Lakes

Lake Sarez, Tajikistan Source: NASA Earth Observatory

While present attention is focused on ethnic and political unrest in Kyrgyzistan, its neighbor, another of the "stans" formed upon dissolution of the old USSR,  presents a problem of a very different kind. Held back by an earthen dam created in a 1911 earthquake, Lake Sarez with more than 15 cubic kilometers of water, threatens catastrophic flooding in downstream areas on the Murghob River, a tributary of the Amur (Oxus) flowing into the Aral Sea (lots more on that later). Erosion and other evidence leads to fears of a break in the earthen dam and subsequent rapid discharge of the water it contains. Slow draining of the lake, which is claimed to hold very pure water, has been proposed.

The Sarez example points  to a basic fact of surface water on earth, lakes usually exist because of episodic geological activity and often have short life spans, in geological terms at least. Earthquake formed lakes can be observed in various places, including Quake Lake in Montana formed behind a landslide caused by a1959 earthquake with its epicenter near Yellowstone National Park. The Great Lakes of the United States and Canada are remnants of the last continental glaciation as are thousands of smaller lakes scattered across the glaciated region from the shore of the Arctic Ocean southward to New York, Minnesota, and Montana. Elsewhere the movement of tectonic plates has created large lakes including Lake Baikal in Siberia and the lakes in Africa's Rift valley. Those lakes usually are longer lived than ones formed by earthquakes or by glacial activity, for a rift continues to widen for millions of years before finally creating a new ocean basin. Finally more than a few lakes are the consequence of animals, including beavers and humans, blocking the flow of streams.Beaver dams can create surprisingly large lakes, and of course damming of streams by humans has created immense reservoirs. 

All lakes are created by dams of one sort or another, conditions which temporarily hold back water from gravitational flow toward the sea.

Lakes die when the dam, natural or man made, breaks, the potential problem with Lake Sarez; when evaporation causes them to disappear; when silt fills their basins, or when organic matter chokes them in a process called eutrophication. Small lakes rarely last more than a few millennia, and some survive for only a century or two.