Reservoir effects on downstream river channel migration

Human occupation and development of alluvial river floodplains are adversel y affected by river channel lateral migration, which may range as high as s everal hundred metres per year. Reservoirs that reduce the frequency and du ration of high flows typically reduce lateral migration rates by factors of 3 to 6. The ecology of riverine corridors is dependent upon the processes of erosion and sedimentation, which lead to lateral migration. Multiple-obj ective use of floodplains adjacent to active rivers therefore requires tool s for assessing the probability and magnitude of channel movements. Existin g approaches for predicting river channel movement may be classified as emp irical or mechanistic, and are inadequate for widespread application. The M issouri River downstream from Fort Peck Dam in Montana, a major alluvial ri ver with flow highly perturbed by regulation, was selected for case study. Maps and aerial photographs were available before and after dam constructio n. This imagery was analysed by digitizing channel centrelines at successiv e coverages under pre-dam and post-dam conditions, and mean migration rates were computed by bend and by reach. The mean rate of channel centreline mi gration fell from 6.6 m yr(-1) to 1.8 m yr(-1) after impoundment. Bend-mean channel activity rates were only weakly correlated with variables describi ng channel form and geometry. Results indicate that flow regulation for flo od control and hydropower production typical of the study reach had profoun d effects on river corridor dynamism, with implications for habitat type di stribution and ecosystem integrity.