Water Quality

When water velocity is slowed, as it is behind a dam, the temperature increases as it’s warmed by the sun. This temperature shift, and associated water quality changes, can make the water uninhabitable for some species during the warmer months. Beaver Brook has many dams, resulting in a compounding effect on temperature increase as the water moves downstream and spends time in the impoundments behind each dam. As temperature increases and water slows, dissolved oxygen decreases. Most aquatic organisms rely on oxygen dissolved in water to breathe, and limited dissolved oxygen can be deadly for aquatic life. Improving water quality does not only benefit aquatic life, however. 

Improved riverine functions

The upstream areas of watersheds supply the downstream estuaries and marshes with sediment, nutrients, and all of the other components necessary for a healthy ecosystem. Dams block natural sediment and nutrient transport, disrupting their conveyance downstream, and often resulting in a buildup behind the dams. Excess nutrients, especially when compounded with warm water, can result in algae blooms, some of which may be toxic for people and animals, and other nuisance aquatic plants.

Flood mitigation 

The dam creates an artificially higher water level upstream. Because these are run-of-river dams, They do not control flooding downstream as some larger dams do. Removal of the dams would both reduce flooding upstream, and remove the risk of a catastrophic failure of the dam; which would release a significant amount of water at once, potentially flooding downstream properties.

Climate resilience and floodplain access

These dams are surrounded by development, which means the river channel upstream is currently constricted on either side. With the removal of the dam, the river width above the dam site will decrease as the natural channel returns, and some  of its historical floodplain will be restored. When large storms occur, the river can then fill the floodplains with water, rather than the surrounding streets and properties. Larger floodplains also slow the speed of the water, which makes it less destructive as it moves downstream. As we expect more intense rainstorms with climate change, allowing the river to utilize these floodplains to manage flood water naturally will be particularly beneficial to the surrounding communities.

Healthier ecosystems 

All of the above benefits together restore a river’s natural  functions, resulting in a healthier ecosystem. When the river benefits, all of the systems that rely on it benefit as well. For example, a migratory bird that passes through the Beaver Brook watershed and is able to feed on fish and insects, then has the nourishment to travel to another completely different ecosystem where the bird contributes to the food web and ecosystem functions. Fish that migrate up through Beaver Brook to spawn also feed bass, bald eagles, seals, whales, tuna, cod, and other fish and birds out in the ocean.  These examples demonstrate how interconnected the watershed is with the greater ecosystem.

Decommission Aging infrastructure

There are over 40 dams in the Beaver Brook watershed. Many of these, including the Collinsville and Navy Yard Dams, are privately owned. These three dams are intermediate-sized, significant hazard dam, meaning that due to its size and the amount of water stored behind it, failure of the dam could result in economic loss, environmental damage, disruption of lifeline facilities, or other impacts. As the dam ages and its condition deteriorates, this risk to the community downstream and the liability to the dam owner become more imminent.

2019 GZA Site Reconnaissance and Conceptual Design Report- Navy Yard Dam

2020 Dracut Hazard Mitigation Plan

2020 Dracut MVP Plan

2020 Stantec Site Reconnassiance and Conceptual Design for Dam Removal- Collinsville Dam

 2021 Merrimack River Comprehensive Plan for Diadromous Fishes: In 2021 the Technical Committee for Anadromous Fishery Management of the Merrimack River Basin, made up of representatives from federal and state agencies in New Hampshire and Massachusetts, completed a comprehensive plan for restoring diadromous fish populations to the Merrimack River Watershed. Removal of Talbot Mills dam was identified as a top priority project resulting in significant potential to restore fish populations to the Merrimack River watershed.