USDA-ARS Channel and Watershed Technology Developed for Assessing Impaired and Unimpaired Watershed Sediment Loadings

Abstract

The development of sediment Total Maximum Daily Loads (TMDLs) requires knowledge of current or impaired conditions and reference or unimpaired conditions of a watershed. The application of USDA watershed technology in a systematic manner provides watershed decision-makers a tool, based on sound-science developed by ARS research, to aid in the TMDL process. This study examined current suspended sediment loads for the James Creek watershed in East-Central Mississippi. These loads were compared with historical flow and sediment-load data from similar, but stable or reference streams from the Southeastern Plains Ecoregion. A weighted-reference condition based on the percentage of drainage area encompassed by the various bedmaterial types produced a reference yield at the effective discharge of 2.2 T/d/km2. Current sediment loads were obtained from a combination of: simulations of flow and sediment transport using the USDA watershed model AnnAGNPS; simulations of channel flow and sediment transport by the USDA channel-evolution model CONCEPTS; and by direct comparisons of measured cross sections from 1967 and 2002. Current suspended-sediment yields at the effective discharge from this approach were 675 T/d/km2, 155 T/d/km2 and 39 T/d/km2, based on data from watershed and channel simulations for the previous 35, 10, or 3 years, respectively. This reflects the stabilization of the channel with time by natural means and to additional grade-control structures installed in 1999. Results of this study indicate that a significant proportion of the sediment in the James Creek watershed emanates from stream channels. Thus, remedial measures may need to be focused on instream erosion control practices, while also working with agricultural producers, to prevent sediment from impairing the watershed.