CONTROLS ON SEDIMENT FLUX AND CHANNEL MORPHOLOGY IN POSTGLACIAL MAINE WATERSHEDS

Rivers in regions formerly occupied by continental ice sheets often flow through mainstem lakes and low-gradient reaches that cause discontinuities in the flux of sediment. In these watersheds, erosion of glacial deposits is a major, often localized, source of sediment to streams. The land use history of Maine includes intensive timber harvest, and associated channel modifications such as dam construction and the removal of roughness elements, which likely influence channel morphology and processes today. In this presentation, I will review recent research in three Maine watersheds (the coastal Narraguagus and Sheepscot rivers and the inland Pleasant River) that focuses on the relationships among longitudinal profiles, fine and coarse sediment supply, land-use change (particularly dam construction and removal), and channel morphology (width, depth and bed grain size). I use lidar-based analysis of watershed characteristics combined with field surveys and studies at a variety of scales. First, studies of marked bedload particles suggest that the frequency and magnitude of bedload transport is reach specific, depending on factors including upstream sediment supply. Second, a simple model for predicting bed grain size is most successful in reaches downstream of mainstem lakes, highlighting the importance of discontinuities on channel form. Third, preliminary work indicates that channel banks upstream of former dam sites on the Sheepscot River are composed of fine sediment deposited in millponds. Finally, I suggest ways that the discontinuities inherent in deglaciated river systems may be exploited in future research focusing on watershed fluxes.