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Title: Suspended-Sediment Transport Rates for Level III Ecoregions of EPA Region 4: the Southeast

item Simon, Andrew

Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 9/4/2007
Publication Date: 9/5/2007
Citation: Klimetz, L., Simon, A. 2007. Suspended-Sediment Transport Rates for Level III Ecoregions of EPA Region 4: the Southeast. USDA-ARS National Sedimentation Laboratory Research Report. No. 55. 135 pp.

Interpretive Summary: Sediment is listed as one of the leading causes of water-quality concern in surface waters of the United States yet little information exists that functionally links sediment-transport rates to aquatic health. To address this problem, historical flow and sediment-transport data from hundreds of sites in the Southeastern United States were analyzed to develop parameters (metrics) such as frequency and duration of sediment concentrations that could be used by aquatic ecologists to develop functional links between sediment and biologic response. Stable sites in the region were used as "reference" sites. The frequency and duration that a given concentration was equaled or exceeded was then calculated to produce a frequency distribution for each site. "Reference" distributions were created for the stable sites in each ecoregion by averaging all of the distributions within an ecoregion. This study presents a method for analyzing historical sediment-transport data from the Southeastern United States that can be used to quantitatively evaluate thresholds for aquatic health in cases where sediment is believed to be the impairing water-quality parameter.

Technical Abstract: Historic flow and sediment transport data from about 750 sites across the southeastern United States were analyzed for the purpose of developing "background" or "reference" rates of suspended-sediment transport by Level III ecoregion. Rapid Geomorphic Assessments (RGAs) were conducted at most sites to determine relative channel stability. Suspended-sediment loads for the 1.5-year recurrence interval flow and on an annual basis were calculated from derived relations between instantaneous discharge and suspended-sediment load. Values were then divided by drainage area to obtain suspended-sediment yield for use in comparing streams of different size. Data from each ecoregion were initially sorted into stable and unstable sites to develop distributions of suspended-sediment yield. The distributions for stable sites were then considered as representative of "reference" sediment-transport rates. The median value or range represented by the 25th and 75th percentiles for stable sites was then identified as the "reference" targets for that ecoregion. Mean annual "reference" yields ranged from about 2-3 T/y/km2 for low relief coastal plain Ecoregions, to 52-79 T/y/km2 for the loess-dominated regions bordering the Mississippi River. "Reference" yields at the 1.5-year recurrence interval flow show a different geographic pattern with the lowest values still along the coastal plain, but peak values in the high relief mountainous ecoregions where flows are particularly flashy. Because "aquatic life support" is a typical "designated use" of surface waters, suspended-sediment data were re-analyzed into parameters such as the frequency and duration of suspended-sediment concentrations, that may be functionally linked with biologic data. This analysis was conducted for all sites and separated by channel stability. "Reference" values for the average frequency that a given concentration is equaled or exceeded, as well as the average number of consecutive days that given concentrations are equaled or exceeded (duration) were established for each Level III ecoregion. These results should be particularly useful to agencies responsible for establishing suspended-sediment "targets" based on direct links to aquatic community health.