Longitudinal variations in channel morphology in postglacial watersheds of the northeastern United States

English Abstract:

Landscapes formerly occupied by continental ice sheets are characterized by exposed bedrock, till and localized thicker glacial deposits. Rivers in Maine, USA have alternating gravel-bedded steep (gradient > 0.002) and finer gradual (gradient < 0.0005) reaches imposed by the glacial geomorphic and geodynamic history of the region. The low-gradient reaches often include mainstem lakes that are infilling with sediment. These stair-stepped longitudinal profiles lead to discontinuities in sediment flux through the channel network, creating a series of downstream-fining sedimentary links (Rice and Church, 1998). In this presentation, I will review the development and testing of a model to predict bed grain size from a high-resolution (lidar) digital elevation model, based on assumptions of threshold single-thread channels with supply-limited conditions. The field area is a set of three watersheds in Maine, USA, where field ground-truth data were collected. My purpose is to explore how the success or failure of the model is influenced by longitudinal controls on bed shear stress or stream power (via slope and drainage area) and sediment supply (via tributary inputs and eroding glacial deposits). The model is most successful sediment-starved steep reaches, such as those downstream of mainstem lakes. The model is least successful in low-gradient, relatively fine-grained (median grain size < 16 mm) reaches. These variations in model success, which are connected to the glacial geology of the region, have implications for fish habitat, stream restoration, and geomorphic response to climate and land-use changes in the region.