Effects of dam removal on river form and process


J E. Pizzuto

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dam removal, dams, morphology, reservoirs, channel, sediment, flow regimes, erosion, restoration, strategy, models, future research

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Dams have a profound influence on fluvial processes and morphology. Reservoirsformed by dams drown river channels and trap sediment. Downstream reaches respond toaltered flow regimes and reduced sediment supply in varied ways (Williams and Wolman1984,Collier et al. 1997) that are difficult to predict, although common responses include erosionand lowering of the channel bed (incision) and development of a coarse-grained surface layer(armor) in the riverbed downstream of a dam.As dam removal continues to gain momentum as a restoration strategy, understanding how ariver changes when a dam is removed is becoming increasingly important. Because few detailedgeomorphic studies of dam removal have been conducted, however, there is little directobservational basis for predicting the geomorphic effects of dam removal. Furthermore, rivers arecomplex and fluvial processes often occur over decades or centuries, so predictions areinherently uncertain.Fortunately for researchers, the processes associated with dam removal also occur naturally. Forexample, after dam removal the sediment fill in an impoundment is likely to becomeincised, and an equilibrium channel with a new floodplain is likely to form as sediment evacuatedduring incision increases the sediment supply to downstream reaches. Natural processes relatedto incision, floodplain formation, equilibrium channel development, and increased sedimentsupply have been widely studied by geomorphologists and engineers, providing useful conceptualmodels for evaluating the geomorphic effects of dam removal (Doyle et al. in press). Thesemodels can rarely be quantified, however, and in many cases the appropriate model for aparticular situation may not be apparent before dam removal. Thus future research will need toconcentrate on discriminating among the myriad possible geomorphic responses to dam removaland improving the quantitative basis for predictions. http://www.ucpressjournals.com/journal.php?j=bio





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