Merrimack Village Dam: The Impact of Removing a Dam in New Hampshire

19 january 2013

Post-dam removal monitoring is ongoing to identify the impacts of removing Merrimack Village Dam in New Hampshire. The dam was removed to restore diadromous fish habitat, improve water quality, restore natural sediment transport and avoid high maintenance costs associated with the aged structure.

In recent years many New England states, including New Hampshire, have been removing low-head dams that no longer serve a useful purpose. To support these efforts, competitive grant funding is available from federal, state and private entities that can be used to partially defray costs associated with feasibility studies and demolition. Most dams eligible for this funding are those along the coast where removal would restore diadromous fish (Atlantic salmon, American shad, river herring and American eel) to former spawning grounds and restore habitat connectivity.

Merrimack Village Dam – the lowermost dam on the Souhegan River in Merrimack, N.H. – was purchased by Pennichuck Water Works in 1964 for use as a supplemental water supply source. These plans never materialized, and in 2004 PWW received a letter from the New Hampshire Department of Environmental Services regarding numerous dam deficiencies. PWW opted to retain Gomez and Sullivan Engineers P.C. of Henniker, N.H., to conduct a feasibility study regarding removal of the dam. Removal achieved many goals: diadromous fish restoration, habitat connectivity, restoration of natural sediment transport, elimination of operation and maintenance costs and removal of a potential safety hazard.

From the onset, this project was a cooperative effort among PWW, state and federal agencies, and many non-profit organizations that invested technical experience and grant funds for the successful removal and restoration of the Souhegan River. In 2008, this dam was removed, thereby allowing diadromous fish to once again ascend the Souhegan River.

Project history and setting

The first documented dam construction on this site at Merrimack Village occurred in 1744 for use as a grist mill. Over the centuries, ownership exchanged hands and modifications were made to the dam and power canal. The river at this site was harnessed to power various saw mills, grist mills, shoe industries and a chemical plant. The most recent changes to the dam occurred circa 1934 when the stone masonry dam was overlain with concrete. The dam was about 20 feet high and 145 feet wide along the arched spillway.

As the local population grew, the dam came to be located along a heavily traveled road in a downtown area of Merrimack. A stone’s throw below the dam is a vintage stone masonry bridge, called Chamberlain Bridge. From this bridge, drivers and pedestrians could easily view the dam. Although the dam was highly visible, the bulk of the impoundment was relatively hidden from view and no development around the impoundment exists. The impoundment had artificially widened as a result of sediment buildup, resulting in the creation of two small islands. Beneath Chamberlain Bridge the Souhegan River cascades over bedrock before slowing into a shallow, sandy pool. From here, the sandy bottom river slowly winds 1,500 feet to the Merrimack River. From just below Chamberlain Bridge to the confluence with the Merrimack River, the Souhegan River is backwatered by the Merrimack River.

Dam deficiencies and funding opportunities

In January 2004, NHDES’ Division of Dam Safety issued PWW a letter of deficiency, stating the dam did not meet current dam safety criterion. Deficiencies included inadequate spillway capacity, deteriorating concrete, inoperable gate structures and excessive leakage due to loose interior stonework on a training wall. The NHDES also indicated in the letter that PWW may opt to remove the dam with assistance from the NHDES’ Dam Removal and River Restoration Program.

A combination of the letter of deficiency, the fact that the dam was not used for water supply, the long-term liability and operations and maintenance costs prompted PWW to consider dam removal. After discussions with NHDES about the Dam Removal and Restoration Program, PWW pursued a study to evaluate the feasibility and costs associated with removing the dam. NHDES also assisted PWW with securing federal, state and private grants to help defray the costs of a feasibility study and potential removal.

Initial discussions between PWW and project partners resulted in the de-cision to conduct a feasibility study to determine if dam repair or removal was the best alternative for the site. Major project partners that provided technical and financial support for removal of the Merrimack Village Dam included NHDES, PWW, U.S. Fish and Wildlife Service, New Hampshire Fish and Game, Conservation Law Foundation, American Rivers, National Oceanic and Atmospheric Administration, Gulf of Maine Council on the Marine Environment, Coastal Conservation Association and Restore America’s Estuaries. PWW contracted with Gomez and Sullivan in 2004 to conduct the feasibility study.

This aerial view of the Souhegan River shows what it looked like before removal of the Merrimack Village Dam. Monitoring is ongoing to better understand the geomorphic response to dam removal.

Feasibility study issues

When evaluating dam removal projects, several resource impacts are considered and can vary depending on site specific conditions. Major issues evaluated by Gomez and Sullivan as part of the feasibility study included:

• Quantity, quality, transport and management of impounded sediment;
• Impacts of dam removal on wetlands and cultural/historical resources; and
• Access to private lands in order to remove the dam.

Given that a dam had been present at this location for centuries, the impoundment was essentially filled with sediment and was artificially widened. The Souhegan River is very sinuous, carving through easily eroded soils that are then transported to the lower Souhegan River. The river is well-known for transporting heavy sediment loads during storm events. The sediment volume in the impoundment was quantified by driving steel rods to the point of refusal every 5 to 10 feet across several transects within the approximate 1,800 foot-long impoundment. The sediment consisted primarily of uniform sand/silt both vertically and horizontally within the impoundment; the gross volume of sediment was about 81,000 cubic yards.