A new multi-year study to determine the health of the Mill Brook in West Tisbury has found the system is struggling, prompting consideration of how to aid the nearly 3,000-acre watershed that feeds Mill Pond.
The town’s Mill Brook watershed management committee presented its findings to an audience of more than 50 people at the West Tisbury library Sunday. The committee largely focused on the Mill Pond along Edgartown-West Tisbury Road, saying it suffers from high temperatures and low dissolved oxygen levels, making the pond inhospitable for native fish and aquatic insects.
“These are really ecologically important and rare species, and habitat quality is clearly very impaired.” said committee member Julie Pringle, who is also a deputy shellfish constable for Edgartown.
The Mill Brook is one of the largest freshwater ecosystems on the Island and is home to brook trout, brook lamprey, American eel and herring. It starts in Chilmark and extends roughly five miles along North Road and State Road until it flows into a cove at Tisbury Great Pond.
A handful of dams create man-made ponds throughout the brook, the last of which is Mill Pond. The pond hosts endangered water-willow stem borer moths which were discovered during a rare species survey conducted in 2021.
The Mill Brook committee formed in 2014 to collect data for management decisions. Sunday’s event was held to go over the new research, collected from 2021 to 2024.
At the onset of the work, the committee set up nine sampling stations along the stream and measured for nutrients, water temperature, pH, dissolved oxygen and conductivity. It continuously measured temperature at six of the stations and dissolved oxygen at Mill Pond.
Water temperatures at the pond regularly exceeded 68 degrees, the upper limit for cold-water fisheries, for nearly the entire period between late June 2023 and mid-September 2023. Temperatures peaked at 84 degrees.
Entomologist Greg Whitmore studied the species along the brook. His report noted a shift from cold-water fauna to warm-water fauna due to impoundments raising the water temperature.
“The temperature increase has a clear negative impact on temperature-sensitive species of macroinvertebrates,” Mr. Whitmore wrote. “Those species adapted to coldwater streams are not able to survive the temperature increase induced by the ponds.”
Members of the committee pointed to the dam at the Mill Pond as the culprit. The committee told the audience on Sunday that dams negatively impact water quality and habitat, which is reflected in high temperatures, low dissolved oxygen and a decrease in biodiversity.
The committee said that the Mill Pond impoundment blocks fish and wildlife passage and is creating excess heat that’s degrading the water quality and habitat.
Removal of the dam has been highly debated by the town for over a decade. It would restore the stream’s natural flow, but drain the existing pond, which has been a fixture in town for hundreds of years.
The committee presented two options for possible town management of the pond.
The first would remove spillway weir boards instead of the entire dam, which runs beneath Edgartown-West Tisbury Road. The boards currently hold the level of water in the pond.
Removal of the spillway weir boards would drain Mill Pond, restore the natural streamflow, allow for fish and wildlife migration and help mitigate high temperatures.
The second option suggested building a by-pass that would cut-off Mill Pond’s access to the stream, separating the two. The pond would be fed by groundwater and the stream would run around it on a natural track.
A similar stream by-pass was built in the Child’s River in Falmouth. The committee said it’s an option that would benefit the ecosystem while preserving the Mill Pond.
At the end of the meeting several members of the public expressed interest in working together to solve the issue. Others were concerned about the future of the Mill Pond.
Sean Conley, the chair of the West Tisbury Historic District Commission, said Mill Pond’s removal would be disgraceful. He said the town should focus on other ponds along the brook that also have high water temperatures.
“It’s already too warm before it gets to Mill Pond,” Mr. Conley said. “So why should we destroy the beautiful Mill Pond until you opt to get the other people on board?”
The committee said the town can only make decisions for the Mill Pond because it doesn’t own the others.
“I think the point is not to throw up our hands and say ‘why bother?’ I think the point is to do what we can, whenever we can, and this is a situation where we can do something and make a massive improvement,” said committee chair Prudy Burt.
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