GEOMORPHIC ADJUSTMENTS TO ALTERED SEDIMENT FLUX AND CONSEQUENCES FOR RIVERINE HABITATS

Impoundments, channelization, and land-use changes have substantively altered sediment budgets in many river systems. Sediment-budget alterations result in excess sediment in some places – for example, deltas at the headwaters of impoundments – and deficits in other places - for example, downstream of dams and in highly efficient channelized segments. In addition to their profound implications for geomorphic processes and associated socio-economic costs, alterations strongly influence riverine habitat characteristics, including quantity, quality, and dynamics of physical habitat. I will present examples of habitat changes related to sediment excess and deficit over a broad range of Midwestern US river sizes, histories, and physiographic contexts. Two important generalizations from these case studies are: 1) ecological restoration and management of rivers must often treat the altered sediment supply as an immutable boundary condition because costs of addressing the source of sediment imbalance are exorbitant, and 2) the ecological effects of altered sediment fluxes can be complex and not necessarily bad. This latter point is an important consideration in design of ecological river restoration projects, particularly as it applies to defining achievable objectives. Understanding trajectories of river system adjustments to sediment imbalances contributes to sustainable designs and management policies.