tems are showing departures from the steady state. Modern watersheds are accumulating more sediment than they pass on. In many rivers a large proportion of the sediment is transported by flood events that prevail only a few days of the year. How the climatic conditions that favor these events relate to the magnitudes and frequencies of erosional and transportation episodes is emerging as an area of great scientific interest.
Erosion by other agents, such as glaciation and wind, plays some part in the degradation of topography, but most eroded material is carried to the sea by rivers either in solution or as detrital sediment. River transport varies enormously with both climate and source area; the Huang He (Yellow River) alone, for example, accounts for 6 percent of the world's total suspended matter river load, because this river drains the readily eroded windblown material of the loess plateau in the Chinese interior. In addition, the beginnings of agriculture more than 4,000 years ago appears to have greatly increased the rate at which sediment is moving into the river.
Although most of the material carried into sedimentary basins is transported by rivers, appreciable amounts are carried by wind and glaciers. Deserts, where wind erosion and deposition in the form of sand dunes are most important, have become the subject of increased research activity. This activity has been sparked by such diverse influences as the rapid extension of the Sahara into the Sahel within the past 30 years, the discovery of wind-made landforms on Mars, and the availability of satellite and radar images of the Earth's remote deserts (Figure 3.4). Again, new techniques of age determination have yielded exciting results.
Recent measurements of the thermoluminesence of rocks from surfaces buried beneath dunes in the high plains of the American West have shown that the dunes were moving in response to desert winds much more recently than had previously been realized. Windblown dust mixed into the deep-sea sediments of the North Pacific helps to show how continental climatic fluctuations in China relate to the orbitally induced climatic changes that are well known from the deep-sea record. On a longer time scale, windblown dust in sediments from the deep Atlantic indicates that the Sahara first became a huge desert about 10-million-years ago, possibly as a result of changes in atmospheric circulation related to the uplift of the Tibetan plateau.
Glaciers and glacial deposits reflect the tremendous fluctuations in the climate of the current ice age. Only 20,000 years ago glaciers extended as far south as New York City, and there may have been as many as a dozen comparable advances and retreats of Northern Hemisphere ice during the past 2.5-million-years. A new research effort to integrate continental and oceanic data from the past 2.5-million-years should produce a picture of how the surface environment adapts to rapid climate change. Glacier ice provides a unique record of short-term change, which is discussed in the part of this chapter dealing with cyclical change.
Lakes represent a peculiar part of the earth system. If the solar-driven heat engine entails erosion of mountains, transit of eroded material from the mountains to the sea, and deposition of sediments at the edges of the continents and on the ocean floor, then lakes represent an interruption to the smooth flow of the system. As such, lakes are usually quite short lived because they fill with sediments. The familiar outlines of North America's Great Lakes are less than 20,000 years old and are unlikely to last more than another 20,000 years. Only a few of the world's existing lakes are more than a million years old, and the oldest, Lake Baikal in Siberia and the Great Rift Valley lakes of East Africa, are found in places where the continents are being ripped apart by tectonic forces.
Lakes and lake sediments provide crucial information about climatic change during the past 2.5-million-years. In recently glaciated areas, some lakes have produced annual cycles in sedimentary layers called varves. Some of these varve-layer exposures indicate thousands of years of continuous deposition. The most productive method of retrieving environmental information from lake deposits is the analysis of fossil pollen, which provides a record of changes in nearby vegetation. In tropical areas, lakes expand during intense monsoonal episodes, while in temperate latitudes high lake levels may indicate more rain or less evaporation. Fluctuations in shoreline elevations are used with the pollen record to reconstruct recent environmental changes.
The long-term rock record contains scattered evidence of ancient lakes; the oldest of these is more than 2-billion-years old, half as old as Earth itself. The study of old lake deposits has become a major research activity. Interest has been stimulated not only by studies of the geologically recent lakes but also by the discovery that lake beds are commonly