introduced, allowing more light to penetrate, algal blooms may occur. Thus, as the Illinois Environmental Protection Agency has pointed out, removing sediments may not be sufficient to ensure high aesthetic enjoyment of water if the nutrients remain in a dissolved state or in sediments on the bottom (Illinois EPA, 1979).
In addition to nutrients, a number of toxic substances are adsorbed on soil particles that move into streams. Among them are metals such as copper, zinc, and lead,d which are known to accumulate in sediment. Mathis and Cummings (1973) found that most metals in the Illinois River occurred in sediments at levels several orders of magnitude greater than the levels in water. Organisms that live in the sediment, such as oligochaete worms and clams, contained higher levels of the metals than did organisms such as fish. Because the chemical environment in the gut of a worm or at the gill surface of a clam is different from that in the sediment or water, it is possible that metals and other toxicants can be mobilized from the sediment and taken up by organisms that ingest sediment or live in contract with it.
Pesticides constitute another group of chemicals that can be taken up by organisms. When pesticides are introduced into an aquatic ecosystem they are stored in the bodies of organisms, where biological amplification may take place as the chemicals move through the food web. Although modern pesticides are formulated to degrade, some of the degradation products are not entirely harmless. Also, much agricultural land still contains persistent pesticides or their metabolites from earlier years (Illinois EPA, 1979). Degradation may take days or weeks, and in the meantime pesticides remain deadly to nontarget species as well as those that were targeted. Every year, fish skills caused by agricultural chemicals are reported either to insurance companies or to the EPA. The two most common causes of these fish kills are runoff of insecticides from freshly sprayed fields. usually when spraying is closely followed by heavy rains, and carelessness on the part of applicators who allow leftover chemicals to drip from their tanks (Illinois EPA, 1979).
Eroded silt also often carries with it organic matter that creates an oxygen demand in the water. Ellis (1936) found that the oxygen demand of organic matter mixed with silt lasted 10 to 15 times as long as the oxygen demand created by the same amount of organic matter mixed with sand.
Butts (1974) found that oxygen demand can increase dramatically when sediment containing organic material and bacteria is resuspended by waves or currents. In the Peoria Pool of the Illinois River, for example, he found that under quiescent conditions the sediment oxygen demand was 2.8 g/m2 per day, but that the demand rose to 20.7