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Sediment is listed as one of the principle pollutants of surface waters in the United States, both in terms of sediment quantity (clean sediment) and sediment quality due to adsorbed constituents and contaminants. We can view sediment-transport rates and amounts as (1) natural or background, resulting from generally stable channel systems, (2) impacted, with greater transport rates and amounts, reflecting a disturbance of some magnitude and more pervasive erosion, and (3) impaired, where erosion and sediment transport rates and amounts are so great that biologic communities and other designated stream uses are adversely effected. Impairment of designated stream uses by clean sediment (neglecting adsorbed constituents) may occur through processes that occur on the channel bed or by processes that take place in the water column. Fully mobile streambeds, and deposition of fines amidst interstitial streambed sands and gravels can pose hazards to fish and benthic macro-invertebrate communities by disrupting habitats, degrading spawning habitat, and reducing the flow of oxygen through gravel beds. Although lethal or sub-lethal thresholds are unknown at this time, high concentrations of suspended sediment, perhaps over certain durations can adversely affect those aquatic species that filter and ingest water. It is critical, therefore, to clearly identify the potential functional relation between an impact due to sediment and the sediment process so that appropriate parameters are analyzed.

Although clean sediment can adversely affect habitat and other designated uses in a variety of ways, this paper will be limited to discussions and analysis of methods and techniques for analyzing impacts due to suspended sediment. The USDA-Agricultural Research Service, National Sedimentation Laboratory (ARS), is conducting analytic research on suspended-sediment transport and clean-sediment TMDL development throughout the United States. This paper, however, focuses on work conducted in the southeastern United States, predominantly in Mississippi where field efforts have been initially focused. The work described in this paper was made possible by USDA-Agricultural Research Service discretionary research funds, a cooperative project with the U.S. Environmental Protection Agency, and support from the Mississippi Department of Environmental Quality.