Abstract
To advance the science of dam removal, analyses of functions and benefits need to be linked to individual dam attributes and effects on downstream receiving waters. We examined 7550 dams in the New England (USA) region for possible tradeoffs associated with dam removal. Dam removal often generates improvements for safety or migratory fish passage but might increase nitrogen (N) flux and eutrophication in coastal watersheds. We estimated N loading and removal with algorithms using geospatial data on land use, stream flow and hydrography. We focused on dams with reservoirs that increase retention time at specific points of river reaches, creating localized hotspots of elevated N removal. Approximately 2200 dams with reservoirs had potential benefits for N removal based on N loading, retention time and depth. Across stream orders, safety concerns on these N removal dams ranged between 28% and 44%. First order streams constituted the majority of N removal dams (70%), but only 3% of those were classified as high value for fish passage. In cases where dam removal might eliminate N removal function from a particular reservoir, site-specific analyses are warranted to improve N delivery estimates and examine alternatives that retain the reservoir while enhancing fish passage and safety.
Highlights
Decisions surrounding the future of dams can be informed by scientific inquiry into functions and values linked to their removal, alterations or maintenance [1,2]
First order streams constituted the majority of N removal dams (70%), but only 3% of those were classified as high value for fish passage
We explore the extent of potential N removal by reservoirs associated with dams through a case study focused on all reported dams (>14,000) in the New England region
Summary
Decisions surrounding the future of dams can be informed by scientific inquiry into functions and values linked to their removal, alterations or maintenance [1,2]. In dam removal decisions, there are the potential benefits of improved migratory fish passage and reduced hazards due to dam failures versus the potential costs of removing water supply storage, flood control, recreational opportunities, pollutant retention, or economic opportunities with hydropower. Dams stymie migratory fish passage, eliminating or degrading vast expanses of aquatic habitats in coastal watersheds [3]. This loss of connectivity between estuaries and watersheds affects biota across multiple trophic levels—resulting in negative consequences for the economics, sustainability and biodiversity of fisheries [4,5,6]. Even when fish passage across dams is promoted through technical structures like Denil fish ladders, poor design, changes in the annual flow regime and water temperatures present challenges to migratory species [7]
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