Trends in loads and concentrations of suspended sedi-ment and suspended sand generally were downward for sta-tions within the Mississippi River Basin during the 60-, 34-, and 12-year periods analyzed. Sediment transport in the lower Mississippi River has historically been, and continues to be, most closely correlative to sediment contributions from the Missouri River, which generally carried the largest annual suspended-sediment load of the major Mississippi River sub-basins. The closure of Fort Randall Dam in the upper Missouri River in 1952 was the single largest event in the recorded historical decline of suspended-sediment loads in the Missis-sippi River Basin.

Knowledge about sediment yields is important in developing management strategies for fluvial systems. The effect of sediment must be considered in the design of river structures and in determining water quality for biotic systems. Changes in sediment transport regimes are difficult to understand or predict due to the complexity of factors that influence sediment flux in fluvial systems. Relationships about sediment source, sinks and transport have long been studied and many of these relationships have been quantitatively and qualitatively defined. However, due to the scarcity of long-term sediment records it is often difficult to test these relationships. This study examines one of these long-term suspended sediment records for the Mississippi River at East Dubuque, Illinois from 1943 to 1996. Daily suspended sediment concentrations from the United States Army Corps of Engineers station at East Dubuque were analyzed for the spring and summer months (March - August). Sediment concentrations were analyzed in terms of average concentration during different hydrologic events at the large basin scale. These events included the spring snowmelt runoff (low and high magnitude) and different intensity storm runoff (low, moderate and high). Additionally, peak sediment concentrations during storm runoff were also analyzed. The general trend for all of these analyses suggests a significant decrease in sediment concentrations from the 1940s to the 1990s. The strongest trends are found in high magnitude snowmelt runoff and in high and moderate magnitude storm runoff. Peak concentrations in storm runoff have decreased from about 1000 ppm in the l940s to about 200 ppm in the 1990s. Average concentrations have likewise decreased from about 200 ppm in the 1940s to 100 ppm in the 1990s. Changes in land management practices are identified as being the primary environmental factor influencing sediment concentrations.

This report presents suspended-sediment concentration and streamflow data, describes load-estimation techniques used in the computation of annual suspended-sediment loads, and presents annual suspended-sediment loads for 48 stream gaging stations within the Mississippi River Basin. Available published, unpublished, and computed annual total suspended-sediment and suspended-sand loads are presented for water years 1940 through 2009. When previously published annual loads were not available, total suspended-sediment and sand loads were computed using available data for water years 1949 through 2009. A table of suspended- sediment concentration and daily mean streamflow data used in the computation of annual loads is presented along with a table of compiled and computed annual suspended-sediment and suspended-sand loads, annual streamflows, and flow-weighted concentrations for the 48 stations.

The Lower Mississippi River, extending from Cairo, Illinois to the Gulf of Mexico, annually transports approximately 170 million tonnes of sediment. Historically, the quantity and calibre of sediment derived from catchment erosion have been affected by changes in land-use and management. For example, soil erosion increased during the 19th and early 20th centuries due to settlement by Europeans and this may have elevated catchment sediment supply to the Mississippi River, while more recently the supply of sediment from tributaries is known to have decreased markedly as a result of river engineering and management. Specifically, the construction of large dams as part of the Mississippi River and Tributaries (MR & T) Project has trapped sediment that would otherwise have been supplied to the Mississippi, particularly by the Missouri River. Marked changes have also occurred in the extent of eroding bankline along the Mississippi and these must have reduced the input of sediment derived from this source. For example, during the last three decades, a sustained construction program of bank revetments and dikes has produced a stable alignment. Given these trends in sediment supply from catchment, tributary and bank sources, it is not surprising that most studies of sediment movement report a large decrease in measured sediment loads at selected monitoring stations along the Mississippi River over the last 50 years (Kesel, 1988; Dardeau and Causey, 1990). However, a case can be made that the bed material load must have increased since the 1940s. This argument is based on analysis of morphological changes observed along the river that have led to an overall increase in slope and available stream power, coupled with the fact that bed material sizes along the river have remained almost constant.