a 320-km (200-mile) reach from the great bend at Hennepin to the conflu- 

 ence with the Mississippi River at Grafton (Figure 1.1) produced 10% of 

 the total U.S. harvest of freshwater fish and two thousand commercial 

 fisherman made a living from the river (U.S. Department of Commerce and 

 Labor 1911). The commercial yield was 24 million pounds annually, or 

 about 178 pounds per acre of permanent water (Lubinski et al . 1981). By 

 the 1950s the yield had dropped to 38 pounds per acre; since the 1970s 

 the yield has been a low 4 pounds per acre, totaling only 0.32% of the 

 total U.S. catch (Sparks 1984). Similar downward trends were recorded 

 over the same period for two other indicators of biological 

 productivity: waterfowl and sport fish populations (Bellrose et al . 

 1979; Sparks 1977; Sparks 1992). Major commercial fish species and the 

 diving ducks feed on bottom-dwelling invertebrates such as clams, 

 snails, aquatic worms, and aquatic insects. In the early 1900's a 

 healthy benthic community contributed to the tremendous production of 

 fish and waterfowl. A major component of that benthic community was a 

 small clam, Musculium transversum (Family Sphaeriidae) . Now, this clam 

 as well as other small mollusks, mayflies, midges, and other burrowing 

 aquatic insects has been virtually eliminated from certain reaches of 

 the Illinois River (Starrett 1972, Anderson 1977, Sparks et al . 1986). 



Declines in the benthic invertebrates of the Illinois River system 

 have been linked to sediment-associated toxicity (Sparks et al . 1981; 

 Blodgett et al . 1983; Sparks 1984). Aquatic sediments can accumulate 

 both inorganic and organic chemicals that are absorbed to particulate 

 matter or are in solution in sediment porewater (Salomons et al . 1987, 

 Tessier and Campbell 1987). Porewater (also called interstitial water) 

 is the water occupying the spaces between the sediment particles. These 



