Dams and Sediment

Assabet Sediment and Dam Removal Study report released

Army Corps of Engineers, Assabet River, Massachusetts: Sediment and Dam Removal Feasibility Study

Click here for OAR’s comments on the draft study. Click here for the Executive Summary of the Study. You can also download the draft study (26 MB).

Dams and phosphorus recycling
The nine dams along the Assabet River turn the river 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. To learn more about what OAR 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. Two currently have Hydropower facilities that are not currently operational. Click here to view a watershed map showing the dams.

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. Stricter phosphorus discharge limits have been set and major investments in upgrading the treatment plants are being made. Since this will not be enough—what next? The TMDL required that in addition to reducing new input of phosphorus, phosphorus recycling from the sediment must be reduced by 90%.

Could removing dams and/or the sediment behind them reduce phosphorus recycling and eliminate the need for even stricter discharge limits?
The State DEP is working with the six Assabet River sewered communities to fund and oversee a study to address this question.

• The $1 million study is funded by 50% state funding with a 50% match of federal funding through the Army Corps of Engineers, who are in charge of the study. A large modeling component was subcontracted to Camp Dresser & McKee.
• Six of the nine Assabet dams are included in the Corps study. Not included in the study are 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.

An article describing the Assabet Dams was published in the June 2008 OAR Newsletter and is available below.