Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 5/15/2009
Publication Date: 5/15/2009
Citation: Klimetz, L., Simon, A., Schwartz, J. 2009. Characterization of suspended-sediment transport conditions for stable, “Reference” streams in selected Ecoregions of EPA Region 8. Laboratory Publication. 61:142. Interpretive Summary: Sediment is listed as one of the leading causes of water-quality impairment in surface waters of the United States, yet little information exists that provides scientifically-defensible method to predict water-quality targets for sediment. In addition, functional links between sediment-transport rates and aquatic health are lacking. To address this problem, historical flow and sediment-transport data from hundreds of sites in EPA Region 8 (Montana, the Dakotas, Minnesota and Wyoming), were analyzed to develop parameters (metrics) that could be used to develop water-quality “target” values (to maintain healthy streams) for sediment across the region. This was accomplished by differentiating between stable and unstable sites and then determining sediment-transport rates for “stable” sites in the region which were used to represent “reference” conditions. Sediment parameters that could be functionally linked to potential biological impairment such as the frequency and duration that concentrations are exceeded, were analyzed with fish data in the Northwestern Great Plains, Ecoregion 43. Results showed that relative channel stability and various characteristics of the sediment-transport regime could be linked to specific functional traits of fish populations. Results of this study provide a method for analyzing historical sediment-transport data that can be used to develop water-quality targets for sediment and to 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 350 sites across the Mountains and Plains region of the 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 3 to 9 T/y/km2 for all ecoregions except Ecoregion 46 which had a mean annual ‘reference’ yield of 0.579 T/y/km2. This pattern of Ecoregion 46 having a ‘reference’ yield at least an order of magnitude less than other ecoregions is true also at the 1.5-year recurrence interval flow; 0.00831 for Ecoregion 46 and between 0.0563 and 0.423 T/d/km2 for other ecoregions. 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 were then used to investigate relations between channel stability and sediment metrics with fish functional-traits information.