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Rivers convey the excess waters of the land areas that they drain to larger bodies of water or directly to the oceans. Collectively, they represent the world\'s WATER RESOURCES, as they carry virtually all the water that is available for human management and use.
The four great civilizations of early human history developed in close dependence on rivers and the fertile, easily worked soils of their floodplains: the Sumerians on the TIGRIS and EUPHRATES rivers in Iraq, the Harrapans on the INDUS in Pakistan, the Chinese on the HWANG HO (\"Yellow River\") and the YANGTZE RIVER in central China, and the Egyptians on the NILE RIVER in Egypt. Rivers and their valleys have continued to play important roles in the course of history; the exploration of much of North America was via river routes, and most of its major settlements are adjacent to rivers.
In addition to providing direct sources of water for domestic, agricultural, and industrial uses, rivers produce energy directly through hydropower generation they also provide the cooling water for many fossil and nuclear-fueled power plants. They serve as transportation routes, as carriers and natural \"treatment plants\" for human wastes, and as the habitats of ecologically, economically, and recreationally valuable fish and wildlife. The continually changing nature of rivers and their often spectacular scenic features make them a source of inspiration for many writers and artists, as well as for more casual observers.
RIVER DISCHARGE, NETWORKS, AND DRAINAGE BASINS
On a global basis, the significance of rivers as a water resource lies not in the amount of water they contain at a given time (only about 0.14% of the Earth\'s liquid freshwater and about 0.01% of all water), but in their average discharge, which is the total volume carried in a given time period. Worldwide, this amounts to about 39,000 cu km per year, or, expressed differently, 28 trillion gallons per day. The flow of the Amazon is about five times greater than that of the second largest river, the Congo, and amounts to more than 17% of the world\'s total river flow. In practice, the discharge of a river at a point is determined by multiplying the surface width by the average depth by the average velocity. In the United States, river discharges are measured and reported for about 16,000 stations by the U.S. Geological Survey.
Terms such as river, stream, and brook have been applied historically; the hydrologic distinction between them is negligible. Although a river is commonly considered a linear feature, in reality it is a treelike branching network. The smallest branches, which do not have tributaries, are designated as first-order streams. Where two first-order streams join, they form a second-order stream; the junction of two second-order streams forms a third-order stream, and so forth. If streams of about 1 mi (1.6 km) in length are considered as first order, the Mississippi River is a tenth-order stream; if smaller brooks and gullies are considered first order, the order of the Mississippi is much higher. A number of studies have shown that in a given region there tend to be three to five times as many streams of one order as of the next highest order. Also, the average lengths of streams and the average size of their drainage basins tend to increase regularly from one order to the next. Surprisingly, these regular relationships have been shown to be the result of random processes operating in the geologic evolution of river networks.
The area of land that contributes water to a river network upstream of a given point on a river is called the drainage basin, or watershed, of the river at that point. Such an area is generally defined on the basis of topography, by tracing drainage divides--ridge lines that separate the area contributing to one river network from that contributing to another--on a topographic map.
In a given region, the average volume of discharge at a given point on a river is directly proportional to the area of the drainage basin above that point. If this discharge is divided by the drainage area, and appropriate conversions of measurement units are made, the discharge can be expressed as a depth per unit time. When expressed this way, the average flow of streams of different sizes can be compared, and streamflow can be directly compared with precipitation (rainfall or melted snow), which is the ultimate source of all river water. For the United States as a whole, the average streamflow rate is about 23 cm/yr (9 in/yr), which is about 30% of the average precipitation of 76 cm/yr (30 in/yr). Average streamflows usually exceed 25 cm/yr (10 in/yr) east of the Mississippi River and rise to 100 cm/yr (40 in/yr) in mountainous areas. The rate for most of the region between the Mississippi and the mountains of the Far West is about 2.5 cm/yr (1 in/yr) but reaches 50 cm/yr (20 in/yr) in higher parts of the Rocky Mountains. In the Pacific Northwest, rates can be as high as 200 cm/yr (80 in/yr).
Water can enter the river network by falling directly onto a water surface, by traveling as overland flow from the surface of the drainage basin, or by moving as subsurface flow beneath the basin. About one-third of the discharge of the world\'s rivers comes from subsurface flow, which is of critical importance in maintaining stream flow between periods of rain and snow melt. Virtually all of the remaining two-thirds comes from overland flow. In arid regions and urbanized areas, almost the entire drainage basin may contribute to this flow, whereas in humid regions generally only the low areas adjacent to stream channels supply it. In these areas, the GROUNDWATER level is typically close to the ground surface and rises due to infiltrating rain or snowmelt. When it reaches the surface, no further infiltration can take place, and subsequent rain or snowmelt on these areas runs off quickly to the stream as overland flow. In the remainder of the drainage basin, particularly if it is forested, the soil is so permeable and the groundwater so far beneath the surface that virtually all the precipitation infiltrates.
RIVER CHANNELS
In general, a river shapes its own channel. Most of the time, however, streamflow is considerably less than the discharge required to fill the channel, and it only occasionally exceeds the channel\'s capacity. This general disparity between streamflow and channel capacity arises because the channel size is determined by moderately large flows, capable of significant erosional work, that occur relatively frequently (every 2 to 3 years); very large floods do much erosional work but are too infrequent to have a long-term effect on channel size, whereas the small flows present most of the time do little work. Because the channel capacity is determined by flows that occur every few years, most rivers naturally overflow their banks every 2 to 3 years.
When the rate at which water is delivered to a channel changes, the river will adjust its discharge by changing its velocity, its depth, and its width. The greatest change is usually in depth, because of both scouring of the channel bottom and adjustment of the level of the water surface. The second largest adjustment to a change in discharge is in velocity, and the smallest is in width. As one proceeds downstream in a river network, however, the increase in discharge almost always accompanying the increase in drainage area is accommodated mostly by an increase in width, with a smaller rate of increase in depth. Thus rivers become relatively as well as absolutely wider as one proceeds downstream. Average velocity also usually increases downstream in a river system, but at a slow rate. Exceptions to the downstream increase in discharge that characterizes most stream networks occur in arid regions. Channels there remain dry for most of the year. During a rainstorm runoff is rapid, sometimes causing a FLASH FLOOD. As the water proceeds along the previously dry channel, it gradually infiltrates, causing discharge to decrease downstream.
The slope of a river channel decreases between its source and its mouth, and the longitudinal profile of a well-established stream is thus a smooth curve, concave upward. The slope also changes with time, in response to changes in the river\'s discharge, sediment load, and BASELEVEL (the level at which it enters a large body of standing water, such as an ocean). During a flood, for example, a river has a high velocity and is capable of carrying more sediment than normal. It therefore cuts down into the river bottom, picking up more sediment and decreasing its slope until its velocity matches its sediment load. When the flood ends and the discharge returns to normal, the river rids itself of the excess sediment by redepositing it in the channel, building up its slope until velocity and sediment load are once more in balance. The river thus constantly tends toward the ideal of a graded stream--one in which slope and velocity are perfectly adjusted to carry the sediment load. Because baselevel, discharge, and sediment load are all highly variable, this process is never complete.
River channels display a variety of patterns when viewed from the air or on a map. A BRAIDED STREAM consists of a network of interconnected channels, with numerous bars and islands between. Generally streams with relatively steep slopes, they carry a considerable load of sand or gravel or both. Meanders are series of rather regularly spaced symmetrical bends that tend to appear in streams with low slopes and channels made of silt and clay. The ratio of the radius of curvature of the bends and of the \"wavelength\" of the bends to the stream width remains quite constant from stream to stream. The exact mechanism that produces meanders is unknown, but they are a common feature of many types of flows, including meltwater streams on glaciers and the Gulf Stream, and are probably due to the action of corkscrewlike currents that exist even in straight flows. Irregularly curved river channels also occur.
SOLID CONCENTRATIONS
Rivers carry dissolved and suspended solids as well as water. In the United States the average concentration of dissolved solids in natural (unpolluted) river water ranges from about 50 mg/l (0.06 oz/gal) in the humid western mountains and the Appalachians up to 1,000 mg/l (1.3 oz/gal) in the arid nonmountainous regions of the West.
The average concentration of suspended solids is 100 mg/l (0.13 oz/gal) or less in humid forests but up to 100,000 mg/l (133 oz/gal) in desert areas.
For a given stream, dissolved-solids concentrations generally decrease as discharge increases, whereas the reverse is true for suspended-solids concentrations. Rivers thus play an important geologic role as the carriers of the products of rock weathering and mechanical erosion.
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