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Title: Suspended-sediment transport rates at the 1.5-year recurrence interval for ecoregions of the United States: Transport conditions at the bankfull and effective discharge?

item Simon, Andrew

Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2003
Publication Date: 3/1/2004
Citation: Simon, A., Dickerson, W., Heins, A. 2004. Suspended-sediment transport rates at the 15-year recurrence interval for ecoregions of the United States. Geomorphology. 58: 243-262.

Interpretive Summary: Sediment is listed as the principle pollutant of surface waters in the United States. The average amount of sediment that is transported varies according to flow rate, regional geology, climate and land use. To aid action agencies in establishing water-quality standards for sediment, it is first essential to determine how much sediment is transported in different regions of the United States. The United States is separated into 84 ecoregions with each ecoregion having similar characteristics of flow, geology, climate, and land use. Historical flow and suspended-sediment transport data from more than 2,900 sites nationwide have been analyzed to develop equations to predict flow and sediment transport rates for each ecoregion. Peak, median concentrations occur in the semi-arid areas of the Southwest while maximum sediment yields (sediment load per unit area) occur in the Mississippi Valley Loess Plains and the Coast Range. Results from this study will provide action agencies nationwide with fundamental information required to establish water-quality targets for sediment.

Technical Abstract: Historical flow and suspended-sediment transport data from more than 2,900 sites across the United States have been analyzed in the context of estimating flow and suspended-sediment transport conditions at the bankfull discharge. This is particularly relevant with the renewed focus on stream restoration activities and the urgency in developing water-quality criteria for sediment. Data were sorted into the 84 Level III ecoregions to identify spatial trends in suspended-sediment concentrations and yields at the bankfull discharge. Arguments are developed that in lieu of form-based estimates of the bankfull level, a flow of a given recurrence interval (Q1.5) is appropriate to estimate channel-forming conditions and the effective discharge. There is sufficient data to develop regional curves for the Q1.5 in all but eight of the ecoregions. At the Q1.5 the highest median suspended-sediment concentrations occur in semi-arid environments (Southwest Tablelands, Arizona-New Mexico Plateau and the Mojave Basin and Range); the highest yields occur in humid regions with erodable soils and steep slopes or channel gradients (Miss. Loess Plains and the Coast Range). Suspended-sediment yields for stable streams are used to determine "background" or "reference" sediment-transport conditions in eight ecoregions where there is sufficient field data. The median value for stable sites within a given ecoregion are generally an order of magnitude lower than for non-stable sites.