Summary

Since 1982, the Occoquan Watershed Monitoring Laboratory (OWML) has, on behalf of the Metropolitan Washington Council of Governments (MWCOG), operated a gaging and automated sampling station at Chain Bridge on the Potomac River. Paced by flow measurements at Little Falls Dam, storm samples are automatically composited by in-field flow weighting. Base flow sampling is typically weekly. This relatively dense sampling strategy has led to a large database upon which accurate direct load computation may be based. It can also be used to compare and contrast other methods that depend on regression. In recent years, the USGS (United States Geological Survey) has developed a modified weighted regression method–called the Weighted Regression on Time, Discharge, and Season (WRTDS)–to address some of the issues with other regression-based fluvial load estimation schemes, and to perform a better trend analysis. This report compares the long term loads and load trends estimated by WRTDS with those estimated by the dense sampling-based direct method that OWML uses.