ABSTRACT

Temporal and spatial variations of processes in the lower 315 mi of the Mississippi River were investigated. The data base consisted of several instantaneous measurements of discharge, suspended-sediment concentration, suspended-sediment load, and percentage of sand in the suspended load. The data were collected periodically by the Corps of Engineers at Tarbert Landing (n=460) between 1972 and 1985, at St. Francisville (n=84) between 1978 and 1986, and at Belle Chasse (n=93) between 1977 and 1986.

Physical factors that contribute to seasonal and annual variations in sediment load and concentration include the magnitude, duration, number, and timing of floods and the background flow conditions. During high-flow years, sediment peaks precede discharge peaks by several months, and depletion occurs during and after the maximum discharge. During low-flow years, sediment peaks occur only shortly before or coincide with discharge peaks. Unexplained sediment peaks during both falling and rising stages may be caused by bank failures or turbulent fluctuations of stream velocity.

Silt-clay and sand concentrations follow different functions with discharge and fit different exponents, with sand being steeper. The total suspended-sediment concentration is a composite of these components. At upstream stations, the silt-clay concentration was depleted and showed a strong nonlinear relationship with discharge, but the sand concentration showed a linear relationship with discharge. Downstream, the silt-clay and sand concentrations showed more linear relationships with discharge. Quadratic power functions produced better correlations than the linear power functions in most cases.