Mill Brook originates in the Chilmark woodlands, and travels approximately four miles, crossing 10 manmade dams of various construction as it wends through West Tisbury, and eventually feeds into Tisbury Great Pond. The ponds created by the dams impede the flow of water, which under the summer sun can spike to almost 90°. Many native fish, in particular brook trout, genetic descendents of fish that fed the Wampanoags, cannot survive those temperatures.
Mill Brook is listed by the Massachusetts Division of Fisheries and Wildlife as a Coldwater Fisheries Resource — a designation given to high-quality streams capable of supporting native species.
Mill Brook 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 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.
For the past five summers, temperature data in Mill Brook has been recorded every 15 minutes, 24/7, using automatic sensors and data loggers placed at 10 locations in the stream. West Tisbury Conservation Commission member Prudy Burt, acting as a private citizen and with assistance from the Sea Run Brook Trout Coalition, landed a grant from the Edey Foundation to purchase the instruments. This year’s readings were taken between June 16 and Sept. 10.
The correlation between Mill Brook temperature spikes and dams is, after five years of study, incontrovertible. The temperature graphs over the five years are practically identical. In 2017, stream temperatures averaged 60° before the dam that creates Fisher Pond, and 70° below it; 60° above Priesters Pond, and 73° below it. Maximum temperatures spiked to a tropical 86° in Priesters Pond, and over 85° in Old Mill Pond. In 2013, temperatures averaged 61° before Fisher Pond, 70° below it; 63° above Priesters Pond, and 73° below it. Maximum temperatures in Priesters Pond reached 89°, and Old Mill Pond topped out at 88°.
In a Sept. 22 letter to the Mill Brook Watershed Study Committee and West Tisbury Conservation Committee, Lloyd Raleigh, who served as the Island’s ecologist for The Trustees of Reservations (TTOR) for eight years, stated that the dams on Mill Brook impede anadromous fish, such as alewives, herring, perch, brook trout, and the American brook lamprey. He recommended a long-term plan to remove the dams. “Clearly the dams are no longer needed functionally, and the native ponds can provide much of the recreational and aesthetic needs of Vineyarders and tourists,” he wrote.
Steve Hurley, southeast district fisheries manager for the Massachusetts Division of Fisheries and Wildlife, has been studying Mill Brook for more than 10 years. In a previous interview with The Times, he said he 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.”
In October, the Conservation Almanac, published by the Vineyard Conservation Society (VCS), stated the Mill Brook ponds “create thermal barriers separating portions of stream that could otherwise be good habitat for heat-sensitive species. It is not nearly as visually apparent as a highway running through a forest, but these hot spots are in effect yet another form of habitat fragmentation.”
VCS ecologist Jeremy Houser told The Times that “habitat fragmentation” is a less obvious complication caused by the dams. “We think stream restoration is about creating more habitat for these anadromous species that need to climb streams, but even if you could magically move the fish to upper parts of the stream that are separated by manmade impediments, there is still an ecological cost to them being separated from each other. Even if the fish could live in the hot water, you’d have the problem of these tiny, isolated populations that are too small to have enough genetic diversity to keep going. Habitat fragmentation is something the VCS is banging the drum about. It’s a broader issue with the pattern of development on the Vineyard. It’s a double-edged sword; we’ve done a nice job hiding development in the woods and leaving a lot of habitat, but we’ve also done a bad job of preventing habitat fragmentation.”
As to VCS taking a more active role in dam removal, Mr. Houser said the VCS role has been and will continue to be focused on public advocacy. “We’ve always favored stream restoration where possible; we’ll support any group that wants to do it,” he said. “We don’t have funding for an ecological restoration project. There are other environmental nonprofits that can do restoration because they own properties where it can be done. It’s an issue that we understand is fraught with the opposition of people who don’t want to remove dams because they like the scenic water features that have been created, and we understand that.”
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. SMF executive director Adam Moore said a basic design for a new, larger culvert at Roth Woodlands has been completed. “Right now the culverts we have are too small and too high,” he said. “We’ve had several grants awarded that we’ve used to design a larger culvert that would meet the Massachusetts stream-crossing guidelines, and we’d like to move forward with that in the not too distant future.”
However, dams aren’t the only manmade obstacles that impede Mill Brook restoration. Extensive local outreach and permitting for the new culvert could take up to two years, in Mr. Moore’s estimation. “We’ve done some outreach, but we need to continue that with some of the downstream neighbors, which include the Dunkls and Island Grown Initiative,” he said. “We need to apply for a permit with the Chilmark Conservation Commission, we need to meet with the Old Farm Road Association, and we would also need to do some more fundraising.”
Mr. Moore said there is not an estimate of what the new culvert will cost, at present.
The placid waters of Mill Pond are at the heart of a simmering environmental and policy stalemate that has roiled West Tisbury politics for a decade.
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.
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, which led to the creation of the Mill Brook watershed study committee (MBWC).
The opening of a farm pond dam on the lower Tiasquam in 2015, written about by Times columnist Matt Pelikan, provides one example of the ecological benefits of dam removal.