Watershed-Scale Assessment of Environmental Impacts and Hazards of Dams

The Department is developing a watershed-scale, GIS-based analytical framework and rating system to (1) assess the environmental impact of dams on wetland and aquatic resources, and (2) characterize the environmental hazard to wetland and aquatic resources from the uncontrolled breach or failure of dams. This system will help focus scarce public dollars on necessary environmental restoration projects through appropriate dam repair or removal. Currently, dam safety hazard ratings focus on damages to people and property. However, there is considerable potential for environmental damages if a dam failure were to release or mobilize contaminants from the sediments or from sources in the inundation zone. In addition, the many impacts that dams have on wetland and aquatic systems have prompted resource agencies to consider removal or breaching of dams in order to restore these impacted systems. Assessment techniques underway consider fragmentation and habitat alteration caused by dams through metrics such as: the length and type of stream habitat above a dam; the area of wetland and length of stream inundated by the impoundment; an estimate of the change in hydrologic residence time or the ratio of the size of the impoundment to stream discharge; and consideration of flow alterations based on the operation of the dam (e.g. flood control, hydropower). These metrics have been applied in multiple watersheds to begin to account for the environmental impacts and hazards of the more than 3,000 dams in Massachusetts