Dams and Sediment

Dams and phosphorus recycling
The dams on the Assabet, Sudbury and Concord Rivers turn the rivers into a series of pond-like impoundments, slowing the flow of phosphorus-rich water, allowing the proliferation of aquatic weeds, and trapping soft sediments (as much as 6-10 feet deep) behind each dam. During the growing season, aquatic plants take the phosphorus directly from the water column and from the sediments to grow to nuisance proportions. In the fall, as the plants die back, the plants and the phosphorus they took up settle back into the sediments, ready to feed another year's growth. This phosphorus recycling comprises a major non-point source of pollution.

Dams also block the movement of fish and other riverine wildlife. Read about what OARS has been doing to restore stream and river continuity. To learn more about what OARS is doing to understand the impact of dams go to River Restoration and Dams.

The dams
Of the nine dams along the Assabet, seven are old mill dams reflecting the river's working history from the time of the Revolutionary war through the industrial revolution. Like most old mill dams in New England, they are small (less than 15 feet tall) and no longer serve their original purpose of powering mills. Some have been built many times over on the same site and are between 83 and 148 years old. The Ben Smith dam in Maynard feeds water to the old Maynard Mills, now Clock Tower Place, which has a non-functioning hydroelectric power facility. The Powdermill dam is the location of Acton Hydro, a small hydroelectric facility. See our watershed map showing the dams. Read Dams on the Assabet: A guided tour in OARS' June 2008 Newsletter.

The Sudbury River has its share of dams too. The Saxonville dam in Framingham powered the milltown of Saxonville in the past, while several upstream dams were built in Ashland, Framingham and Southborough to create water supply reservoirs for the city of Boston. The Concord River is impounded in North Billerica by the Faulkner (Talbot Mill) dam where the river intersects with the Middlesex Canal, and further downstream by Centennial and Middlesex dams in Lowell. A feasibility study for restoring fish passage at the Concord River dams was completed in 2016 and is available here. In addition there are many millponds on tributaries to the three rivers, such as the dams upstream and downstream of the Wayside Inn on Hop Brook in Sudbury.

Solutions being studied
To improve water quality, the Assabet's nutrient study (known as a TMDL Total Maximum Daily Load) showed that reducing the phosphorus discharge from the wastewater treatment plants will help reduce the recycling but not enough. Phase 1 of stricter phosphorus discharge limits have been set and major investments in upgrading the treatment plants have been made to meet these limits. If this is not be enough to allow the river to meet its water quality standard, what next? The TMDL required that in addition to reducing new input of phosphorus, phosphorus recycling from the sediment must be reduced by 90% in order to meet standards.

Could removing dams and/or the sediment behind them reduce phosphorus recycling and eliminate the need for even stricter discharge limits?

The State DEP worked with the six Assabet River sewered communities to fund and oversee a study to address this question.

  • The $1 million study was funded by 50% state funding with a 50% match of federal funding through the Army Corps of Engineers, who were in charge of the study. A large modeling component was subcontracted to Camp Dresser & McKee.
  • Six of the nine Assabet dams were included in the Corps study. Not included in the study were two flood control dam (Nichols Dam in Westborough, and Tyler Dam in Marlborough) and a breached (Damonmill Dam in West Concord) that have little impact on water quality.

The Assabet Sediment and Dam Removal Study was published in September 2010, entitled "Assabet River, Massachusetts: Sediment and Dam Removal Feasibility Study."

Army Corps of Engineers Sediment and Dam Study webpage.
OARS' comments on the draft study.
Executive Summary of the Final Study.