The Effects of Dams on Watershed Fragmentation and Riparian Disconnectivity Across Multiple Scales

Dartmouth College, Hanover NH

Investigators

Abstract

More than 80,000 dams exist in the U.S., and the proliferation of large dams for hydropower and flood control has led to significant hydrologic, geomorphic, and ecological adjustments in river systems. Recent geomorphic research has documented profound and sustained changes resulting from flow regulation, including changes in channel properties, sediment transport, and reduced channel complexity leading to impaired ecological habitat both within the channel and across the broader riparian zone. Several important research questions have emerged concerning the magnitude and direction of impacts across lateral and longitudinal scales caused by the interactive effects of flow regulation and the progressive downstream tributary inputs of both water and sediment. This research project will focus on subwatersheds of the Connecticut River valley, an area significantly affected by historical flow regulation and impoundment by both flood control and hydroelectric dams. The investigators will quantify (1) the length scale of impact of impoundment laterally across floodplains and progressively downstream of dams; (2) the geomorphic adjustments associated with tributary inputs; and (3) the changes in riparian community structure resulting from impoundment and downstream flow contributions. The project specifically focuses on the length scale of impact imposed by these dams through their associated flow reductions, their resulting changes to the sediment discharge regime by reservoir trapping, and the combined interactive effects of dams on sediment transport and storage as well as flow, especially as part of the progressive downstream inputs of water and sediment by tributaries. Through the use of fallout radionuclides, especially 210-Pb inventories on floodplains, stage-discharge flow modeling, and riparian vegetation sampling, this project will quantify the increased connectivity downstream of dams associated with increased tributary flow. To capture the effect of tributary inputs on sediment discharge, a field design will be employed to measure (1) the activity of 7-Be as a fingerprinting of sediment and (2) changes in bed elevation along the longitudinal profile. The gradient and aggradational adjustments then will be linked to the sediment residence time through the use of 7-Be. This research project will elucidate the links between hydrologic changes in flow and inundation patterns, sediment inputs, and ecological responses. Results from this research can be used as a template to gage the lateral and longitudinal impacts of impoundment and can thus help provide the approaches for ecological amelioration. Moreover, from a geomorphic perspective, the project results will help ascertain the morphologic structure of watersheds and the fundamental ways that tributary inputs influence mainstem characteristics in both regulated and unregulated conditions.