Water Resources of the Caribbean
Relating chemical and physical erosion, in Chemical Weathering Rates of Silicate Minerals
Robert F. Stallard
U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303-1066, USA
Two great classes of material move through river systems-- solutes and solids. This discussion is dedicated to analyzing the processes that relate solute transport to solid transport under natural condition and under the influence of human activities. This topic has received surprisingly little recent attention from geochemists, despite its importance in discussions of a quarter of a century ago. The concept was embodied in the title of one the great geochemical texts, Garrels and Mackenzie's (1971a) "Evolution of Sedimentary Rocks," in which the chemical evolution of the major elements of the Earth's crust through geologic time was examined. Many geomorphologists working at the time were concerned with the degree to which tectonic processes and human activities had affected the discharge of sediments from rivers into the oceans (Schumm, 1963; Judson, 1968; Holeman, 1968; Meade, 1969; Ahnert, 1970). Geochemists took an active part in the interpretation of these observations (Garrels and Mackenzie, 1971b). Among geologists and geomorphologists the interest has continued (Stallard, 1995).
Subsequent time saw the enthusiastic embrace of small-watershed studies by ecologists and geochemists. Such studies permitted the examination of the complex array of processes that interact to control the composition and discharge of substances exported from small watersheds. Sediment discharge was never a major focus of these studies, because of two factors. First, unlike solutes, for which the bulk of transport is during low to normal flow, most solid transport is at high to extremely high flow. High flows arise during "events"-- typically major storms and snow melt. Event sampling is often difficult, if not dangerous, and automatic samplers are best used. Second, sediments are difficult to sample representatively.
Funding for sediment research is largely derived from the need to mitigate sedimentation or erosion problems such as the reduction of reservoir capacity to unacceptable agricultural-soil loss. The watersheds examined tend to be larger than those studied by landscape ecologists and geochemists. As such, they differ from the small watersheds in one fundamental aspect-- considerable quantities of sediment are stored in larger watersheds. Sites of storage include the base of hillslopes (colluvium), within channels and floodplains (alluvium), and in lakes and reservoirs (lacustrine sediments). For solutes, storage is typically not so important.
Stallard, R.F., 1995, Relating chemical and physical erosion, in Chemical Weathering Rates of Silicate Minerals: Washington, D.C., Mineralogical Society of America, Reviews in Mineralogy 31, p. 543-564.