at which sufficient light penetrates to enable plants to grow). The sediments in southern and midwestern rivers that drain agricultural areas serve as sinks for nutrients, particularly phosphorus, which nourishes the new plants once there is sufficient light. In the Mississippi River, for example, extensive new plant beds are located near Montrose, Iowa, just upstream of Lock and Dam 19 (Sparks et al., 1990). In the long-term view, however, these plant beds are only temporary because they will continue to collect sediment until they become higher than mean water level, at which time they will begin providing habitat for terrestrial creatures.
Much of the prime agricultural land in the Midwest and the South is located on alluvial floodplains that developed over thousands of years and supported bottomland hardwood forests. Before the forests were removed, the floodplains served as sedimentation basins and nutrient sinks. Wilkin and Hebel (1982) found that sediment settled in forested floodplains and forested stream borders at the rate of 10 to 20 tons per acre per year. Where the floodplain had been cleared for row crops, sediment was being eroded from the floodplain at a rate of 15 to 60 tons per year. In an agricultural watershed, stream-bank erosion and resuspension of sediment contributed the major portion of annual stream yields of sediment (Sharpley and Syers, 1979). These sediments carry with them the nutrients that make the floodplains desirable for agriculture. By chemically analyzing eroding stream bank soils, Roseboom (1987) determined that bank erosion yielded approximately half of the total phosphate, ammonia, and nitrogen in a channelized floodplain stream in central Illinois.
Once it has returned to the water, sediment can serve as either a source or a sink for nutrients, depending on conditions such as pH, temperature, oxidation-reduction potential, and the amount of nutrients present in the water. For example, phosphorus in the water and phosphorus carried into the water on sediments will come into equilibrium. If plants take up the phosphorus in the water, the sediment can supply more. If there is an excess of dissolved phosphorus, the sediment will take it up (Illinois EPA, 1979); Froelich, 1988). Ammonia supplies nitrogen, another nutrient for aquatic plants; however, it constitutes a greater problem than phosphorus because it is toxic to fish and other animals (Roseboom and Richey, 1977; Thurston et al., 1981).
In extremely turbid waters the presence of these nutrients may not be evident because light is insufficient for plant growth. However, should turbidity be somewhat reduced either by natural processes such as low flow or by reductions in the amount of sediment being