The Geomorphic Response of Gravel‐Bed Rivers to Dams: Perspectives and Prospects: Fid-Bau Portal

The Geomorphic Response of Gravel‐Bed Rivers to Dams: Perspectives and Prospects

Grant, Gordon E.

Predicting changes downstream of a dam to a river's planform, hydraulic geometry, grain size distribution, and slope represents both a real world management problem and a fundamental test of geomorphic science. We consider the current state‐of‐science for predicting downstream geomorphic response, and whether the distinctive aspects of gravel‐bed rivers might constrain the wider range of potential responses to impoundment, improving quantitative prediction. Analytical approaches based on how dams change flow regimes, transport competence, and capacity clearly improve estimates of the direction, trend, and magnitude of downstream channel adjustments, including predictions of direction and magnitude of bed incision, channel armoring, channel width, and changes at tributary junctions. Bed incision on most gravel‐bed rivers below dams is modest, typically on the order of several metres or less. Mixed grain‐sizes typical of gravel‐bed streams create the opportunity for textural adjustments that would not be present if the grain‐size distribution were more homogeneous. Predicting the timescale over which armoring develops, resulting grain size of the bed, and longitudinal extent of armoring below dams are less certain. New approaches to evaluating channel changes at tributary junctions below dams based on the calibre of the introduced sediment and ratio of mainstem to tributary competence also look promising. Although predicting downstream response of rivers to impoundment remains a challenge, these new tools provide a strong foundation for moving forecasts of fluvial response out of the realm of simple conceptual approaches, and toward more technically defensible predictive models.