Analysis of suspended-sediment concentrations and discharges at four long-term sediment stations in central and southern Illinois, 1975–92 water years

The U.S. Geological Survey and the U.S. Army Corps of Engineers, St. Louis District, have jointly operated four sediment stations in central and southern Illinois since May 1975—Illinois River at Valley City, Kaskaskia River at Cooks Mills, Kaskaskia River near Venedy Station, and Big Muddy River at Murphysboro. A comprehensive analysis of the historical data from these sediment stations was done to determine changes in the concentrations or amounts of suspended sediment transported in the streams.

Generally, the highest suspended-sediment concentrations were found in the Illinois River at Valley City (the station with the largest drainage area), and the lowest concentrations were in the Kaskaskia River at Cooks Mills (the station with the smallest drainage area). Suspended-sediment concentrations were typically high in the spring and summer (March through August) and low in the fall or winter (September through February). The seasonal Kendall test for trends indicated a statistically significant downward trend in suspended-sediment concentrations at three of the sediment stations, including a downward trend of 5.50 milligrams per liter per year in the Illinois River at Valley City.

Median suspended-sediment discharges at the four sediment stations ranged from 47.1 to 3,260 tons per day and corresponded to the size of the drainage areas. The largest median suspended sediment yield, 0.12 tons per square mile per day, was in the Illinois River at Valley City. Suspended-sediment discharges during the spring were larger than during other seasons. The seasonal Kendall test for trends indicated a statistically significant downward trend in suspended sediment discharges at two of the sediment stations (Kaskaskia River at Cooks Mills and Big Muddy River at Murphysboro).