Magnitude, Frequency and Duration Relations for Suspended Sediment in Stable (Reference) Southeastern Streams: Metrics for Linking Suspended Sediment with Aquatic Health

Sediment is listed as one of the leading causes of water-quality impairments in surface waters of the United States. A water body becomes listed by a State, Territory or Tribe if its designated use is not being attained, i. e., impaired. In many cases, the prescribed designated use is aquatic health or habitat, indicating that Total Maximum Daily Loads (TMDL) targets for sediment should be functionally related to this use. TMDL targets for sediment transport have been developed for many Level III ecoregions over the past several years using suspended-sediment yield (load per unit drainage area) as a metric. Target values were based on data from "reference" streams or reaches, defined as those exhibiting geomorphic characteristics of equilibrium where no temporal trends of net erosion or deposition occurred over a period of years. This approach has proved useful to some states developing TMDLs for suspended sediment. Although one can use this methodology to differentiate between sediment-transport yields between stable and unstable streams in a given ecoregion, one cannot conclude that if a stream exceeds the target range, the aquatic ecosystem will be adversely impacted. This is mainly due to the lack of data and knowledge on the functional links between suspendedsediment transport rates and ecologic function in streams. To address this problem, historical flow and sediment-transport data from hundreds of sites in the Southeastern United States were re-examined to develop parameters (metrics) such as frequency and duration of sediment concentrations. These relationships could then be applied by aquatic ecologists to develop functional links between sediment and biologic response. Sites determined as geomorphically stable from field evaluations and from analysis of gauging- station records were sorted by Level III ecoregion. Mean-daily flow data obtained from the U. S. Geological Survey were applied to sediment-transport rating relations to determine suspendedsediment load for each day of record. 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 at specified exceedance intervals. As with the "reference" suspended-sediment yields, there is a broad range of frequency and duration distributions that reflect the hydrologic and sediment-transport regimes of the ecoregions. Ecoregions such as the Mississippi Valley Loess Plains (#74) maintain high suspended sediment concentrations for extended periods whereas coastal plain ecoregions (#63 and 75) show much lower concentrations.