LONGITUDINAL VARIATIONS IN CHANNEL MORPHOLOGY IN POSTGLACIAL WATERSHEDS OF THE NORTHEASTERN UNITED STATES: IMPLICATIONS FOR CHANNEL RESPONSE TO DISTURBANCES (Invited Presentation)

Rivers flowing through landscapes formerly occupied by continental ice sheets have non-equilibrium conditions over millennial timescales. In particular, such rivers are characterized by bedrock knickpoints, incision of glacial deposits, and filling of depressions such as lakes and wetlands. Rivers in many parts of New England, USA have alternating gravel-bedded steep (gradient >0.002) and finer gradual (gradient <0.0005) reaches that reflect these variations. 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). Channels are relatively stable in many places, particularly where sediment supply is low, downstream of depocenters. Places with a large supply of glacial sediment, such as incised outwash plains, have more dynamic channels with meanders and avulsions. In this presentation, I will review several recent research projects that explore aspects of how these variations in sediment supply lead to differing responses to disturbances. First, I will present 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 model is most successful sediment-starved steep reaches, such as those downstream of mainstem lakes. 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. Second, I will look at how variations in sediment supply influence the presence or absence of legacy sediment in New England river valleys, contrasting the relatively high-gradient South River watershed of western Massachusetts, with low-gradient coastal rivers. Finally, I will discuss how understanding of the post-glacial trajectories of rivers can be incorporated into decision-making processes related to valley land use and dam removal.