Dams provide recreational and water supply benefits, but can also pose safety risks and negatively impact coastal ecosystems (Sayville Dam, Barkhamsted CT)

Method Description for Existing Dams

A comprehensive geodatabase with multiple attributes associated with all known dams in New England was created. ArcGIS Version 10.3.1 was used to perform all Geographic Information System (GIS) analyses. A complete list of all known dams in New England from databases maintained by each of the six New England states became the starting point for this analysis. These combined databases included the geospatial locations and hazard classifications for 14,291 dams—well in excess of the 4075 New England dams estimated from the 2013 U.S. Army Corps’ National Inventory of Dams, which focuses on dam safety. These state-based databases were then compiled into one feature class in a geodatabase.

NEST decided it was important for all the dams in the New England Dams Database to be at the same scale and selected NHDPlusV2 as the scale of reference (1:100,000).  A “Snap” analysis was completed within ArcGIS to align the data to the rivers (i.e., flowlines) within 60 m of each dam. Dams that were not within 60 m of the NHDPlusV2 flowlines were eliminated from the dataset. Martin and Apse (2011) used a similar approach with a 100 m tolerance range to associate dams to the NHDPlus. We took a more conservative approach to minimize incorrectly identifying dams associated with small tributaries that might be adjacent to one of the streams depicted on the 100,000 scale hydrography.

Grist Mill, Plymouth MA

Once the full set of dams associated with NHDPlusV2 flowlines was obtained, ArcGIS was used to perform a “Near” analysis using NHDPlusV2 data to determine if a reservoir identified by the NHDPlusV2 data was within 30 m of the dam.  Each dam is identified in the Database as being associated with a reservoir or not.  If a reservoir is there, its name, size, and other attributes are listed.

Watersheds of each dam were delineated with the National Hydrography Dataset USGS NHDPlusV2 Basin Delineator software; watershed area was calculated for each watershed. For points that failed to process using Basin Delineator, ArcHydro version 1.4 tools were used in conjunction with ESRI’s ArcMap 10.2.2 geographic information system. Watersheds were checked for accuracy using the NDHPlusV2 catchments. Pond area, stream order, USGS Hydrologic Unit Codes and other attributes from NHDPlusV2 for each dam and its associated reservoir were obtained. This dataset was then manually curated to eliminate duplicate entries and sites with obvious anomalies, such as no recorded watershed area.  Land use and land cover of each watershed were also determined from NLCD.

If a dam was listed in the Northeast Aquatic Connectivity Tool (NCAT) database, various attributes were incorporated into the New England Dams Database. 

See the full metadata record for a listing of all attributes in detail in the New England Dams Database.

Martin, E. H.; C. D. Apse.  Northeast Aquatic Connectivity: An Assessment of Dams on Northeastern Rivers. The Nature Conservancy, Eastern Freshwater Program: Brunswick, ME, USA, 2011.

Method Description for Removed Dams

In 2018, a significant expansion of the database occurred with the addition of removed dams and removed dams watersheds. The source of these removed dams data is the American Rivers’ Removed Dams Database. Watersheds were then modeled using ArcGIS 10.5.1 software and the Esri Cloud.  The source pour points for this modeling were the removed dam locations from the American Rivers database. 

For a full method description and other metadata for the removed dams and removed dams watersheds, please see the removed dams metadata record.