If you think the water level of the Foss Reservoir in Framingham is lower than usual, you are correct. It's about 10 feet lower.
And that's by design: the Massachusetts Water Resources Authority has temporarily drawn down the reservoir to combat the spread of an invasive aquatic plant known as Eurasian watermilfoil.
It's the first time the MWRA has used this form of weed control at Foss Reservoir, but the strategy is practiced at other reservoirs, ponds, and lakes around the state. The premise is simple: Dropping the water level exposes the weeds during the winter, when freezing temperatures can kill the plants.
Lowering water levels is "part of our ongoing effort to battle invasive species that impact water quality," said Frederick A. Laskey, executive director of the MWRA, which began the practice at Chestnut Hill Reservoir in 2009. "It allows us to avoid using herbicides."
Drawdowns have also been used by communities to control weeds at Forge Pond and Nabnasset Lake in Westford, Lake Maspenock in Hopkinton, Fort Meadow Reservoir in Marlborough, and Bare Hill Pond in Harvard.
"The problem right now is we're waiting for cold weather," said Jonathan Yeo, director of water supply protection for the state Department of Conservation and Recreation. To be effective in weed control, he said, a deep freeze is needed for a couple months.
Eurasian watermilfoil — a plant with soft feathered leaves that was once used in aquariums — "is our number one culprit" among invasive species, Laskey said. Since it was first documented in the United States in the 1940s, it has spread rapidly in lakes and ponds throughout the country. The pesky weeds can clump together to form dense mats on the surface of the water, impeding water flow and blocking sunlight from reaching other plants.
In an effort to curb the weed's growth, the MWRA began drawing down both the Foss Reservoir in Framingham and the Chestnut Hill Reservoir on Nov. 2, with the goal of decreasing their water levels by 10 feet. The Foss reservoir reached the 10-feet mark on Nov. 30.
"It's pretty noticeable," said MWRA spokewoman Ria Convery.
Both bodies of water are part of the MWRA's emergency back-up water supply system. The water levels will be restored early next year, though the exact timing will depend on the temperatures and amount of snow that falls.
A dam along Route 9 controls the water in the Foss Reservoir, Yeo said. During the drawdown, the water was released into Framingham's lower reservoirs, which flow downstream to the Sudbury River.
Richard Nota, Harvard's public works director, said the winter water levels have been lowered at Bare Hill Pond by as much as 6 1/2 feet. As far as controlling weeds go, he said, "it's been very helpful."
Fort Pond Reservoir in Marlborough — which is not used for drinking water — has been drawn down by 4 feet this winter and will return to normal levels in March.
But drawdowns are not a silver bullet, by any means.
John Westerling, director of public works in Hopkinton, said Lake Maspenock was lowered by 8 feet this year.
But largeleaf pondweed is causing the most problems, and the plant, which is a native species, is "not affected by the drawdowns or freezing."
"It is growing unabated," he said.
In the spring, there was a proposal to use herbicide to kill the weeds, but it was not approved by Town Meeting, he said. A weed management committee has been formed in Hopkinton to examine the issue. "We're looking at all options," he said.
In Groton, water levels at Knops Pond and Lost Lake are lowered by 30 inches. "We have been doing that for decades to prevent ice damage to docks and retaining walls," said Art Prest, chairman of the town's Lost Lake Watershed Committee, in an e-mail. "This also allows shoreline residents to repair their walls and docks in preparation for winter."
Unfortunately the drawdowns did not reduce the growth of invasive weeds, Prest said. "In the early 2000s we tried lowering the water level by 5 feet but were stopped when several homeowners claimed that their wells went dry."
They then tried a different approach, and used an herbicide. "We did this in 2013 and were delighted with the results," he said.
Julie Wood, director of projects for the Charles River Watershed Association, said drawing down can potentially have a negative impact on other species. But invasive weeds, allowed to grow unabated, "can really really degrade your water body," she said. "Each management technique has its benefits and drawbacks."