Channel Adjustments to Dams in the Connecticut River Basin: Implications for Forested Mesic Watersheds

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Through an analysis of at-a-station channel and reach-averaged fluvial properties, we document the stream channel, riparian, and hydraulic adjustments associated with six dams in the Upper Connecticut River basin. For the reach-scale geomorphic analysis, we measured channel planform properties of five tributaries and one mainstem section using geographic information systems of available aerial photographs that pre- and postdate the dams. For the at-a-station morphological adjustments, we used field-measured U.S. Geological Survey stream gauge data of channel hydraulic geometry and discharge. Width, mean depth, cross-sectional area, mean velocity, roughness, and bed elevation were assessed for changes subsequent to dam closure while the reach-scale metrics, sinuosity, and wetted perimeter width, were measured by aerial photography. Corresponding to relatively minor changes in the frequency and magnitude of channel-maintaining flows, we observed no significant differences between pre- and postdam channel morphology in the mainstem section. In the tributaries, where major hydrologic alteration has occurred, we observed some statistically significant adjustment at both cross-section and reach scales. These effects, however, have been relatively minor, and are not consistent across dams in the magnitude, direction, or type of change observed. In highly forested upland New England watersheds, characterized by modest sediment loads, stable channels, and relatively low storage/runoff ratios, channel changes in response to hydrologic alteration appear to be small and contingent on local conditions and landscape history. These responses are in strong contrast to impacts in other geographical regions, and underscore the importance of considering regional hydroclimatic context in large-scale assessments of the impacts of dams.