Development of Methodology to Reduce Suspended Sediment Sample Collection on the Missouri River at Sioux City, Iowa

The U.S. Army Corps of Engineers, Omaha District, in cooperation with the U.S. Geological Survey, collects suspended sediment samples twice weekly on the Missouri River at Sioux City, Iowa. An assessment of the suspended sediment data collection program was undertaken to determine if it was possible to reduce the sampling frequency, or even to halt sampling for specific periods of time, without compromising the precision and accuracy of annual suspended sediment discharge calculations. The study evaluated the adequacy of the existing data and this data was used to evaluate the effectiveness of different sampling strategies. The recommended sampling strategy was a seasonal, flow-stratified suspended sediment sampling design having the following properties: 1) suspended sediment samples that span the entire streamflow range for low, moderate and high water discharges; 2) more suspended sediment samples concentrated at streamflow intervals having the largest fraction of the total suspended sediment discharge; 3) the number of annual suspended sediment samples necessary to observe shifts in the suspended sediment rating curves that can occur in response to changes in the watershed or river; and 4) an adequate number of samples that will maintain or improve the precision and accuracy of the suspended sediment rating curves. The proposed sampling design would require significantly fewer annual sampling events than the current twice-weekly sampling scheme.