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

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 1.5-year recurrence interval flow (Q1.5). This is particularly relevant with the renewed focus on stream restoration activities and the urgency in developing water-quality criteria for sediment. Arguments are developed that in lieu of form-based estimates of say the bankfull level, a flow of a given recurrence interval (Q1.5) is more appropriate to integrate suspended-sediment transport ratings for the purpose of defining long-term transport conditions at a site. At the Q1.5 the highest median suspendedsediment concentrations occur in semi-arid environments the highest yields occur in humid regions with erodible soils and steep slopes or channel gradients. 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.