Guiding decisions on the future of dams: A GIS database characterizing ecological and social considerations of Dam decisions

In the United States and elsewhere, there are a growing number of dams which have exceeded their design life and will need to be repaired or removed in the coming decades. Most of these dams no longer serve their original purpose and removal can provide ecological benefits and eliminate future maintenance costs and hazards. However, many decision-makers have been ill-prepared by community resistance to proposals to remove dams. Given the number of dam removal initiatives that have failed or been delayed due to community resistance, both ecological and social attributes of dams need to be better mapped and conveyed in understandable ways. The goal of this study was to support future decisions regarding dams by 1) developing a set of metrics to assess the social and ecological dimensions of dams, and 2) using these metrics to develop a GIS database, for the 1000+ dams in the Narragansett Bay/Rhode Island area of southern New England. The database characterizes the ecological benefits of dam removal or modification, in terms of fish passage, and the social dimensions that may need to be considered when engaging a community in discussions about the future of a dam. Our emphasis was on small-head dams (i.e. <5 m tall) which comprise most dams in the study area. We created social value metrics that used GIS data to assess dams and their impoundments for potential benefits to waterfront properties, history, sense-of-place, and recreation. We modeled our ecological metrics and ranking system after the Nature Conservancy's Northeast Aquatic Connectivity study which considered factors relating to river connectivity and watershed quality. We evaluated our social and ecological metrics using case studies of dams in the study area that had been previously removed or modified. We assumed that both sets of dams were ecologically important, but the modified dams had higher social value that prohibited their removal. Dams that had been removed or modified were both ranked as high priority in terms of value for fish passage, particularly for diadromous fish. Dams that were modified to include fish passage had substantially larger impoundments, more waterfront properties, and more features associated with recreational or cultural value (e.g. boating opportunity, visibility, etc.). Our social metrics were consistent with expectations based on the limited case studies (7 removals, 19 modifications) available in the study area. We made the dam assessment metrics readily accessible to stakeholders through an interactive ArcGIS Online web map.