Effects of Dam Removal on River Form and Process

Dams have a profound influence on fluvial processes and morphology. Reservoirs formed by dams drown river channels and trap sediment. Downstream reaches respond to altered flow regimes and reduced sediment supply in varied ways (Williams and Wolman 1984, Collier et al. 1997) that are difficult to predict, although common responses include erosion and 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 a river changes when a dam is removed is becoming increasingly important. Because few detailed geomorphic studies of dam removal have been conducted, however, there is little direct observational basis for predicting the geomorphic effects of dam removal. Furthermore, rivers are complex and fluvial processes often occur over decades or centuries, so predictions are inherently uncertain. Fortunately for researchers, the processes associated with dam removal also occur naturally. For example, after dam removal the sediment fill in an impoundment is likely to become incised, and an equilibrium channel with a new floodplain is likely to form as sediment evacuated during incision increases the sediment supply to downstream reaches. Natural processes related to incision, floodplain formation, equilibrium channel development, and increased sediment supply have been widely studied by geomorphologists and engineers, providing useful conceptual models for evaluating the geomorphic effects of dam removal (Doyle et al. in press). These models can rarely be quantified, however, and in many cases the appropriate model for a particular situation may not be apparent before dam removal. Thus future research will need to concentrate on discriminating among the myriad possible geomorphic responses to dam removal and improving the quantitative basis for predictions.