There's been a dam on the Charles River just upriver from Watertown Center almost as long as there's been a Watertown, but it could soon be removed to improve the environment and reduce the risk of flooding upriver from the dam.

According to Charles River Watershed Association spokesperson Alexandra Ash, the Watertown Dam, originally built in 1632, has "significant hazard potential," meaning a failure in the dam may cause loss of life and damage homes, industrial or commercial facilities or roads in Watertown and beyond.

According to the CRWA, removing the Watertown Dam would restore the free flow of water and sediment. and allow fish and other wildlife to travel between portions of the river and can help migratory fish populations rebound.

Removing a dam eliminates expensive maintenance costs and removes the risk of people being injured around a deteriorating dam. The Massachusetts Department of Conservation and Recreation’s Office of Dam Safety has rated three of the dams on the Charles as having high hazard potential, eleven of the dams as having significant hazard potential, and five of the dams as having low hazard potential.

There are currently 109 dams in the Charles River watershed, 23 of which are on the Charles River. 13 of the dams on the Charles are owned by the state, three are owned by local towns and cities, one is owned by the U.S. Army Corps of Engineers, and six are privately owned.

Dam harms environment, say experts

Currently, while some Charles River dams provide flood control, many of the dams on the Charles River bring more problems than benefits.

Built to generate power for industry in the 18th and 19th centuries, dams served an important role in the history of the Charles River watershed. However, dams slowed the flow of the Charles River, hampering its ability to filter out bacteria and clean itself. In some cases, according to the CRWA, dams also prevented migratory river herring from reaching upstream reaches of the river to spawn.

According to CRWA scientist Elisabeth Cianciola, in a preliminary evaluation of management options for the Watertown Dam in 2011, Stantec Consulting Services, Inc. determined that river flow is impacted for approximately one mile upstream of the dam, around the location of the Bridge Street bridge.

"The reduced movement of water upstream of the dam allows water temperatures to reach higher levels than we would expect to see if the river flowed unobstructed, creates an environment that is conducive to the growth of invasive aquatic plants, and has allowed sediment to accumulate behind the dam," she said.

With dam removal, Cianciola said, water and sediment would be able to continually move through the project area as they would naturally, which could reduce flood risk upstream of the dam.

Fish such as alewife, blue-back herring, and American shad would be able to access spawning grounds upstream of the existing dam and less vulnerable to being eaten by gulls that visit the dam.

Additionally, canoeists and kayakers would also be able to boat from the Charles River Dam in Boston to the remnants of the Bleachery Dam in Waltham without having to carry their craft around the dam.

"It would be reasonable to expect that because water would no longer pond behind the dam structure, the width of the river upstream of the dam would decrease," she said. "The elevation of groundwater on the existing riverbank areas would likely decrease, making them drier, and consequently, it may be necessary to vegetate the newly-exposed ground in the existing river bed with wetland plants in order to avoid a loss of wetlands in the project area."

Feasibility study planned

According to the Charles River Watershed Association, the Massachusetts Division of Ecological Restoration, CRWA and Department of Conservation and Recreation will be conducting a feasibility study this year that will assess the possible removal of the Watertown Dam.

In December 2016, the Watertown Dam was added to the list of designated priority projects of the Division of Ecological Restoration at CRWA's recommendation.

The feasibility study will look at costs and benefits of removing the dam and will also consider alternatives to removing the dam.

Cianciola said the timeline for the feasibility study would depend on the funding structure that DER and DCR decide to use for the project.

"If the feasibility study, hydrologic analysis, and design are funded as a single project, the feasibility study likely would not start for another six months or so while we line up funding for the project," she said.

Potential pollution, urban site pose challenges

In 1918 and then again in 1955 the Watertown was severely damaged by flooding and a hurricane, respectively. It wasn’t until 1962, when the Mass Dept. of Conservation built a new dam in the spot as part of a flood control project.

The dam hasn’t been updated since 1972, when a new fish ladder was put in.

As for possible challenges associated with the project relate to the urban nature of the site, Cianciola said turning the Watertown Dam into an active construction site would likely have temporary impacts on access to the Charles River Greenway paths on either side of the river and the existing pedestrian bridge.

"Because a dam has existed at this site since the late 1700’s, the sediment that has accumulated behind the dam may contain pollutants that were dumped into the river before environmental regulations existed," she said. "It would be necessary to determine whether any of the accumulated sediment needs to be dredged from the river, and, if so, where and how the sediment will be contained or disposed."

In December of last year, the first of these dam removal projects started in Bellingham as workers began to take down the Old Mill Dam. According the CRWA, officials currently anticipate that the dam removal and dredging will be largely completed by February, with additional site stabilization and planting to be performed in the spring of 2017.