FATE OF ORGANIC-RICH SUSPENDED SEDIMENT DURING SMALL TO INTERMEDIATE STORMS

Increasing awareness of the important role of suspended sediment in the transport of nutrients and pollutants and the degradation of downstream channel, reservoir, and estuary habitats is driving the need for a better understanding of the sources, transport, and fate of suspended sediment. The organic content of suspended sediment suggests it could be an important agent in the coupling of biogeochemical cycles between continents and ocean, yet recent findings suggest that riverine carbon discharged to the oceans is only a fraction of that entering rivers from terrestrial ecosystems. We take advantage of a recently-developed methodology that combines measurements of the mass balances of the fallout radionuclide (FRN) Be-7 and the stable isotopes of hydrogen in water to quantify fine sediment transport and fate during individual storm events. Specifically, determine the seasonal variation in the ratio of suspended sediment transported out of the watershed to that deposited on the channel bed and margins during small to intermediate storm events. We focus on small to intermediate storms because they, on average, transport the largest portion of the total sediment load. To constrain possible sources of the suspended sediment, we also seek to use the different decay rates of two FRNs (Be-7 and Pb-210) to constrain the atmospheric exposure age of the suspended sediment. We find that during the spring and fall when suspended sediment fluxes are greatest, only a small fraction (less than ~ 20%) of the suspended sediment is exported from the watershed. The vast majority of the suspended sediment is deposited on the channel margins, much of it only after it has been at least partially decomposed by invertebrates and microbes within the channel. Thus while in-stream organic carbon storage is limited, the channel still serves an important role in the decomposition of organic carbon and as a source of organic carbon to the channel margins. Understanding the seasonality in sediment export is important as on-going and projected changes in climate, especially in the northeast US, is preferentially increasing the frequency of warm season high discharge events.