DECREASING SUSPENDED SEDIMENT CONCENTRATIONS IN THE UPPER MISSISSIPPI RIVER

U.S. Geological Survey and U.S. Army Corps of Engineers records indicate that suspended sediment concentrations (SSC) have been decreasing steadily in recent decades in the Upper Mississippi River. Stations in Minnesota, Iowa, and Illinois with 20-50 years of record clearly show decreasing trends in both mean annual SSC and peak annual SSC. The greatest decrease has occurred for peak SSC values associated with spring snowmelt floods. Prior to 1980, peak SSC values in excess of 1000 mg/L were not uncommon in March and April. Since 1990, however, only a handful of events have generated SSC values in excess of 100 mg/L. Mean annual SSC values have decreased from roughly 100 mg/L in the 1940s, to about 50 mg/L in the 1970s, to about 25 mg/L by the year 2000.

The most likely causes of the decreasing SSC trend are improved land management and soil conservation practices that have been adopted since the 1930s. The closure of the navigation lock and dam system in 1939 has probably also contributed to the trend by trapping suspended sediment in newly-created backswamp environments and navigation pools. Decreasing suspended sediment trends are also evident in some tributary records, suggesting that conservation practices are more significant than the lock and dam system in decreasing SSC values on the Mississippi River.

The geomorphic effectiveness of extreme floods has been diminished by the SSC trend, allowing a greater proportion of the annual sediment load to be carried by moderate flows. The SSC trend is also initiating significant geomorphic change on the Mississippi River floodplain by reducing overbank sedimentation. Cs-137 analysis on floodplain sediments indicates that average vertical accretion rates decreased by about 40% between the periods 1954-1964 and 1964-2001. Despite the recent decreasing trend, modern accretion rates of about 9 mm/yr on low-lying floodplain surfaces are still much greater than the average long-term accretion rates of about 1 mm/yr for the last 2500 years.