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Publication Abstract

The Sediment Budget of the Illinois River: 1981-2015 Demissie, Misganaw, Elias Getahun, Laura Keefer, 2016  Illinois State Water Survey, Champaign, IL,  ISWS RI-122    Full Text Available

Many major streams in Illinois flow 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 is an important ecological resource by providing a significant habitat for fish, waterfowl, and other birds and animals.

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 has gradually been improving. However, ongoing issues associated with erosion and sedimentation are recognized as primary environmental problems in the Illinois River valley. The main source of sediment to the Illinois River valley is soil erosion in the watershed land areas, streambanks, and bluffs. 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 period of sediment data records used in the analysis ranges from 1 year to 34 years, with the majority of the gaging stations having 5 or more years of record. Sediment rating curves that relate daily sediment load to daily water discharge were developed for each sediment monitoring station based on existing data. An improved rating curve procedure that uses nonlinear regression has already been developed in a previous Illinois State Water Survey study, and this procedure minimizes the inherent underestimations of the linear regression -based sediment rating method. The resulting sediment rating curves were used to fill missing data gaps in those monitoring stations with observed sediment load data, allowing the computation of annual sediment loads at each of these stations. Subsequently, regional annual sediment load estimation equations were then developed using annual sediment loads and discharge data from the monitoring stations. The data points coalesced into four different annual sediment yield equations, which were then used to calculate annual sediment yields generated by tributary streams in the Illinois River basin. The sediment budget analysis was performed for a 35 -year period from water year 1981 to 2015. Tributary streams of the Spoon and La Moine Rivers had the highest sediment yield rates. The main stems of the Spoon, La Moine, and Vermilion Rivers had the second highest sediment yield rates, followed by the Sangamon, Iroquois, and Des Plaines Rivers.

Sediment yield calculations allowed the construction of a quantitative sediment budget for the Illinois River valley. The regional sediment load estimation equations were used to calculate the sediment inflows into the Illinois River valley from tributary streams. The sediment outflow from the Illinois River valley was determined from data collected by the USGS at the Valley City and Florence monitoring stations. On average, an estimated 12.9 million tons of sediment were delivered annually to the Illinois River valley from 1981 to 2015, of which 5.2 million tons was the average annual sediment outflow from the Illinois River at Valley City during the same period. This resulted in an estimated average annual deposition of 7.7 million tons (i.e., 60 percent of the total sediment delivered to the valley ) of sediment delivered from tributary streams to the Illinois River valley. The total amount of sediment deposited in the Illinois River valley may be even higher since the estimated amount does not include sediment loads generated by bank and bluff erosion along the main stem of the Illinois River.

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