TERRESTRIAL SEDIMENT YIELD AND THE LIFETIMES OF RESERVOIRS, LAKES, AND LARGER BASINS

Water reservoirs, lakes, and larger basins, including their drainage a reas, represent sedimentologically closed to semi-closed denudation-ac cumulation systems. The mean rates of mechanical denudation, DRme, and elastic sedimentation, SRme, are related by the ratio of the drainage /lake area, A(d)/A(1). If the latter is known, DRme (or the specific s ediment yield SY in t per km(2)/a) can be calculated from SRme, or vic e versa. The best data for modern SY mainly come from the sediment fil ls of artificial reservoirs. Small drainage areas of mountainous regio ns show SY values up to two orders of magnitude higher than lowlands a nd approximately one order higher than larger regions of mixed relief. This is also true of arid to semi-arid zones which often provide appr oximately as much sediment (SY) as humid temperate and even tropical z ones of comparable relief. Lithology and climate (river runoff) also m ay play some role for SY from catchments of limited size. The importan ce of these factors is exemplified by perialpine lakes and two East Af rican lakes. Sediment yields gained from some large reservoirs compare well with long-term denudation rates derived from geological studies (e.g., the Tarbela dam reservoir along the Indus River). In many other cases, human activities have raised SY by factors of 2-10; locally up to >100. Artificial reservoirs in mountainous regions with SY in the range of 300-2000 t per km(2)/a tend to become filled within several t ells to hundreds of years; some have even shorter lifetimes. Perialpin e lakes of the Alps and British Columbia are strongly affected by delt a prograding and have lifetimes mostly between 15 and 40 ka. Closed la ke systems in deep morphological depressions (Lake Bonneville, Aral Se a, northern Caspian Sea) have a high potential for sediment storage up to the level of spillover and therefore can persist over long time pe riods. Basins with markedly subsiding basin floors (lakes of the East African rift zone, the southern Caspian Sea, and the Black Sea, both o n oceanic crust) can survive for many Ma in the future, despite relati vely high terrigenous input.