Non-point Source Pollution

Despite significant progress in addressing point source discharges, sedimentation remains a critical problem. In 1982 it is estimated that 15 million tons of new sediment ends up in the river system every year (Friederici 1992). According to a report completed in 1992, the problem of soil erosion is getting worse, and is now the number one problem in the Illinois River valley. The sedimentation rate is higher in 1992 than in earlier periods (Demissie, et al. 1992). The main sources of sediment are river basin, stream bank, and bluff erosion.

A study conducted by the ISWS in 1992 estimates sediment yields for the tributary streams of the Illinois River. The sediment yield of a river basin is the amount of sediment that eventually leaves a river basin and is available for deposition at other locations. According to Demisse, et al., sediment yield is typically a small fraction of the total gross erosion in the river basin, which includes
sheet, rill, gully, streambank, and streambed erosion. Not all of the eroded soils in a river basin are transported and delivered to streams out of the river basin; some are moved from one place to another within the same river basin.

It is important to determine how much sediment is being delivered into the valley from tributary streams in order to evaluate the magnitude and pattern of sedimentation in the Illinois River and its backwater lakes. Sediment yield calculations are based on suspended sediment data collected by the USGS. Its methodology is as follows:

Sediment rating curves that relate daily sediment load with daily water discharge were developed for each of the sediment monitoring stations. The sediment rating curves were then used to calculate annual sediment yields from all the tributary streams. The annual sediment yields were then used to develop regional equations for annual sediment yields. These regional equations were then used to calculate annual sediment yields by tributary streams into the Illinois River valley (Demisse 1992).

Tributary streams of the Spoon and La Moine rivers have the highest sediment yield rates. The main stems of the Spoon, La Moine, and Vermilion Rivers have the highest sediment yield rates, followed by the Sangamon, Iroquois, and Des Plaines rivers.

The Spoon River is the number one sediment producer in the Illinois River basin based on average sediment yield for the 10-year period examined. On average, the Spoon River produces 2.7 million tons of sediment per year. The Sangamon River is number two with a total yield of 1.6 million tons, and the La Moine is third at 1.4 million tons. For the Spoon River, the annual sediment yield is 1,500 tons per square mile, followed by the La Moine River with 1,000 tons per square mile, the Mackinaw River at 735 tons per square mile, and the Vermilion River at 700 tons per square mile (Demisse 1992).