LINKING FLUVIAL GEOMORPHOLOGY WITH LARGE WOODY DEBRIS: IMPLICATIONS FOR BIOGEOCHEMICAL HETEROGENEITY IN A HEADWATER STREAM, OHIO, USA

Stream metabolism and biogeochemical dynamics are intimately linked with channel morphology, but studies that link these parameters, particularly in relation to large woody debris (LWD), are relatively rare. Here we integrate remote sensing methods with in-channel geomorphic surveys and biogeochemical monitoring to characterize three reaches of Raccoon Creek in Licking County, Ohio. Geomorphologic and biogeochemical characterizations were conducted above and below each LWD accumulation. This multivariate approach allowed us to link LWD with watershed-scale dynamics and ecosystem function. At each site, we conducted cross-channel elevation profiles and calculated discharge using both flow meter and dilution gauging methods. Elevation trends were confirmed with GIS using LiDAR digital elevation models. Discharge volume increased with distance downstream, and scaled inversely with sediment retention and bar formation. We found that Raccoon Creek became increasingly wide and entrenched with distance downstream, and had fewer in-channel depositional surfaces above LWD. GIS and drone imagery suggests sites below LWD, even in widened and entrenched reaches, experienced bar and backwater pool formation. Cross-channel flow meter measurements from below LWD show variations in longitudinal distribution of flow velocity gradients. These observations suggest LWD influences channel bed and bank geomorphology by enhancing hydraulic complexity and sediment storage variability. Larger drainage areas contain wider riparian buffers and reduced sediment delivery, which may reflect patterns of land use. In contrast, upstream reaches contain less developed riparian zones, and are closer to sediment sources. These observations suggest differences in sediment retention rates may reflect land use. Geomorphic observations were supplemented by subaqueous biogeochemical characterizations. Our observations suggest that low velocity pools below LWD accumulations support enhanced heterotrophic activity. These findings are consistent with previous studies, which suggest that LWD enhances geomorphologic and biogeochemical stream heterogeneity.