Mill Brook begins its journey in the Chilmark woodlands and travels approximately four miles, crossing 10 dams of various construction as it passes through West Tisbury, before it flows into Town Cove and feeds into Tisbury Great Pond. Along that journey, the stream, habitat to native brook trout that are the genetic descendents of fish that fed the Wampanoags and greeted the first Mayhews, reaches upwards of 80° Fahrenheit in several spots, a temperature range that means death to the Island’s native coldwater species.
Recently compiled data from water monitors placed at 10 locations this summer revealed that between June 1 and September 30, maximum water temperatures ranged from near 70° above Fisher Pond to near 90° below Priester and Mill ponds.
Mill Brook is nourished by a watershed that encompasses some 3,400 acres in Chilmark and West Tisbury. It begins its visible life in the 26-acre Roth Woodlands off North Road in Chilmark, which is owned and managed by the Sheriff’s Meadow Foundation (SMF), a private Island land conservation organization.
A berm used to create Old Farm Road is pierced by a culvert that SMF said is too small and too high, and now impedes fish and water flow, creating a small pond to the west. SMF has received a state grant to replace the culvert and allow the water to flow freely. The goal is to improve stream connectivity, recognized by land managers across the country as an increasingly important element in managing water resources.
Mill Brook flows easterly and parallels North Road moving
through dam-created impoundments that date back to the town’s industrial age: Fisher Pond, Crocker Pond, and Priester Pond. In each case, the properties are protected by Island conservation organizations which have varying degrees of responsibility for the dams. The Vineyard Conservation Society holds a conservation restriction over Fisher Pond; The Trustees of Reservations holds a conservation restriction over Crocker Pond; and the Martha’s Vineyard Land Bank owns Priester Pond.
The brook turns south at the intersection of North and County roads, and passes over a small earthen berm. The next obstruction is a dam in a narrow stone culvert on private property that creates what is known as Albert’s Pond and blocks fish passage.
Just south of Scotchman’s Lane, a diversionary dam siphons off water into a canal built in 1906 to create Parsonage Pond. Over the years the canal has deteriorated, and with it the pond, however one estimate is that at this stage Mill Brook loses 10-20 percent of its water flow to the canal.
The most visible portion of Mill Brook is scenic Mill Pond, the artificial impoundment created by a dam under West Tisbury Road that generations ago helped power mills on the site. A second diversion dam below Mill Pond creates Maley’s Pond, also known as Factory Pond.
The placid waters of Mill Pond are at the heart of a simmering environmental and policy debate that has roiled West Tisbury politics for almost 10 years.
One group of townspeople wishes to dredge the pond, which now has an average depth of about two feet. Another group wishes to see the dam removed and the brook restored to its natural state. It is the only dam along the course of the brook with a fish ladder, installed to facilitate the passage of herring, which still return to the stream to spawn.
A June 2011 dam-removal-study commissioned by the Division of Ecological Restoration that looked at dam removal placed the cost of dredging at $500,000 and full dam removal at $500,000. A less expensive alternative that would achieve some of the same results as full dam removal, removing the boards in the spillway, was pegged at $25,000.
In April 2014, annual town meeting voters said no to funds to dredge Mill Pond, and unanimously voted in favor of a Mill Brook watershed study that would look at the broader picture, not just the pond.
More than one year ago, West Tisbury selectmen appointed a Mill Brook watershed study committee (MBWC). The study includes the cooperation of local organizations, private volunteers, and biologist and trout expert Steve Hurley, Division of Fisheries and Wildlife Southeast District Fisheries manager.
Since 2013, water-temperature data recorders have been placed at key locations along the length of the brook. The most recent data was collected and compiled last week, and requested by The Times.
A graph shows the mean, or average, temperature and maximum recorded from June 1 to September 30 for 10 locations: Roth Pond tributary (61.95/73.0); below Roth Pond (69.91/86.26); above Fisher Pond (60.41/69.56); below Fisher Pond (68.52/78.34); Witch Brook (62.91/75.25); unnamed tributary Priester Pond (61.09/72.18); below Priesters Pond (74.52/89.18); Scotchman’s Road (69.92/81.64); below old Mill Pond (72.50/89.73); head of Town Cove (67.38/82.09).
The range of temperatures reflects the effects of blocking the stream. “It just shows the impact of ponds on stream temperatures,” Mr. Hurley told The Times in a phone conversation Friday. “Typically the groundwater is fairly stable, and that’s why most of the trout streams with a lot of groundwater have pretty stable temperatures. When you put an impoundment on a stream, that exposes more surface area to the sun; it warms up the water.”
Mr. Hurley is not surprised by the data. “Typically what we expect when you put an impoundment on a river is you get higher temperatures in the summer, and lower temperatures in the winter,” he said.
All of which has an effect on the ecology of the brook. Many native fish, in particular brook trout, cannot survive temperature extremes. “Temperature is one of the major factors that affects fish species composition,” Mr. Hurley said.
“When you get the warmer temperatures, you are going to drive out, or eliminate, the trout, and you will allow species like largemouth bass, which is an introduced species, to flourish.”
In the case of Mill Brook, native species include brook trout, American brook lamprey, American eels, and white perch. Mr. Hurley said there is no question that the dams along Mill Brook have had a detrimental effect on native species. Water temperatures close to 90 degrees, as have been recorded in recent summers, would spell death to any cold water species, he said.
Last March, in response to a February request from the watershed committee for useful information and data, Brendan O’Neill, executive director of the Vineyard Conservation Society (VCS), spoke in measured tones about Mill Brook.
“Regarding the Mill Brook watershed big-picture, restoring stream connectivity for ecological purposes is currently a priority regional and national conservation goal,” Mr. O’Neill said. “So it makes sense for it to be a planning goal on the local level as well. VCS and colleagues support such restoration as a mid/long-term goal for all the Island’s watersheds, including the Mill Brook.”
“SMF is taking positive steps in this direction with its culvert work at the Roth Woodlands sanctuary. Our field ecologist at the Woods Preserve endorsed a process of educating landowners over time and working with engineers to identify strategies to remedy fish hindrances, noting that the ‘factory ponds’ are a relatively recent artifact of the early settlement period. As far as the fate of the Mill Pond itself, we remain intrigued by the potential ‘win-win’ floated by DFW’s Steve Hurley when he suggested that stream restoration doesn’t preclude engineering a separate pond and town park at that location. VCS hopes that your work will help test the feasibility of that idea.”
The win-win Mr. O’Neill referred to is a third option that has surfaced in the ongoing discussion: create a bypass channel that would allow the brook to bypass the pond, thus allowing it to flow freely — a critical element in water-temperature management.
“Basically you would allow most of the flow from the river to pass to the side of the pond,” Mr. Hurley told The Times. “Typically, they’re called bypass channels. And what that does is allow the cooler temperatures to pass by and enter the area below the pond. And if you can adjust it just right, you can still maintain the water levels in the ponds without overspilling.”
“Essentially, the pond is isolated from the stream, which allows the unimpeded flow of water,” Mr. Hurley said.
The natural order
Mr. Hurley expresses no hesitancy about the dam on Mill Pond. “If you got rid of the dam, that would eliminate all of the problems, at least from the trout’s point of view,” he said.
Mr. Hurley sees no environmental benefit to the dams along Mill Brook. “There is plenty of other habitat for pond species in the area,” he said, “whereas the cool water habitat is pretty limited in Eastern Massachusetts. That is one of our more limited habitats — these cold water streams — we have an abundance of warm water habitat.”
What would be the natural progression if the dams were to be removed? “Basically, you would reestablish what was originally there, which was a riparian stream corridor,” he said. ”You would generally have low shrubs or trees along a stream. You don’t have the open water — people tend to like the aesthetics of the open water — but in terms of ecosystem functioning, the river is the natural order of things.”
The opening of a farm pond dam on the lower Tiasquam last year provides one example. The area of the pond has revegetated.
Good and bad dams
The aesthetic consideration is a principal concern for West Tisbury selectman Richard Knabel, who told The Times he was unaware of the bypass option described by Mr. Hurley but anticipates it will be one of the options examined by the watershed committee. Mr. Knabel said he is not surprised by the temperature readings, which he ascribes to slow water flow, but remains passionate in his defense of the dam that formed Mill Pond.
It is a view that puts him at odds with Riverkeeper, a Hudson River environmental organization that the retired professor of physical science led as president for five years and on whose board he serves. Riverkeeper supports state efforts to restore free-flowing tributaries to the Hudson River by removing old dams and reinstalling culverts.
Asked about the seeming contradiction, Mr. Knabel said, “There’s a difference between supporting dam removal and supporting all dam removal in all places regardless of the circumstances. I think there are distinctions to be made about particular dams and locations, and I don’t see any contradictions in there at all.”
Mr. Knabel said there are dams that serve a purpose and those that no longer do and probably should be removed. Asked to describe the purpose of the Mill Pond dam, he said, “It’s aesthetic; it’s historic; it’s beautiful; because if it were removed, what we’d have there would be something awful. I’ve seen the pictures of what happens when you take out dams and try to restore some of those areas, and there’s nothing attractive about it at all. I think aesthetics are important, and they certainly are in this case.”