DER’s Restoration Potential Model Tool Description

The purpose, use and methodology behind the RPM Tool

Purpose

The Division of Ecological Restoration developed the Restoration Potential Model (RPM) Tool to provide easily-accessible information about the ecological benefit of removing any known dam in the Commonwealth. Dam removal is most ecologically beneficial when a dam is the primary stressor on the river. When a river is also impacted by other stressors (e.g., poor water quality, chronic flow alteration from water withdrawals, or multiple passage barriers) the benefit realized from dam removal may be diminished. By evaluating key indicators of river stress, the RPM Tool helps to identify those dams that are the primary stressor on a river. The Tool also estimates how dam removal may positively affect the surrounding watershed and unique biological habitats (e.g., BioMap2, coldwater and headwater streams, and coastal stream habitats) including the number of river-miles that may be re-connected after removal. The combined analysis of river stressors and potential dam removal benefits results in a relative ranking of ecological benefit potential for each dam if it were removed.

Use

The Tool provides consistent, statewide, GIS-based indicators of ecological benefit from dam removal that can be used to help evaluate and prioritize river restoration efforts, support grant proposals, and inform communications. The Tool provides one of many sources of information that DER and other organizations may consider when evaluating and prioritizing projects. It is also useful as a repository of key indicator data for river ecological condition and as a means to communicate information about additional stressors on river ecosystems at dam sites. The Tool can also be used in climate adaptation planning, particularly in relation to habitat improvement and wildlife resilience that may benefit as a result of dam removal.   

Limitations

The evaluation of potential dam removal projects requires careful consideration of many factors that this Tool does not include. Other factors may include the condition of the dam, its current use, engineering and technical issues unique to the dam, site-specific habitat quality, availability of funding and community support, and whether the dam owner is interested in dam removal. The RPM Tool ecological benefit rankings should not be interpreted or represented as a comprehensive assessment of dam removal benefits, costs, or feasibility; or as an endorsement by DER for the removal of any particular dam.

Model Development

Modelbuilder Models: The RPM Tool was developed in two major phases, watershed delineation and ArcGIS Modelbuilder models. Modelbuilder was used in part to automate the analysis but also to ensure that a detailed record of the analysis was preserved. 

Watershed delineation: We delineated the boundaries of contributing watersheds and subwatersheds for all dam points and then used the resulting watershed polygons to calculate indicators of each dam’s impact on the environment. The “subwatershed” is the portion of each dam’s watershed excluding the watersheds of any other upstream dams; in other words, it is the watershed of a given dam extending to the next upstream dam or dams (as applicable). The watersheds and subwatersheds were delineated using ArcHydro and the New England method, which requires several steps to prepare the Digital Elevation Models (DEMs) so that they match the vector hydrography data. DER used the 5 meter statewide DEM supplemented by the 10 meter USGS data for the areas of the Major Watersheds that lie outside the state.  Hydrography data: These data are based on the 1:24,000 National Hydrography Dataset (NHD) hydro line data.  These data include the stream centerlines that transect hydro polygons such as ponds and lakes. The NHD data set was used to determine the “connected stream network” (i.e., network of continuous, free-flowing main-stem and tributary streams). See additional information on the NHD data set.     

Dam data: The locations of the dams are based on a GIS feature class developed by the Massachusetts Office of Dam Safety and downloaded from MassGIS in February, 2012. DER added and removed dams from this data set based on institutional knowledge of dam locations and/or completed dam removals. Also, only the main dam structure on an impoundment was used in the model. Dikes and other structures that do not obstruct a river channel were marked as ‘secondary’ structures and not modeled. The Massachusetts data were supplemented by GIS data of dam locations from the surrounding states, New Hampshire, Rhode Island, Connecticut, Vermont and New York. The dams in other states were not modeled, but they were used to determine the extents of the subwatersheds of Massachusetts dams. See additional details on this data.   

Environmental indicators used in the Tool

The Restoration Potential Model scoring of dams is based on the following indicators of environmental impact and stream health.

1. Watershed Position: There are three watershed position settings with unique ecological characteristics that particularly benefit from dam removal: head-of-tide habitats, coastal stream habitats, and headwaters ecosystems.  The Tool adds a scoring weight for dams located in these settings when calculating the estimated ecological benefit of removal.
2. Ecological Integrity:  The Tool uses four indicators to measure the ecological integrity of stream reaches that would be affected by a dam removal.
   1. BioMap2 habitat in the dam’s upstream or downstream connected stream network, which indicates the presence of rare species and aquatic habitats of high ecological value and integrity (see more information about Biomap2);
   2. Percent cover of impervious surfaces in the watershed upstream from the dam, with lower percentages indicating better water quality and stream habitat health;
   3. Presence of mapped coldwater habitat in a dam’s upstream or downstream connected stream networks, indicating waters in which the maximum mean monthly temperature generally does not exceed 68°F and are capable of supporting year-round populations of cold water stenothermal aquatic life such as trout, as defined by the Massachusetts Department of Environmental Protection (310 CMR 4.00);
   4. Alteration of August stream flow due to water withdrawals, with higher flow alterations indicating higher stress on the aquatic ecosystem and lower ecological integrity. See more information about this indicator.
3. Connectivity:  The tool measures the improvement in connectivity that would be gained if the dam were removed.  For instance, a dam removal with a 2-mile connected stream network upstream and a 5-mile connected stream network downstream would result in a net connectivity improvement of the lesser of the two connected networks, or 2 miles in this example.  

Here is how each of these indicators is scored:

1. Watershed Position – 20 points possible, with points given for head of tide habitats, coastal stream habitats, and headwaters streams.
   1. Coastal stream habitat. The first dam from the ocean receives 10 points; the second dam receives 5 points; and the third dam receives 3 points.
   2. Headwaters habitat. Based on watershed area; dams that have an upstream watershed area of less than 5 square kilometers receive 5 points.
   3. Head-of -tide habitat. Head of tide dams are those at an elevation of 5 meters or less and receive 5 points.
2. Ecological Integrity – 55 points possible
   1. Biomap.  If the dam’s upstream or downstream connected network intersects with BioMap Aquatic Core or BioMap Critical Natural Landscape Aquatic Buffer – 10 points
   2. Coldwater.  If a coldwater stream segment intersects the connected stream network immediately upstream or downstream of the dam – 10 points
   3. Impervious surfaces.  Based on the percentage of the watershed area upstream from the dam that is covered by impervious surfaces (e.g., roads, parking lots, driveways, and rooftops).
      Percent area covered	Points
      <3	20
      3-6	15
      6-12	5
      12	0

   4. Stream flow. Based on estimated August Flow reduction. Dam that have less flow alteration in the upstream watershed score higher than those with more flow alteration. Dams outside of the area for which estimated flow reduction data is available (near the coast or in the Connecticut and Merrimack major watersheds) receive 10 points. Since there is no stream flow data for these dams, it is assumed that they are neither seriously flow impaired or completely unimpaired. This parameter does not incorporate any flow alteration that may be caused by management of dams for water supply, recreation, or other purposes. See additional information. 
      % Reduction of August stream flows due to water withdrawals	Points
      0-10	15
      10-25	10
      25-55	5

3. Connectivity – 25 points possible

1. Based on the net gain in connected stream length when the upstream and downstream networks are re-connected by dam removal (including mainstem and tributaries). For the first dam from the from the ocean, the upstream stream network length is used because the downstream length extends into the ocean and is difficult to quantify. 

Connected miles gained	Points
<2	0
2-3	5
3-5	10
5-10	15
10-20	20
>20	25

Additional Displayed Information

The RPM Tool displays additional information about the dam in relation to its location, regulating authority, ownership, and hazard classification. The information displayed is based on the GIS feature class developed by the Massachusetts Office of Dam Safety and downloaded from MassGIS in February 2012. See additional details.