Challenges for Diadromous Fishes in a Dynamic Global Environment

How Dammed Is Your Watershed: First Approximation of an Index to Relative Dammed-ness of U.S. Watersheds

Mark A. Cantrell and Amanda K. Hill

doi: https://doi.org/10.47886/9781934874080.ch77

We describe an index of “dammed-ness” of waters of the United States, with an emphasis on the Atlantic coast of North America where there are many efforts to restore diadromous fishes. We compared the relative degree of dammed-ness at multiple scales, with specific examples where we are involved with diadromous fish restoration. This first approximation provides a quick means of ranking watersheds and evaluating dams as environmental stressors on diadromous fishes. Use of this dammed index can provide insights to relative conditions of subbasins when prioritizing diadromous fish restoration efforts, both within and among basins or regions. We offer additional metrics in the dammed index as a source for prioritization of restoration efforts, as identification of problem areas, and for understanding the seriousness of the challenges facing fishery managers on the Atlantic coast of the United States.

The dammed index is based on comprehensive data in the National Inventory of Dams, maintained by the U.S. Army Corps of Engineers (USACE 2005). We used a geographic information system to determine the dammed index, which lists and ranks watersheds classed by hydrologic unit codes (#d- HUCs) according to several metrics. The first group of metrics that we have developed are related to dam infrastructure: dam density, storage volume, and damless areas are summarized by two d-HUCs, four d- HUCs, six d-HUCs, and eight d-HUCs. Watersheds are ranked according to dam density and placed into five dammed categories: extreme, high (many, large), moderate (transitional), low, and few (or small). We also considered that as the combined storage volume of dams increased, so did the potential for streamflow regulation in the watershed. Similarly, as the total surface area of reservoirs within a HUC increase, so does the potential for thermal and chemical alteration, as well as potential reduction of streamflow by evaporative losses, in the watershed. These categories provide a framework for understanding potential watershed impacts to diadromous fish conservation at multiple scales. Dams are often cited as an impediment to diadromous fish restoration, usually because dams can block or slow fish spawning runs (Freeman et al. 2003). The dammed index provides a broad means of consideration of the storage capacity of dams as well as the relative position of these reservoirs within the subbasins. The dammed index can be used to evaluate diadromous fish restoration efforts among watersheds. The dammed index can be used to compare or guide diadromous fish restoration for subbasins within larger basins to prioritize restoration efforts (Hill 2009, this volume).