Lakes, Aquifers, and Karst Hydrology

Not every river system reaches the ocean. An endorheic, or closed, drainage basin is one in which water flowing into it has no outlet to the sea, instead terminating in an inland lake, marsh, or area of evaporation and infiltration. Endorheic basins are especially common in arid and semi-arid regions, where evaporation rates are high enough to balance the inflow of water. The basin surrounding the Caspian Sea, fed primarily by the Volga River, is the largest endorheic system in the world. The Great Basin of the western United States, encompassing much of Nevada and parts of neighboring states, is another major example, its rivers terminating in shallow lakes or salt flats such as the Great Salt Lake. Lake Chad, in the Sahel region of Africa, sits at the center of its own large endorheic basin and has shrunk dramatically over recent decades due to reduced rainfall and rising water demand. The Aral Sea, once the fourth-largest lake in the world, has similarly shrunk to a small fraction of its former size after Soviet-era irrigation projects diverted the rivers that once sustained it.