Soil Erosion and Sedimentation (continued)

For decades, politicians and the public ignore the problem of soil erosion. The initial concerns relate to the loss of farmland productivity, but in 1940, the Illinois State Planning Commission releases a report that identifies soil erosion as a serious problem to the lower Illinois and Mississippi rivers and their tributaries. Table 40a identifies soil erosion by counties in the Illinois River valley. The degree of the problem is classified as destructive, serious, harmful, or negligible. Most counties upstream of Peoria show only minor problems with soil erosion; ranges with a combination of both “destructive” and “serious” are as low as 6.6% to 19.9%. The problem of soil erosion increases downstream of Peoria, with the ranges as low as 1.5% in Mason County to as high as 61.1% in Brown County. Several other counties show severe soil erosion problems: Pike County has 54.3% of the county classified as destructive and serious, and Schuyler County is 39.7%.

Table 40a: Soil Erosion Occurring in Counties in the Illinois River Valley, South from Bureau Junction (Illinois State Planning Commission 1940).

In parts of the Illinois River valley, seven of the original 13 inches of topsoil have been lost in the past 150 years. These soils are most vulnerable in the spring when the land is bare of vegetation. For example, when Richland Creek is running low, sediment concentrations are 2 milligrams per liter of water. During times of heavy rainfall, however, sediment concentrations can be as high as 8,200 milligrams per liter. The main channel of the Illinois River has less concentrated sediments than its tributaries. A typical reading at Chillicothe is 490 milligrams per liter. Because of the main channel’s greater water volume, however, the river’s total sediment load is enormous. The Illinois River can carry an estimated 93,800 tons of soil per day (IDNR 1998).

In 1976, Lee and Stall calculate a theoretical annual loss of sediment in the Illinois River and estimate that 62% of the silt comes from the slopes of the tributary rivers and the parent valley; 38% comes from upland areas. In 1976, it is estimated that 11 million metric tons (12.1 million tons) of sediment are transported out of the Illinois River into the Mississippi River at Grafton, leaving 14 million metric tons (15.4 million tons) deposited in the backwater areas and non-leveed areas of the floodplain (Bellrose 1983). Table 40b illustrates the impact these sediments have on the backwater lakes, in some cases using multiple data sources. Average annual sediment deposits range from 0.41 centimeters (cm) to 3.06 cm. The overall average annual sediment deposits, weighted based on the area of the lake basins, is 1.24 cm.

Table 40b: The Total Sediment and Yearly Amount of Sediment in Certain Bottomland Lakes of the Illinois River Valley (Lee & Stahl 1976)