A lake is a body of standing water with an upper surface that is exposed to the atmosphere and does not have an appreciable gradient. Ponds, marshes, and swamps with standing water can all be included under the definition of a lake. Lakes receive water from streams, overland flow, and ground water, and so they form part of drainage systems. Many lakes lose water at an outlet, where water drains over a dam—natural or constructed— to become an outflowing stream. Lakes also lose water by evaporation. Lakes, like streams, are landscape features but are not usually considered to be landforms.
Lakes are quite important as sources of fresh water and food, such as fish. They can also be used to generate hydroelectric power, using dams. And, of course, lakes and ponds are sites of natural beauty. Lake basins, like stream channels, are true landforms, created by a number of geologic processes and ranging widely in size. For example, the tectonic process of crustal faulting creates large, deep lakes. Lava flows often form a dam in a river valley, causing water to back up as a lake. Landslides can also suddenly create lakes.
Where there aren't enough natural lakes, we create them by placing dams across the stream channels. Many regions that once had almost no lakes now have many. Some are small ponds built to serve ranches and farms, while others cover hundreds of square kilometers. In some areas, the number of artificial lakes is large enough to have significant effects on the region's hydrologic cycle. On a geologic time scale, lakes are short-lived features.
Lakes disappear by one of two processes, or a combination of both. First, lakes that have stream outlets will be gradually drained as the outlets are eroded to lower levels. Even when the outlet lies above strong bedrock, erosion will still occur slowly over time. Second, inorganic sediment carried by streams enters the lake and builds up, along with organic matter produced by plants and animals within the lake. Eventually, the lake fills, forming a boggy wetland with little or no free water surface. Many former freshwater ponds have become partially or entirely filled by organic matter from the growth and decay of water-loving plants.
Lakes can also disappear when the climate changes. If precipitation is reduced, or temperatures and net radiation increase, evaporation can exceed input and the lake will dry up. Many former lakes of the southwestern United States that flourished during the Ice Age have now shrunk greatly or have disappeared entirely. The water level of lakes and ponds in moist climates closely coincides with the surrounding water table (Figure 14.29). The water surface is maintained at this level as ground water seeps into the lake and as precipitation runs off.
THE GREAT LAKES
The Great Lakes—Superior, Huron, Michigan, Erie, and Ontario—along with their smaller bays and connecting lakes form a vast network of inland waters in the heart of North America (Figure 14.30). They contain 23,000 km3 (5500 mi3) of water—about 18 percent of all the fresh, surface water on Earth. Only the polar ice caps and Lake Baikal in Siberia have a larger volume. Of the Great Lakes, Lake Superior is by far the largest. In fact, the volume of the other Great Lakes combined would not fill its basin. The Great Lakes watershed contains a population of about 33 million people—22.8 million Americans and 9.2 million Canadians. The lakes are an essential resource for drinking water, fishing, agriculture, manufacturing, transportation, and power generation.
The Great Lakes are largely the legacy of Ice Age glaciation, formed in a low interior basin of old, largely sedimentary rock. During at least four major periods in the last two million years, ice sheets advanced over this basin, scouring the rocks and lowering the surface by as much as 500 m (1600 ft) below the surrounding terrain. As the continental ice sheets of the last glacial advance retreated, water filled these depressions, creating lakes dammed by glacial deposits and melting ice. With the final melting of the ice and a slow, gentle uplift of the terrain, the lakes eventually acquired their present shapes and configurations.
Because of their position close to centers of population and agricultural development, the Great Lakes have suffered significant water pollution. Lake Erie, with the smallest water volume and its heavily developed coastal region, was hard hit in the 1960s and 1970s. Persistent organic compounds have also been a source of concern. These include organic substances largely of industrial origin that are long-lasting, highly mobile in the aquatic system, and toxic in very small amounts.
Many of these compounds accumulate up the food chain as predators consume contaminated prey. In 1987 an American and Canadian commission identified 43 “Areas of Concern,” with 28 in the United States and 15 in Canada. Remedial action plans were proposed and implemented, and many of the sites have experienced much improvement.
SALINE LAKES AND SALT FLATS
In arid regions, we find lakes with no surface outlet. In these lakes, the average rate of evaporation balances the average rate of stream inflow. When the rate of inflow increases, the lake level rises and the lake's surface area increases, allowing more evaporation—striking a new balance. Similarly, if the region becomes more arid, reducing input and increasing evaporation, the water will fall to a lower level.
Salt often builds up in these lakes. Streams bring dissolved solids into the lake, and since evaporation removes only pure water, the salts remain behind. The salinity, or “saltiness,” of the water slowly increases. Eventually, the salinity level reaches a point where salts are precipitated as solids.
Sometimes the surfaces of such lakes lie below sea level. An example is the Dead Sea, with a surface elevation of ?396 m (?1299 ft). The largest of all lakes, the Caspian Sea, has a surface elevation of ?25 m (?82 ft). Both of these large lakes are saline. Another saline inland lake is the Aral Sea.
In some cases the lake is missing. In regions of high evapotranspiration and low precipitation, we find shallow empty basins covered with salt deposits instead of lakes. These are called salt flats or dry lakes. On rare occasions, these flats are covered by a shallow layer of water, brought by flooding streams.
Desert irrigation is as old as civilization itself. Two of the earliest civilizations—Egypt and Mesopotamia— relied heavily on irrigation with large supplies of water from nondesert sources. For Egypt and Mesopotamia, the water sources of ancient times were the rivers that cross the desert but derive their flow from regions that have a water surplus. These are referred to as exotic rivers because their flows are derived from an outside region.
Irrigation systems in arid lands can suffer from two undesirable side effects: salinization and waterlogging of the soil. Salinization occurs when salts build up in the soil to levels that inhibit plant growth. This happens when an irrigated area loses large amounts of soil water through evapotranspiration. Salts contained in the irrigation water remain in the soil and increase to high concentrations.
Salinization can be prevented or cured by flushing the soil salts downward to lower levels by the use of more water. This remedy requires greater water use than for crop growth alone. In addition, new drainage systems must be installed to dispose of the excess salt water. Waterlogging occurs when irrigation with large volumes of water causes a rise in the water table, bringing the zone of saturation close to the surface. Most food crops cannot grow in perpetually saturated soils. When the water table rises to the point at which upward movement under capillary action can bring water to the surface, evaporation is increased and salinization is intensified.