rivers

rivers A river is a natural stream of water that flows from a higher to a lower elevation across a land surface. Rivers comprise only 0.0001 per cent of the water at the Earth's surface. This is a small but very important fraction, since rivers transport most of the water as well as dissolved and suspended matter from the continents to the oceans.

Rivers result from the run-off of water from the land surface. This run-off is derived from water that reaches the land surface in the form of precipitation (rain, hail, sleet, snow, etc.). Some of the water that falls on the land surface is lost by evaporation, some of it forms surface flow, and some of it sinks into the ground to become groundwater. In order to have river run-off, precipitation must exceed evaporation. There are two major belts on the Earth where precipitation exceeds evaporation. One is near the Equator, where there is both high rainfall and high evaporation but where rainfall exceeds evaporation. This results in large rivers such as the Amazon in South America, and the Congo in Africa. The second belt is in the temperate zone, which is characterized by generally adequate rainfall and lower evaporation rates. Two major rivers originating in this area are the Yangtze-kiang in Asia and the Mississippi in North America.

The composition of major dissolved solids in river water is affected by atmospheric sea salts, evaporation, chemical weathering of rocks, temperature, the rate of water circulation, relief, vegetation, and pollution. Small rivers are more diverse in composition than large rivers: the size of a river catchment tends to moderate variations by integrating different environments. The chemical composition of world average river water, both actual and ‘natural’, is given in Fig. 1. The ‘natural’ figures are corrected for pollution. Calcium is the most abundant cation and bicarbonate is the most important anion. The total content of dissolved solids of the unpolluted world average river is about 100 mg l⁻¹. Of the major elements, pollution contributes most to sulphur, chlorine, and sodium.

Sea salt in the atmosphere affects the chemistry of surface waters, particularly when they drain from crystalline rocks near the ocean. The concentration of sea salt in precipitation decreases rapidly inland and with increasing altitude. The atmospheric contribution of sea salt as a percentage of actual world average river water is shown in Table 1.

Of the rain that falls on the continents, more than half is returned to the atmosphere, either by direct evaporation or by transpiration through plants. The net effect of evaporation on river water is to remove pure water from solution, so that the concentrations of all dissolved components tend to increase. Although evaporation takes place in all climates, it is only in relatively arid regions that it is of great importance for the composition of river water. The average concentrations in river water are about 20 times those in rain. The increase in concentration due solely to evaporation is only twice that of the concentration in rainwater. The difference is primarily due to contributions from rocks and soil during chemical weathering, and to pollution.

Rocks can be arranged according their tendency to chemical weathering or dissolution in increasing order as follows: granites, gneisses, mica-schists, gabbros, sandstones, volcanic rocks, shales, serpentines and amphibolites, carbonates, and evaporites. The effect of the chemical weathering of rocks on the composition of river water is to integrate the chemical weathering tendency of each rock-type, its areal extension, and the amount of runoff from each rock-type. The contributions of the three main rock-types, carbonates, silicates, and evaporites, are shown in Table 1. The contribution of evaporite rocks is significant despite the fact that they cover only 1–2 per cent of the surface of the continents. This is due to the rapid dissolution of evaporites. Carbonates dominate world river chemistry. They dissolve rapidly, they are extensive, covering about 16 per cent of the surface of the continents, and they are located where runoff is abundant, primarily in the temperate regions. The bicarbonate (HCO₃⁻) derived from weathering comes from two sources. One is the carbon in carbonate minerals, such as calcite and dolomite. The other is the result of the reaction of carbon dioxide dissolved in soil water and groundwater with carbonate and silica minerals. The carbon dioxide is derived almost entirely from the bacterial decomposition of organic matter in soil. Since the organic matter in soil gains its carbon from the atmosphere by photosynthesis, the ultimate source of much of the HCO₃⁻ in river water is the atmosphere. Of the 61 per cent of HCO₃⁻ derived from weathering of carbonates, 34 per cent comes from calcite and dolomite, and 27 per cent from soil CO₂. For silicates, however, all 37 per cent is derived from soil CO₂ (Table 1).

Table 1 Sources of major elements in world river water (percentages of actual concentrations)

		Weathering
Atmospheric
Element	cyclic salt	Carbonates	Silicates	Evaporites	Pollution
Other sources of river SO42−: natural biogenic emissions to atmosphere delivered to land in rain, 3 per cent; volcanism, 8 per cent; pyrite weathering, 11 per cent. (From Berner, E. K. and Berner, R. A. (1996.)
Ca2+	0.1	65	18	8	9
HCO3−	≪ 1	61	37	0	2
Na+	8	0	22	42	28
Cl−	13	0	0	57	30
SO42−	2	0	0	22	54
Mg2+	2	36	54	≪ 1	8
K−	1	0	87	5	7
H4SiO4	≪ 1	0	99+	0	0

The dissolution rates of rock-forming minerals and their solubility are temperature-dependent: the higher the temperature, the faster the dissolution. The solubility of silicates increases with increasing temperature, but the solubility of carbonates decreases with increasing temperature. The weathering of silicate rocks is more temperature-dependent than the weathering of carbonates.

Great physical relief results in intense physical erosion and rapid exposure of fresh reactive rock for chemical weathering, but it provides less contact time between water and rocks. Because the removal of weathered material by transport is rapid with high relief, chemical weathering in the soil is incomplete, and soils are thin. With low relief, chemical weathering is much more complete and thick soils develop over the underlying rocks so that variations in rock-type become less important for the composition of rivers draining the soil.

Vegetation has two opposing effects on the composition of rivers. It increases chemical weathering by supplying CO₂ and organic acids to soil waters. However, vegetation also prevents physical weathering by stabilizing the soil and thus decreasing the exposure of bedrock to chemical weathering. The effects of rock-type, temperature, relief, and vegetation on the composition of river water are often hard to separate in a particular area.

Pollution can be an important source of some ions and major elements, such as sulphate (SO²⁻₄), chlorine (Cl), and sodium (Na) (Table 1). Population density, type and state of industry (as reflected in energy consumption per capita), and land use affect river chemistry. In general, the major element chemistry of rivers draining highly populated areas is subject to significant variation.

The three major nutrients in river water are carbon, nitrogen, and phosphorus. Carbon in the form of carbon dioxide (CO₂) is generally abundant and readily available. Nitrogen gas (N₂), the most abundant atmospheric gas, must be ‘fixed’ or combined with hydrogen (as NH₄⁺) or oxygen (as NO₃⁻), in order to be used by plants and organisms. Nitrogen is fixed by micro-organisms and lichens; ‘fixed’ nitrogen is dissolved in precipitation; and nitrogen is ‘fixed’ industrially to produce fertilizers. The concentration of phosphorus in rainfall is low. The chief sources of phosphorus in river water are the weathering of rocks and the leaching out of fertilizers from agricultural land. In general, human activities have raised the levels of dissolved phosphorus and nitrogen to twice their natural values. In Western Europe and North America, however, concentrations of dissolved phosphorus and nitrogen in rivers have increased by factors of 10 to 50. Within individual watersheds, the pollution of rivers is proportional to the population density and the energy consumption.

The trace elements in river waters can be derived from the weathering of rocks, from volcanic eruptions, or, increasingly, from human activities. The processes controlling the concentration of trace elements in river waters are poorly understood. Even the average natural concentrations of dissolved trace elements in river waters are highly debated. Samples can easily be contaminated, detection limits of the analytical methods are sometimes higher than the natural levels, and few major pristine rivers have been studied. The reported average natural concentrations have been declining in recent decades, primarily because of improved sampling and analytical techniques. Considerable amounts of trace elements in river waters are adsorped on to suspended inorganic and organic material in the water.

Sigurdur Reynir Gislason

Bibliography

Berner, E. K. and and Berner R. A. (1996) Global environment: water, air, and geochemical cycles. Prentice-Hall, New Jersey.