This investigation was Phase III of a three-phase study of the sediment regime of the Mississippi River Basin. Phase I resulted in the publication of inventory of Sediment Sample Collection Stations in the Mississippi River Basin. The Phase II study identified a downward trend in Mississippi River suspended-sediment loads that apparently began around the middle of the 20th century. The current study deals with suspended-sediment sampling, analysis, and load-computation procedures used at key stations on major streams in the Mississippi River Basin and the possible influence of these procedures on the downward trend. Keywords: Suspended sediments; Flood control; Rivers/capacity quantity; Sediment transport; Sediments/sampling/analyzers.

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.

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.