susceptible to erosion for almost two-thirds of the year, including the period of heaviest rainfall in the early spring (Havera and Bellrose, 1985).
Stream modifications undertaken by farmers also contributed to sediment loading. Farmers channelize streams to improve drainage so that they can get on the fields earlier in the year and to straighten field borders to make it easier to use large equipment. Channelization shortens the stream length and thereby increases the slope. The water moves at greater velocity, gains erosive power, and tends to erode its bank and bed. Leedy (1979) estimated that more than 50 percent of the annual sediment yield of Illinois streams comes from bank and bed erosion. Because the rate of fall of the tributaries is approximately a foot per mile, they deliver most of their sediments to the Illinois River, where it settles out because the mainstem falls at only one-tenth of a foot per mile (Mills et al., 1966). Channelization extended to the marshy or forested deltas where tributaries enter the Illinois River, so these areas no longer trap sediments before they enter the river (Roseboom et al., 1989). Finally, approximately half the floodplain of the Illinois River was drained and leveed for agriculture (Bellrose et al., 1983; Thompson, 1989), so that sedimentation was concentrated in the remaining overflow areas, lakes, and backwaters.
The end result of these changes was an increasing sediment loading on the river, evidenced by sedimentation rates in a mainstem lake (Peoria Lake) that were twice as high in 1965 to 1985 (1.44 percent of the lake volume is lost per year) as in 1903 to 1965 (Demissie, 1989). Of an estimated 27.5 million tons of sediment delivered to the Illinois mainstem annually, approximately 12.1 million tons are delivered to the confluence with the Mississippi: the remaining 15.4 million tons are deposited in the remaining unleveed floodplain and backwaters (Lee, 1989). If this amount were spread evenly over the remaining floodplain, it would aggrade at the rate of 0.19 inch per year (Lee, 1989).
Once the sediment is in the river and backwaters it is resuspended by boat-and wind-driven waves and currents. The fine-grained sediments take 7 to 12 days to settle out, following a windstorm (Stall and Melsted, 1951). Because the average recurrence interval of moderate to strong winds in Illinois is less than 7 days, the river and its backwaters tend to remain turbid (Jackson and Starrett, 1959). Commercial and recreational boats not only cause bank erosion, but also resuspend and relocate sediments (Bhowmik and Schicht, 1980). By using infrared photography, Karaki and van Hoften (1974) showed