Non-point Source Pollution (continued)

Numerous other sedimentation-related studies have been conducted of Illinois’ rivers, including the Illinois River and its tributaries. The following is a summary of the findings from these reports:

• Over 50% of the annual sediment yield of Illinois streams results from stream bed erosion (Roseboom 1986);
• Only a small fraction of soil eroded from upland row crop fields actually reaches an Illinois stream (Roseboom 1986);
• Total lead (Pb) concentrations in the Illinois River peak between 1920 and 1980; these concentrations sometimes exceed consensus-based Probable Effects Concentration levels for freshwater sediments (Machesky et al, 2005);
• Local tributaries contribute 65% of the total sediment load to Peoria Lake in 1989 (White 2005);
• Channel and near-channel sources of sediment have a higher rate of sediment deliver to water bodies than do other areas within a river basin (White 2005);
• Stream bank erosion can contribute a significant amount of sediment to rivers (White 2005):
  1. In the Court Creek basin, streambank erosion constitutes more than 50% of the sediment yield.
  2. More than 40% of the sediment in the Spoon River results from bank erosion.
  3. Ninety percent of channel sediments eroded from unstable stream systems originate from streambanks.
• In 1990, the average capacity loss of the 60 backwater lakes associated with the Illinois River is 70%, with some losing as much as 100% of their capacities (Bhowmik 1999).

Studies of Peoria Lake by researchers at the ISWS indicate the dramatic changes that occur from 1903 and 1985. The lake has lost about 68% of its 1903 average capacity and the average depth has reduced to 2 feet compared to the 1903 depth of 8 feet. It is estimated that this lake lost an additional 15% or more of its capacity by late 1999. Tables 39a and 39b show these changes in volume and depth over this period.

Table 39a: Volume of Peoria Lake at 440 feet msl (Bhowmik 1999)

Table 39b: Average Depth of Peoria Lake at 440 feet msl (Bhowmik 1999)