed is between 1.2 and 1.8 million square miles, or about 41% of the total land area of the lower forty-eight states. The Mississippi River is considered the largest river in the United States by the measure of the average volume at its mouth. (See Table 3.1.) The "Mighty Mississippi," as it is often called, discharges its water into the Gulf of Mexico at an average rate of 593,000 cubic feet per second.


Unlike rivers, lakes are depressions in the earth that hold water for extended periods. Reservoirs are human-made lakes created when a dam is built on a river. They are generally used to store water for uses such as drinking, irrigation, or producing electricity. Often, reservoir areas are used for recreation as well. Some ponds are made for livestock watering, fire control, storm water 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 and build up.

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.

Largest rivers in the United States in length and average discharge volume at mouth
RiverLocation at mouthAverage discharge at mouth (cfs)Lengtha (ml.)
Note: cfs is cubic feet per second.
aIncluding headwaters and sections in Canada.
bBelow Mississippi diversion, without headwaters.
Source: J.C. Kammerer, "Largest Rivers in the United States in Discharge, Drainage Area, or Length," in Water Fact Sheet: Largest Rivers in the United States, U.S. Department of the Interior, U.S, Geological Survey, May 1990, (accessed January 4, 2007)
St. LawrenceCanada348,0001,900
OhioIL, KY281,0001,310
TennesseeKY68,000 886
MobileAL67,200 774
KuskoswimAK67,000 724
CopperAK59,000 286
AtchafalayaLA58,000 140b
StikineAK56,000 379
SusitnaAK51,000 313
TananaAK41,000 659
SusquehannaMD38,200 447
WillametteOR37,400 309
NushagakAK36,000 285

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.


According to the U.S. Environmental Protection Agency (EPA), in The Hydrologic (Water) Cycle (2007,, the large freshwater lakes of the world contain about 30,000 cubic miles of water and cover a combined surface area of about 330,000 square miles. In "NatureWorks: Lakes" (2007,, New Hampshire Public Television indicates that by surface area Lake Superior, which is located on the U.S.-Canadian border, is the largest freshwater lake in the world. Lake Baikal in Asiatic Russia, however, is the deepest freshwater lake in the world, while Oregon's Crater Lake is the deepest in the United States.

Harvey A. Bootsma and Robert E. Hecky report in "A Comparative Introduction to the Biology and Limnology of the African Great Lakes" (Journal of Great Lakes Research, 2003) that the large lakes of Africa (Victoria, Tanganyika, and Malawi) contain about 32% of the volume of all freshwater lakes on Earth (7,062 cubic miles [mi3] or 29,435 cubic kilometers [km3] of 21,832 mi3 or 91,000 km3). The North American Great Lakes (Superior, Michigan, Huron, Erie, and Ontario) hold slightly less, or about 25% of the volume of the freshwater lakes (5,472 mi3 or 22,807 km3). Lake Baikal holds slightly more water (5,662 mi3 or 23,600 km3) than the North American Great Lakes, holding about 26% of the volume of Earth's freshwater lakes. Together, these nine lakes hold 83% of the volume of all freshwater lakes.

The Geological Survey notes in Where Is Earth's Water Located? (August 28, 2006, that the saline (saltwater) lakes of the world contain almost as much water as the freshwater lakes (20,488 mi3 or 85,400 km3). Rene P. Schwarzenbach, Philip M. Gschwend, and Dieter M. Imboden, in "Ponds, Lakes, and Oceans" (Environmental Organic Chemistry, 2003), indicate that of that volume, however, nearly 92% is in the Caspian Sea (18,761 mi3 or 78,200 km3), which borders Russia and Iran. Most of the remainder is in lakes in Asia. North America's shallow Great Salt Lake is comparatively insignificant; it varies in depth and volume with climatic conditions, so exact comparisons are difficult.


People have always congregated on the shores of lakes and rivers. Benefiting from the many advantages of nearby water sources, they established permanent homes, then towns, cities, and industries. 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 wastes 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 wereand some still arecontaminated to the point of ecological deaththat is, that they are 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.

The CWA was enacted by Congress in 1972 in response to growing public concern over the nation's polluted waters. Although the Federal Water Pollution Control Act was enacted originally in 1948, it was amended many times and then reorganized and expanded in 1972. It continues to be amended almost every year.

The original 1948 legislation was intended to eliminate or reduce the pollution of interstate waters and improve the sanitary condition of surface and underground waters. However, on June 22, 1969, the Cuyahoga River in Cleveland, Ohio, burst into flames, the result of oil and debris that had accumulated on the river's surface. Clearly, the legislation was not achieving its desired goal.

The objective of the CWA was to "restore and maintain the chemical, physical, and biological integrity of the Nation's waters." Primary authority for the enforcement of this law lies with the EPA.

The CWA 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 CWA with reversing, in a single generation, what had been a decline in the health of the nation's water since the mid-nineteenth century.

Assessing and Monitoring the Quality of Water

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. The EPA met this mandate by researching and compiling the National Water Quality Inventory Report to Congress every two years. Along with reporting to Congress, the purpose of the biennial reports was to inform the public about water quality in the United States. The National Water Quality Inventory reports characterized water quality, identified water quality problems of national significance, and described programs implemented to restore and protect the nation's water.

In the 2000 report Water Quality: Key EPA and State Decisions Limited by Inconsistent an Incomplete Data (March 2000,, the U.S. General Accounting Office (GAO; now the U.S. Government Accountability Office) stated that "the National Water Quality Inventory does not accurately portray water quality conditions nationwide." The GAO noted that states collectively assess only a small percentage of waters and that monitoring assessments and the interpretation of assessment results varied across states. Thus, the GAO indicated that "the information in the Inventory cannot be meaningfully compared across states." In addition, the data collected by the states were often insufficient to enable them to pinpoint and clean up their water quality problems.

As a result, the EPA, states, tribes, and other federal agencies began collaborating on a new process to monitor the nation's waterways. Following the publication of the 2000 National Water Quality Inventory (August 2002,, the EPA entered a transition period in the gathering and analysis of water quality data in nationally consistent, statistically valid assessment reports. Its new reporting schedule is discussed in "Schedule for Statistically Valid Surveys of the Nation's Waters" (December 5, 2005, As of early 2007 the only new reports available were The National Coastal Condition Report II (2005) (December 2004, and The Wadeable Streams Assessment: A Collaborative Survey of the Nation's Streams (December 2006, The Wadeable Streams Assessment serves as a basis for most of the data in this chapter, and the National Coastal Condition Report II is used in Chapter 6.

In "An Introduction to Water Quality Monitoring" (March 21, 2007,, the EPA reports that the five major purposes for water quality assessment and monitoring were to:

  • characterize waters and identify changes or trends in water quality over time;
  • identify specific existing or emerging water quality problems;
  • gather information to design specific pollution prevention or remediation programs;
  • determine whether program goalssuch as compliance with pollution regulations or implementation of effective pollution control actionsare being met; and
  • respond to emergencies, such as spills and floods.

Agriculture Takes up the Challenge

According to the 2002 Census of Agriculture (March 20, 2006, by the U.S. Department of Agriculture (USDA), about 938 million acres, or roughly half of the continental United States, is used for agricultural production. Cropland accounts for 46% of the acreage, and 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 source pollutants enter bodies of water over large areas rather than at single points. Nonpoint sources of water pollution include agricultural runoff and soil erosion.) This situation presents a challenge to water quality management efforts.

The agricultural community shares in the growing national concern over water quality degradation. 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 in soil erosion of U.S. cropland between 1982 and 2003. (See Figure 3.1.) In 1982 a total of 3.1 billion tons of cropland eroded from the nation's agricultural areas. By 2003 this figure had dropped to 1.7 billion tons, a 43% decrease. The USDA, with state and local agencies, is providing technical assistance and financial incentives through many 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.


What Are Wadeable Streams?

Wadeable streams are exactly what their name suggests: flowing bodies of water in which people can walk throughout. Researchers can perform tests and take samples of water in wadeable streams without a boat. The EPA notes in the Wadeable Streams Assessment that approximately 90% of stream and river miles in the United States are wadeable streams. Thus, an assessment of the water quality of the nation's wadeable streams is an excellent indicator of the water quality of much of the flowing freshwater in the country.

The Wadeable Streams Assessment covers streams within the lower forty-eight states. Wadeable stream assessments in Alaska, Hawaii, Puerto Rico, and Guam are not included in the 2006 report, but will be included in future reports. The EPA's assessment reports on the water quality of wadeable streams nationally and by dividing the nation into three regions: the West, the Plains and Lowlands, and the Eastern Highlands. (See Figure 3.2.) The West includes mountainous and dry regions; the Eastern Highlands are the mountainous regions east of the Mississippi River; and the Plains and Lowlands include low-elevation areas of the East and Southeast and the plains areas (vast grassland regions). The sampling areas were selected using techniques that provided a random sample having the full range of variation of the wadeable streams of the United States.

Indicators of the Biological Health of Freshwater Streams

To determine the biological health of freshwater streams, EPA researchers examined the biological condition of the aquatic macroinvertebrates living there. Aquatic macroinvertebrates are animals without backbones that live in water and can be seen with the naked eye, such as certain fly larvae, worms, and beetles. The number and types of the aquatic macroinvertebrates living in a stream reflect the biological condition of the water, because the organisms are exposed to the pollutants and various other stressors in the water. Certain species of macroinvertebrates can survive only in freshwater of good quality, whereas others can survive in good-, fair-, or poor-quality water. Fish live on the macroinvertebrates in a stream, so the presence of certain species of fish reflects not only the conditions in which certain fish species survive but also the presence of certain types of aquatic macroinvertebrates on which they typically feed.

In a simplified example, if researchers find a thriving community of mayfly larvae, riffle beetles, and trout, then the water quality is likely to be good. If these organisms are absent and crayfish, dragonfly nymph, and clams predominate, then the water quality is likely fair. An abundance of aquatic worms, leeches, black fly larvae, and catfish signal poor water quality. However, organisms that survive in fair- or poor-quality water can also survive in good-quality water, so determining not only the presence but also the relative abundance of species of organisms inhabiting the stream sample area is important.

Factors Responsible for Diminished Water Quality

For the 2006 Wadeable Streams Assessment, EPA researchers also measured factors responsible for diminished water quality, which are called aquatic indicators of stress, stressor indicators, or, simply, stressors. Some stressors are naturally occurring, and some are the result of human activity. According to the EPA:

Most physical stressors are created when we modify the physical habitat of a stream or its watershed, such as through extensive urban or agricultural development, excessive upland or bank erosion, or loss of streamside trees and vegetation. Examples of chemical stressors include toxic compounds (e.g., heavy metals, pesticides), excess nutrients (e.g., nitrogen and phosphorus), or acidity from acidic deposition or mine drainage. Biological stressors are characteristics of the biota that can influence biological integrity, such as the proliferation of non-native or invasive species (either in the streams and rivers, or in the riparian areas adjacent to these water bodies).

Water of good quality has low levels of pollutants and other stressors, and a high level of dissolved oxygen. Fair water quality has a higher level of pollutants and other stressors than good-quality water, and a lower level of dissolved oxygen. Poor water quality has even higher levels of pollutants and other stressors, and even lower levels of dissolved oxygen.


Figure 3.2 shows a summary of the condition of wadeable streams across the country in 2006. The streams in the West had the best water quality, with 45.1% in good condition, whereas 29% of the streams of the Plains and Lowlands were in good condition, and 18.2% of those in the Eastern Highlands were in good condition. Overall, only 28.2% of the streams in the coterminous (lower forty-eight) states of the United States were in good condition.

The EPA notes that the water quality of the streams of the Eastern Highlands was of the greatest concern. Not only were a mere 18.2% in good condition but also over half (51.8%) were in poor condition. This compares to 40% in poor condition in the Plains and Lowlands, and 27.4% in poor condition the West. Overall, 41.9% of the nation's streams were in poor condition in 2006.

Macroinvertebrate Index of Biotic Condition

The Macroinvertebrate Index of Biotic Condition is a statistical measure that provides a total score based on six characteristics of the macroinvertebrates found in a particular water sample. This total score is one indicator of water quality. Researchers determine which species of macroinvertebrates are present in a particular water sample, the proportion of each, the level of diversity of species within the sample (high-quality water has a high level of diversity), and the feeding habits, habitats, and pollution tolerance of the species present. Each of these characteristics is an indicator of water quality, and together they provide a snapshot of the macroinvertebrate "naturalness" in the portion of the stream tested. The researchers then use this total score, factor in the stream length represented by the study site, and use the data from all the study sites to compile the Macroinvertebrate Index ratings for the stream miles of a region and for the nation.

Figure 3.3 shows the Macroinvertebrate Index of Biotic Condition for the coterminous states as well as for the Eastern Highlands, the Plains and Lowlands, and the West. Nationally, 28.2% of the total miles of wadeable streams was in good condition in terms of macro-invertebrate life, 24.9% was in fair condition, and 41.9% was in poor condition. The Eastern Highlands had the most stream length in poor condition: 51.8%. The West had the most stream length in good condition: 45.1%.

Chemical Stressors

Four chemical stressors were assessed for the 2006 Wadeable Streams Assessment : phosphorus, nitrogen, salinity, and acidification. The levels of these stressors in the samples were compared with data from a set of "least disturbed" reference sites in each region to develop regional thresholds for all indicators.


Phosphorus and nitrogen are plant nutrients. When phosphorus and nitrogen enter bodies of waterusually as runoff from fertilized land, leaking septic systems, or sewage dischargesthey promote aquatic plant and algal growth. Plant and algal growth can become excessive, a process called eutrophication. This excessive growth can result in waters clogged with plants and algae, which can look unsightly, slow water flow, and interfere with swimming and fishing. Mats of algae can grow on the surface of the water, blocking light to plants beneath. When these plants die, bacteria degrade them, using oxygen in the process and diminishing the concentration of oxygen in the water available for aquatic macroinvertebrates and fish. With lowered dissolved oxygen in the water, many fish and invertebrates die, worsening the situation.

Phosphorus is a common component of fertilizers and was routinely found in laundry detergents until the industry removed phosphates from its products in 1994. However, phosphates are still found in dishwashing detergents and in some cleaners. These phosphates wash down the drain during or after use and enter either septic systems or sewage treatment plants. From there, the phosphates can end up in bodies of water as they leach into the ground from septic systems or are discharged into streams with treated wastewater from sewage treatment plants. Agricultural runoff containing phosphate fertilizers is also a common source of added phosphates in bodies of water. The EPA summarizes in the Wadeable Streams Assessment by stating that "high phosphorus concentrations in streams may be associated with poor agricultural practices, urban runoff, or point-source discharges (e.g., effluents from sewage treatment plants)."

Figure 3.4 shows the percent of stream miles with low, medium, and high levels of phosphorus compared to regional references. "Low" means the concentrations were most similar to the reference (most natural) condition. "Medium" and "high" had statistical bases, but they can be thought of as above the regional reference (medium) and much above the regional reference (high). Nationally, nearly one-third (30.9%) of all stream miles had high levels of total phosphorus. However, nearly half (48.8%) of all stream miles had low levels of this chemical stressor.

Regionally, the highest percentage of stream miles with high levels of phosphorus was in the Eastern Highlands, where 42.6% of all stream miles had high levels of the chemical. (See Figure 3.4.) The Plains and Lowlands had 24.9% of its stream miles high in phosphorus, whereas the West had the least number of stream miles with high levels of this chemical stressor: 18.5%.

Nitrogen is another plant nutrient, as noted previously. It finds it way into streams primarily from agricultural runoff (it is found in fertilizer), wastewater and animal waste (it is a waste product from the digestion of protein), and atmospheric deposition (it is released into the air when fossil fuels, such as gasoline and coal, are burned). Nitrogen is particularly important as a contributor to the rapid, excessive growth of algae along coastal waters and in estuaries, where freshwater meets the saltwater of the ocean.

Figure 3.5 shows the percent of stream miles with low, medium, and high levels of nitrogen compared to regional references. The statistics are similar to those for phosphorus. Nationally, nearly one-third (31.8%) of all stream miles had high levels of total nitrogen. However, close to half (43.3%) of all stream miles had low levels of this chemical stressor.

Regionally, the highest percentage of stream miles with high levels of nitrogen was in the Eastern Highlands, where 42.4% of all stream miles had high levels of the chemical. (See Figure 3.5.) The Plains and Lowlands had about one-quarter (27.1%) of their stream miles high in nitrogen, whereas the West had the least number of stream miles with high levels of this chemical stressor: 20.5%.


Excessive salinity in freshwater streams generally occurs because water is lost from the stream, not because excessive salts enter the stream. This happens when the evaporation rate of stream water is high. Already high salinity from evaporation can be made higher by repeated water withdrawals for irrigation or other purposes.

Figure 3.6 shows the percent of stream miles with low, medium, and high levels of salinity compared to regional references. Nationally, 3% of all stream miles had high salinity conditions, whereas 82.5% had low salinity conditions.

Regionally, the highest percentage of stream miles with high salinity conditions was in the Plains and Lowlands, where 5% of all stream miles had high levels of salts. (See Figure 3.6.) The West had only 2.6% of its stream miles experiencing high salinity, whereas the Eastern Highlands had the least number of stream miles with high levels of this chemical stressor: 1.3%.


Stream acidification means that the water has become more acidic than is natural. Figure 3.7 shows that the pH of a healthy lake (or stream) is about 6.5. The pH scale shows levels of acidity. This scale is numbered from zero to fourteen, with a pH value of seven considered neutral. Values higher than seven are considered more alkaline or basic; values that are lower than seven are considered acidic. Pure, distilled water has a pH level of seven.

The pH scale is a logarithmic measure. This means that every pH drop of one is a tenfold increase in acid content. Therefore, a decrease from pH six to pH five is a tenfold increase in acidity; a drop from pH six to pH four is a hundredfold increase in acidity; and a drop from pH six to pH three is a thousandfold increase.

"Clean" rainfall has a pH of 5.6. It is not neutral because it is not pure water; it accumulates naturally occurring sulfur oxides and nitrogen oxides as it passes through the atmosphere. In comparison, acid rain (or acid deposition) has a pH of about 4.2 to 4.4. The introduction of large volumes of acid deposition can over time increase the acidity of a body of water by as much as a hundredfold.

One of the main components of acid deposition is sulfur dioxide from the burning of fossil fuels, mainly from auto exhaust and coal-burning power plants. (See Figure 3.8.) As sulfur dioxide reaches the atmosphere, it becomes sulfuric acid when it joins with hydrogen atoms in the air. Nitric oxide and nitric dioxide are the other major components of acid deposition. Like sulfur dioxide, these nitrogen oxides are produced from the burning of fossil fuels. They rise into the atmosphere and oxidize in clouds to form nitric acid.

Figure 3.8 illustrates how sulfur and nitrogen oxides are carried into the air to become acid deposition. Gases and particulate matter are carried into the atmosphere, where they mix with moisture and other pollutants to form dry (aerosols, particles, and gases) and wet (fog, hail, rain, sleet, snow, dew) acid deposition. Wet deposition returns to the earth as precipitation, which enters the water body directly, percolates through the soil, or becomes runoff to nearby water bodies. Dry deposition builds up over time on all dry surfaces and is transported to water bodies in runoff during periods of precipitation or falls directly onto a water surface.

Figure 3.9 shows acidification in U.S. streams. In the Wadeable Streams Assessment, the EPA states that "about 2% of the nation's stream length (14,763 miles) is impacted by acidification from anthropogenic [humanrelated] sources. These sources include acid deposition (0.7%), acid mine drainage (0.4%), and episodic acidity due to high-runoff events (1%). Although these numbers appear relatively small, they reflect a significant impact in certain parts of the United States, particularly in the Eastern Highlands region, where 3.4% of the stream length (9,396) is impacted by acidification."

Physical Habitat Stressors

Freshwater streams are the physical habitats (natural homes) for a variety of plants and animals. The physical characteristics of a stream can be changed by human activities, and those changes can be stressors for the organisms that live there. EPA researchers assessed these physical characteristics of wadeable streams: streambed sediments, in-stream fish habitat, riparian vegetative cover, and riparian disturbance.


Water and sediments drain into streams as the result of a number of human-related activities, including agriculture, road building, construction, and the grazing of farm animals. Drainage of water and sediments into a stream can affect its size and shape. In addition, the size of the sediment particles can affect the streambed. If sediment particles are large, the stream may not be able to move them downstream, so they eventually accumulate in the streambed, changing habitats. If the particles are small but excessive, the stream may not be able to move them as well. Fine sediments that are left in the streambed can begin filling in the habitat spaces between stones and boulders on the stream bottom. Suspended fine sediments can block sunlight to aquatic plants and abrade the gills of fish. All these occurrences can negatively affect macroinvertebrates and fish. (See Figure 3.10.)

The EPA notes in the Wadeable Streams Assessment that "25% of the nation's stream length (167,092 miles) has streambed sediment characteristics in poor condition compared to regional reference condition. Streambed sediment characteristics are rated fair in 20% of the nation's stream length (132,197 miles) and good in 50% of stream length (336,197 miles) compared to reference condition. The two regions with the greatest percentage of stream length in poor condition for streambed sediment characteristics are the Eastern Highlands (28%, or 77,381 miles) and the Plains and Lowlands (26%, or 63,958 miles) regions, whereas the West region has the lowest percentage of stream length (17%, or 26,522 miles) in poor condition for this indicator." (See Figure 3.11.)


Streams and rivers that have diverse and complex habitats support a diversity of fish and macroinvertebrates. Such habitats include undercut banks with exposed tree roots, brush and large pieces of wood within the stream, and boulders within the stream and at the stream bank. Cover from overhanging vegetation also affects stream habitats. When humans use streams, they often change these complex habitats to simpler ones, which often results in a reduction in the diversity of the organisms living there.

Figure 3.12 shows that 19.5% of the stream miles across the nation had poor in-stream habitat conditions. About one-fourth (24.9%) had fair conditions, and about half (51.5%) had good conditions. The highest percentage of stream miles with poor in-stream habitat conditions was in the Plains and Lowlands, with 37% of stream miles rated poor. The West was next, with 12.3% of stream miles rated poor, and the Eastern Highlands had only 8.2% of stream miles rated poor. Nonetheless, the West had the greatest number of stream miles with instream habitats rated good: 66.4%.


The word riparian means the banks of a body of water, such as a river or stream. In the Wadeable Streams Assessment, riparian vegetative cover refers to the amount and type of vegetation growing on or next to stream banks; it is an indicator of the health of a stream. Complex, multilayered riparian vegetation helps maintain the health of the stream by reducing runoff from the surrounding land, preventing stream bank erosion, supplying shade, and providing food and habitats in the form of leaf litter and large wood. As with in-stream habitats, riparian coverage is often changed, or simplified, by humans. EPA researchers assessed the ground layer, woody shrubs, and canopy trees of the riparian cover of streams.

Figure 3.13 shows that 19.3% of the wadeable stream miles nationally were in poor condition "due to severely simplified riparian vegetation," 28.3% were in fair condition, and 47.6% were in good condition. Regionally, the Plains and Lowlands had the greatest percentage of stream miles in poor condition (26%) with respect to riparian coverage. The Eastern Highlands followed with 17.6% in poor condition, and the West had the least number of stream miles in poor condition: 12.2%.


As mentioned earlier, human activities can change or disturb the riparian vegetative cover. The closer potentially disturbing human activities take place to the stream bank, the more likely they are to cause riparian disturbance. To determine riparian human disturbance, EPA researchers tallied eleven forms of human activities and disturbances along sections of streams and weighted them according to how close they were to the streams.

Figure 3.14 shows that nationally one-fourth (25.5%) of stream length had high riparian disturbance when compared to reference sites. Nearly half (46.8%) had fair riparian disturbance, and one-fourth (23.6%) had low riparian disturbance. The EPA notes in the Wadeable Streams Assessment that "one of the striking findings [was] the widespread distribution of intermediate levels of riparian disturbance," both nationally and regionally. Furthermore, the EPA states, "It is worth noting that for the nation and the three regions, the amount of stream length with good riparian vegetative cover was significantly greater than the amount of stream length with low levels of human disturbance in the riparian zone. This finding warrants additional investigation, but suggests that land managers and property owners are protecting and maintaining healthy riparian vegetation buffers, even along streams where disturbance from roads, agriculture, and grazing is widespread."

Relative Extent and Relative Risk of Stressors

Figure 3.15 shows a list of stressors caused by human activity, the relative extent to which each affected the freshwater streams of the United States in 2006, and the relative risk they posed to macroinvertebrates.

The bar graph on the left shows what percentage of stream length nationally each stressor affects and what its relationship is to the other stressors. Each stressor is ranked according to the proportion of stream length that was in poor condition nationally. Excessive nitrogen levels affected the greatest percentage of stream miles at 31.8%. Phosphorus, the other plant nutrient assessed, was a close second, affecting 30.9% of stream miles nationally. Excess salinity and acidification affected the least percentage of stream miles, 2.9% and 2.2%, respectively.

The bar graph on the right shows the national relative risk values for each stressor. A relative risk value of 1 means no association between the stressor and the biologic health of wadeable stream macroinvertebrates. Thus, acidification, with a relative risk value of 1, does not pose a risk to stream macroinvertebrates. Values greater than 1 suggest a positive association; the higher the number, the greater the risk. As such, the poor condition of streambed sediments poses the greatest relative risk to stream macroinvertebrates of all the stressors assessed. Excessive phosphorus and nitrogen pose the next greatest relative risks, respectively.


The older EPA water quality reports were set up quite differently from the Wadeable Streams Assessment, which is typical of the new reports published by the EPA on water quality. In the 2000 National Water Quality Inventory report, a use was designated for surface water bodies in each state. The state then established water quality numeric and narrative criteria to protect each use. More than one designated use was frequently assigned to a water body. Most water bodies were designated for recreation, drinking water use, and protection of aquatic life.

The 2000 National Water Quality Inventory assessed 43% of the nation's 40.6 million acres of lakes, ponds, and reservoirs. Forty-seven percent were found to fully support their designated uses. (See Figure 3.16.) However, 8% of the lake acres were threatened. Of the lakes assessed, 45% could only partially support their designated uses.

Leading Pollutants/Stressors in Lakes, Ponds, and Reservoirs

A lake's water quality reflects the condition and management of its watershed. In 2000 elevated levels of plant nutrients (phosphorus and nitrogen) were identified as the most common stressors, contributing to 50% of the impaired water quality in lakes. Figure 3.17 shows the top stressors and the percentage of impaired lake acres affected by each.

Metals were the second most prevalent stressor, 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. Mercury generally enters the water from the air, often released from the smokestacks of 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. (Figure 3.17.)

Sources of Pollutants/Stressors in Lakes

As in the case of streams, agricultural runoff was a significant source of pollution for lakes, affecting 41% of impaired lake acres in 2000. (See Figure 3.18.) 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. (See Figure 3.18.) These modifications, resulting from regulation of the flow of water, 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.


The Great Lakes Environmental Research Laboratory reports in "About Our Great Lakes: Great Lakes Basin Facts" (June 18, 2004, that the Great Lakes basin, which is shared with Canada, is home to thirty-five million people. The lakes provide drinking water for about forty million people. The five lakes are the largest surface area of freshwater in the world, at ninety-five thousand square miles. The water in the Great Lakes accounts for 90% 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 ten thousand miles long and equal to about one-quarter of the earth's circumference. International shipping on the Lakes annually transports two hundred million tons of cargo, and sport fishing contributes $4 billion to the economy.

The prosperity of the Great Lakes region, however, has taxed its ecological health. 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, the watershed still contains many ecologically rich areas. In "About Our Great Lakes: Ecology" (June 2, 2004,, the Great Lakes Environmental Research Laboratory indicates that approximately thirty-five hundred species of plants and animals inhabit the Great Lakes basin.

Great Lakes Water Quality Agreement

In 1972 the United States and Canada entered into the Great Lakes Water Quality Agreement, which is a worldwide model for cooperative environmental protection and natural resource management. The agreement 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 and the most recent was in late 2006.

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.

The report published after SOLEC 2004, State of the Great Lakes 2005 (2005, Report/English%20Version/Complete%20Report.pdf), presents the following mixed news about the chemical, physical, and biological integrity of the waters of the Great Lakes basin ecosystem. Some of the good features identified include:

  • Persistent toxic substances are continuing to decline.
  • The Great Lakes are a good source for treated drinking water.
  • Total forested land in the Great Lakes basin appears to have increased in recent decades. Approximately 50% of the Great Lakes basin is covered by forest.
  • Bald eagles are continuing to nest and fledge along the Great Lakes shorelines.
  • Lake trout stocks in Lake Superior have remained self-sustaining.
  • Natural reproduction of lake trout is evident in Lake Ontario and in isolated areas of Lake Huron.
  • Mayfly (Hexagenia ) populations have partially recovered in western Lake Erie and in the Bay of Quinte, Lake Ontario.
  • Phosphorus targets have been met in Lakes Ontario, Huron, Michigan and Superior.

Some of the negative features identified include:

  • Nonnative species are a significant threat to the ecosystem and continue to enter the Great Lakes (aquatic and terrestrial species).
  • Scud (Diporeia ) populations continue to decline in Lakes Michigan, Ontario, and Huron.
  • Type E botulism outbreaks, resulting in the deaths of fish and fish-eating birds, have recently been detected in a few locations along the Lake Ontario shoreline, and minor outbreaks are continuing in Lake Erie.
  • Groundwater resources are being negatively impacted by development, withdrawal and agricultural drainage.
  • Long-range atmospheric transport is a continuing source of contaminants to the Great Lakes basin.
  • Native mussel populations continue to be decimated as a result of invasive zebra mussels.
  • Land use changes in [favor] of urbanization along the shoreline continue to threaten natural habitats in the Great Lakes and St. Lawrence River ecosystems.
  • Some species of amphibians and wetland-dependent birds are showing declines in population numbersin part due to wetland habitat conditions.
  • Phosphorus levels are still above guidelines in Lake Erie.


When fish or shellfish in particular locations contain harmful levels of pollutants, the state issues advisories to recreational fishermen against eating the fish. Commercial fishing is usually banned. Since 1993 the EPA has compiled these advisories annually and made them available to the public. Fish advisories are advice to limit or avoid eating certain fish.

Figure 3.19 shows the number of advisories against eating fish or wildlife reported by the states to the EPA in 2004. These advisories are specific as to location, species, and pollutant. Some advisories caution against eating any fish from a particular location, whereas 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 in 2004 included warnings that women who are pregnant or nursing and young children should avoid eating certain kinds of fish. Consuming mercury can damage the developing nervous systems of babies and children.

In 2004 fifty states, the District of Columbia, American Samoa, Guam, the Virgin Islands, and Puerto Rico reported 3,221 fish and wildlife consumption advisories. (See Figure 3.19.) The EPA indicates in "Bioaccumulative Pollutants" (2004, that the bioaccumulative chemicalsmercury, polychlorinated biphenyls, chlordane, dioxins, and dichloro-diphenyltrichloroethane (DDT)cause most advisories. These pollutants are called bioaccumulative because when ingested by certain species of fish and waterfowl they are not metabolized and excreted from the organism. Instead, they are stored in the fatty tissues and remain there. As more chemical is ingested, more accumulates in the organism.

The use of polychlorinated biphenyls, 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. According to "Fishing Warnings up Due to Mercury PollutionEPA" (Reuters News Service, August 25, 2004), U.S. coal-burning power plants are the largest source of mercury in the United States, releasing about forty-eight tons of the toxin annually. Federal standards require a 70% reduction in mercury emissions by 2018, but some states are passing legislation to reduce emissions much sooner.

Recreational Water-Associated Outbreaks

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, and thirteen states and tribes identified over two hundred sites where recreation was restricted at least once during the reporting cycle. Local health departments closed many of those sites more than once. Pathogens (disease-causing organisms) 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.

Eric J. Dziuban et al., in "Surveillance for Waterborne Disease and Outbreaks Associated with Recreational WaterUnited States, 20032004" (Morbidity and Mortality Weekly Report, December 22, 2006), list the incidence of disease outbreaks caused by recreational water contact. During this two-year period twenty-six states and the territory of Guam reported sixty-two outbreaks involving nearly twenty-seven hundred people. Of the sixty-two recreational waterborne disease outbreaks reported, thirty involved gastroenteritis. There were fifteen such outbreaks in 1999 and twenty-one in 2000. Figure 3.20 shows the number of waterborne disease outbreaks due to recreational water use annually from 1978 to 2004, with a breakdown by illness.

As part of the Beaches Environmental Assessment and Coastal Health (BEACH) Act of 2000, Congress 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 were to be used to help in the development of the new guidelines for recreational water.

The first report, Implementing the BEACH Act of 2000: Report to Congress (, was published in October 2006 by the EPA. The EPA summarizes the following achievements:

  1. States have significantly improved their assessment and monitoring of beaches; the number of monitored beaches has increased from about 1,000 in 1997 to more than 3,500 out of approximately 6,000 beaches, as identified to EPA by the states for the 2004 swimming season.
  2. EPA has strengthened water quality standards throughout all the coastal recreation waters in the United States; the number of coastal and Great Lakes states with up-to-date water quality criteria has increased from 11 in 2000 to 35 in 2004.
  3. EPA has improved public access to data on beach advisories and closings by improving its electronic system for beach data collection and delivery systems; the system is known as "eBeaches." The public can view the beach information at
  4. EPA is working to improve pollution control efforts that reduce potential adverse health effects at beaches. EPA's Strategic Plan and recent National Water Program Guidance describe these actions to coordinate assessment of problems affecting beaches and to reduce pollution.
  5. EPA is conducting research to develop new or revised water quality criteria and more rapid methods for assessing water quality at beaches so that results can be made available in hours rather than days. Quicker tests will allow beach managers to make faster decisions about the safety of beach waters and thus help reduce the risk of illness among beachgoers.