Suspended sediment trapping and Export, Mink Brook, Hanover, NH

We combine measurements of the fallout radionuclides 7Be and 210Pb and the stable isotopes of hydrogen in water with a conceptual model of suspended sediment trapping by channel margins to quantify suspended sediment mobilization and storage in a stream and its channel margins during individual intermediate-sized storm events with recurrence intervals of a few months or less. We found that across all events the channel bed was a source of organic-rich suspended sediment and the channel margins a sink and observed strong seasonal variations in the export of suspended sediment. The increasing export of suspended sediment through the spring and summer may result from the in-channel decomposition of organic debris as indicated by its changing bulk δ13C composition and changing relationship between turbidity and suspended sediment concentration. The trapping of suspended sediment limits the transport distance of suspended sediment, which, given sufficient discharge to fully suspend the sediment, is nearly independent of stream discharge for sub-bankfull discharges. An analysis based on hydraulic geometry scaling is consistent with empirical observations that the characteristic transport length decreases with increasing watershed area. Simultaneously, limited available data indicate that the fractional size of the channel margins where deposition occurs decreases with increasing watershed area. The increasing transport length and decreasing fractional margin area results in a systematic decoupling of the channel from terrestrial organic matter inputs.