Erosion Control and Reservoir Deposition

Abstract

The path of sediment from hillslope to ultimate deposition involves many storage sites on hillslopes, streambeds, and in valley alluvium. The spatial and temporal distributions of such storage sites, some of which may involve decades to centuries, are poorly understood. Predictions of the impact of conservation measures on river systems thus involve significant uncertainty. The universal Soil Loss Equation contains parameters difficult to evaluate and subject to considerable error at drainage basin scales. Isotope data indicate sediment delivery over the U.S.A. of only 4–5% of particles contaminated from air over a long period of time. Ten-fifteen percent delivery at drainage areas greater than 100 mi² is not uncommon. Land use changes including revegetation demonstrably reduce sediment yields, as much as ten-fold in places, but large reductions are generally confined to small. drainage areas of uniform land use. At roughly 1000 mi², with mixed land uses, sediment yields converge to a regional average in temperate U.S.A.

Reservoir storage represents an efficient sediment control technique in some terms. However, reservoir lifetimes where capacity/watershed ratios are small (≤ 20) may be decades. Only large downstream reservoirs have lifetimes of centuries. Long-term sediment control clearly demands land management, even though socially and economically more difficult than reservoir construction. Precise identification of sources and connective links of sediment delivery may lessen the total area requiring careful land management.