Seasonal and Longitudinal Variations in Suspended Load Connectivity Between River Channels and Their Margins.

Input of organic matter into stream channels is the primary energy source for headwater ecosystems and ultimately carbon to the oceans and hence is an important component of the global carbon cycle. Here, we quantify organic‐rich fine sediment mobilization, transport, and storage in a Strahler fourth‐order stream during individual intermediate‐sized storm events. By combining measurements of fallout radionuclides (FRNs) 7Be and 210Pb and stable water isotopes with a conceptual model of suspended load trapping by channel margins, we find that the channel bed was consistently a source of suspended load to the channel margins. Relative to storage on the channel margins, suspended load export increased through the spring and summer, perhaps related to the in‐channel decomposition of organic debris as indicated by its FRN exposure age and changing bulk δ13C composition. Trapping of suspended load by riparian margins limits sediment transport distances, which, given sufficient discharge to fully suspend the load, is nearly independent of stream discharge for sub‐bankfull discharges. Limited data indicate that the fractional size of the channel margins where trapping occurs decreases with increasing watershed area. Increasing transport length and decreasing fractional margin area with increasing watershed area results in a systematic downstream decoupling of the channel from local terrestrial organic matter exchange. These findings provide a framework for understanding suspended load dynamics in formerly glaciated regions where sediment production and fluxes are generally low and thus the annual input of organic debris is a major component of suspended load budget. Plain Language Summary: The decomposition of organic‐rich debris (leaves, twigs, etc.) within stream channels serves as an important organic carbon source to stream margins or banks. During moderate storm events, we observed that the channel bed was consistently a source of organic‐rich suspended load that is then trapped by the channel margins. Through spring and summer less of the suspended load is trapped by the margins, increasing the fraction of the suspended load exported. This decreased trapping and increased export may be related to changes in the character of the suspended load due to the in‐channel decomposition of organic debris. Trapping of suspended load by channel margins limits the transport distance of suspended load, systematically decoupling the channel from the channel margins with increasing watershed size. These findings provide a framework for understanding suspended load transport in formerly glaciated regions where the annual input of organic debris is a significant component of suspended load budget. Key Points: We observe seasonal variations in the ratio of suspended load exported from the watershed versus stored in channel marginsGreater fractional area of deposition along margins of headwaters facilitates trapping of suspended load that limits suspended load exportIncreasing transport length with increasing watershed area systematically decouples the channel from terrestrial organic exchange [ABSTRACT FROM AUTHOR]