TOWARDS A GENERAL MODEL OF CHANNEL CHANGE DOWNSTREAM FROM DAMS

Prediction of channel change downstream from dams has been an elusive goal of geomorphologists and engineers, yet such prediction is required where new dams are built and where river rehabilitation is pursued. Channel change is driven by the balance between the capacity of stream flow to transport sediment and the magnitude of the sediment supply available for transport. Conditions of sediment deficit or surplus can be approximated, to first order, by Henderson's (1966) quantification of Lane's (1955) conceptualization of the problem. Applied to the prediction of channel change downstream from dams, Henderson's (1966) relation becomes S _postS _pre ^-1 = {(qs _postqs _pre ^-1)(q _preq _post ^-1)(D _postD _pre ^-1) ^0.75} ^0.5 where S is the slope necessary to transport the sediment supplied at rate qs, of grain size D, with the available flow q, and where the subscripts _pre and _post indicate conditions before and after the dam. Values of S _postS _pre ^-1<1 anticipate evacuation of sediment, and values >1 anticipate accumulation of sediment. Computation of S _postS _pre ^-1 for 61 reaches covering 4000 km of the large, dam-impacted rivers of the western U. S. shows that more than 60% of these rivers are in sediment deficit. The greatest uncertainties in utilizing this approach arise from (1) the discharge that serves as an index of all changes in flow regime, (2) the grain size of the post-dam supply, and (3) the temporal domain of the supply – the permanent post-dam supply from tributaries or the decadal scale transient supply derived from sediment evacuation nearer the dam. Despite the inherent limitations associated with development of any metric that describes average channel response to dams, this metric is useful in comparing regional differences in the magnitude of perturbations caused by dams. More precise prediction or characterization of channel change necessitates metrics that integrate data describing the entire post-dam flow regime, the temporal and size characteristics of the sediment supply, longitudinal changes in surface and subsurface bed material, and characteristics of the post-dam riparian vegetation community.