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Publication Abstract

The Sediment Budget of the Illinois River. Demissie, Misganaw, Renjie Xia, Laura Keefer, and Nani Bhowmik, 2004  Illinois State Water Survey, Champaign, IL,  ISWS CR 2004-13    Full Text Available
Many major streams in Illinois drain into the Illinois River, which drains nearly half of the state. The Illinois Waterway with its system of locks and dams links Chicago and the Great Lakes to the Mississippi River, and thereby to the Gulf of Mexico. This linkage has significant transportation and commercial values for the state and the nation. In addition, with its numerous backwater lakes, wetlands, and floodplain forests, the Illinois River valley provides a significant habitat for fisheries, waterfowl, and other birds, and animals, making it an important ecological resource.

The Illinois River's environment has been subject to many impacts associated with watershed development, including waste discharges from urban areas, water-level control for navigation, and sediment and chemical inflow from agricultural lands. Water quality of the river was severely degraded for several decades prior to the 1970s when environmental regulations were enacted to control pollutant discharges. Since then the river water quality gradually has been improving. However, problems associated with erosion and sedimentation have not been improving and are recognized as the primary environmental problem in the Illinois River valley. The main sources of sediment to the Illinois River valley are watershed erosion, streambank erosion, and bluff erosion. The contribution of watershed erosion to the sedimentation problem in the Illinois River valley can be quantified by analyzing the sediment yields of tributary streams that drain into the valley. The contribution of bank erosion along the Illinois River and bluff erosion along the Illinois River valley are much more difficult to quantify at present because of the lack of data.

Sediment yields from tributary streams of the Illinois River were calculated based on suspended sediment load data collected by the U.S. Geological Survey (USGS). The duration of the sediment data ranges from one to 20 years, with most of the stations having less than five years of record. Sediment rating curves that relate daily sediment load and daily water discharge were developed for each sediment monitoring station based on existing data. Because rating curves often underestimate sediment yield, an improved rating curve procedure was developed to minimize the underestimation. Sediment rating curves then were used to calculate annual sediment yields from all tributary streams for which sediment load data were available. Annual sediment yields then were plotted against the annual water discharge to develop regional equations for annual sediment yields. The data points coalesced into four different annual sediment yield equations, which were then used to calculate annual sediment yields by tributary streams into the Illinois River valley. A 20-year period (1981-2000) was used for the analysis. Tributary streams of the Spoon and LaMoine Rivers had the highest sediment yield rates. The mainstems of the Spoon, LaMoine, and Vermilion Rivers had the second highest sediment yield rates, followed by the Sangamon, Iroquois, and Des Plaines Rivers.

Sediment yield calculations were used to construct a quantitative sediment budget for the Illinois River valley. By using the four group equations developed from observed data, the sediment inflow into the Illinois River valley from tributary streams was calculated. The sediment outflow from the Illinois River valley was determined from data collected by the USGS at the Valley City monitoring station. On average, it was estimated that 12.1 million tons of sediment were delivered to the Illinois River valley annually during the period 1981-2000, and the average annual outflow of sediment from the Illinois River at Valley City was 5.4 million tons. This resulted in an estimated average annual deposition of 6.7 million tons of sediment delivered from tributary streams to the Illinois River valley. Because of improved datasets, the present estimate of sediment deposition for the 1981-2000 period is about 18 percent lower than the estimate for the 1981-1990 period. However, the total amount of sediment deposited in the Illinois River valley was probably even higher than our estimate because of the contribution of bank and bluff erosion along the mainstem of the Illinois River, which were not included in these calculations.



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