How will removing the dam affect wetland systems upstream of the dam?

The wetland system of the Lower Pawcatuck River watershed encompasses approximately 2,000 acres of forested, shrub and emergent wetlands based on information available in National Wetland Inventory (NWI) maps and state (RIGIS) land cover maps. Of these wetlands, some are located directly along the impounded Pawcatuck River upstream of Potter Hill Dam. Others are situated in the landscape where surface water and groundwater may hydrologically connect the wetlands to the river.

The project partners engaged the University of Rhode Island, Department of Natural Resources Science (NRS) to conduct a wetland assessment of existing conditions, and how wetlands may change by removing the dam. This work is ongoing and is expected to be presented and summarized in fall 2021. To date, NRS professors and students have improved on the accuracy of the wetland type mapping, and are thoroughly assessing soils and hydrology along the lower river. This information is needed to predict changes to wetlands such as one wetland type converting to another wetland type (e.g., emergent marsh to shrub wetland) with dam removal. Once the NRS study is completed, the data results are expected to be used in the regulatory permit applications for the project.

Removing the Potter Hill Dam will restore the ability of the upriver wetlands to temporarily store flood flows and trap sediments that are suspended in the river during high flow events. These will result as the impoundment that inundates former vegetated wetlands is lowered. Currently, the dam is artificially holding water in areas that would otherwise provide temporary flood storage capacity. Based on outcomes from past dam removal projects in New England and elsewhere, it is expected that up-gradient vegetated wetlands will adjust to the change in water depth and return to their natural composition, relatively rapidly (within 1-2 growing seasons/years).

The project engineers have also modeled the river by updating a “HEC RAS” model of the Pawcatuck prepared by the Federal Emergency Management Agency. The model includes use of bathymetric data (the river bottom elevations collected by professional land surveyors and other experts) and long-term river flow data from government flow gauges on the river to predict how water surface elevations will seasonally change with dam removal. The study includes a number of cross-sections of the lower river to help predict water surface elevations with the lowering of the impoundment and changes in the river channel.

Results of the HEC RAS study were presented during the initial two public informational meetings, and will be included as part of the regulatory permit applications for the project. The project engineers continue to work collaboratively with NRS faculty and students to review environmental data collected for the Lower Pawcatuck River, and to thoroughly analyze what changes are expected to occur.

Show All Answers

1. Why is the dam being removed?
2. Could the dam be repaired or partially removed?
3. Could a design similar to that of the Bradford fish passage project be used for the Potter Hill project?
4. What alternative designs have been considered and why was the proposed design selected?
5. What safety hazards are associated with the mill and dam structures?
6. How will removing the dam affect water levels in the river?
7. Will my household well be affected?
8. Many properties along the Pawcatuck River have private drinking water wells, especially on the Hopkinton side of the river. How many homeowners’ wells could be affected by the predicted drop
9. Will Westerly’s municipal water wells adjacent to the Pawcatuck River be affected?
10. How will removing the dam reduce flood risk to upstream and downstream properties? Will larger flood events (i.e., the 500-year recurrence flood) be worse upstream or downstream following dam removal?
11. Will removal of the dam cause increased flooding to downstream areas, including downtown Westerly?
12. How will removing the defunct dam and mill affect public access to and uses of the river?
13. How will removing the dam affect fish and wildlife?
14. How will removing the dam affect wetland systems upstream of the dam?
15. Is the proposed design accounting for climate change projections and worst-case future hydrologic scenarios?
16. Who is the project team?
17. What are the next steps?
18. Where can I get more information on the project?