1 DEVELOPING LINKAGES BETWEEN SEDIMENT LOAD AND BIOLOGICAL IMPAIRMENT FOR CLEAN SEDIMENT TMDLs by Roger A . Kuhnle, Andrew Simon, and Scott S. Knight, USDA-Agricultural Research Service, National Sedimentation Laboratory, P. O. Box 1157, Oxford, Mississippi 38655-1157, email: kuhnle@sedlab.olemiss.edu Wetlands Engineering & River Restoration Conference 2001 ASCE, August 27-31, 2001, Reno, Nevada Abstract Clean sediment has been identified as the largest named pollutant in the 303(d) listed sites in the United States. The methods used by states to list streams as impaired by sediment is variable. Standard scientifically-based assessment tools are needed to determine the likelihood streams are impaired by clean sediments. In this study, linkages were sought between sediment indices and biologic indices for streams with detailed records of flow discharge, suspended sediment transport , and biological data to use as analogues in the evaluation of sites lacking detailed data. Preliminary analyses show that as durations of suspended sediment concentration at or above 1000 mg/l increase the total number of organisms and the number of taxa tend to decrease for benthic organisms. The data for this determination was from streams in the Mississippi Valley Loess Plains in the state of Mississippi. Introduction Excessive erosion, transport, and deposition of sediment in surface waters is a major problem in the United States. The 1996 National Water Quality Inventory (Section 305(b) Report to Congress) indicates sediments are ranked as a leading cause of water quality impairment of assessed rivers and lakes. The impact of sediment in many of these listed streams is from too much or too little clean sediment. Clean sediment is defined here as sediment uncontaminated by other substances. Methodologies are needed to evaluate the likelihood that a given stream is impaired by clean sediment and the sediment conditions for an unimpaired stream (reference conditions) are needed to serve as a target for restoration. It is generally accepted that a significant change in the amount of sediment in the water column or in the movement of the sediment on the bed surface of a stream is detrimental to aquatic organisms. Except in a small number of cases, such as for salmonid fish, the magnitude of the change in suspended sediment or movement of the bed material, necessary to significantly impair the biota is not known. This type of information is necessary to determine whether a given departure of sediment load from a defined stable reference condition is sufficient to impact the designated use of a stream or river. Water quality standards are set by States, Territories, and Tribes. They identify the uses for each water body, for example, drinking water supply, contact recreation (swimming), and aquatic life support, and the scientific criteria to support that use. The Clean Water Act, section 303, establishes the water quality standards and Total Maximum Daily Load (TMDL) programs.