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.
FRESHWATER VERSUS SALTWATER.
The large freshwater lakes of the world contain nearly 30,000 cubic
Largest rivers in the United States
SOURCE: "Largest Rivers in the United States," in Water Fact Sheet: Largest Rivers in the United States, U.S. Geological Survey, 1990
||Location at mouth
||Average volume at mouth (cfs)
|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.