The parade of nor’easters that’s pummeled Martha’s Vineyard has filled Island great ponds to capacity, and in some cases, well past it.
In addition to the prodigious amounts of rainfall, the storms, aided by astronomical high tides, have produced significant amounts of “overwash,” surges of ocean water flowing into the ponds. The soaking rains have also pushed up groundwater levels, leaving precipitation with nowhere to go.
The water level in Chilmark Pond is the highest shellfish constable Isaiah Scheffer has seen it in his 11 years on the job.
Tisbury Great Pond in West Tisbury has been overflowing its banks for several weeks.
Quansoo parking lot is a wading pool.
The winter entrance to Long Point is flooded and impassible.
Adam Moore, executive director of Sheriff’s Meadow Foundation and resident in the area for 10 years, said he’s never seen the water levels in the area this high.
A caretaker who asked not to be named told The Times he knows of at least 20 houses on the pond that have flooded basements, and that a number of homeowners are dismayed that the pond hasn’t been opened yet.
Several times a year, a channel is excavated in great pond barrier beaches to allow the water that builds up in the pond to drain into the ocean, to allow tidal flow to flush the pond with salt water, and in the spring, allow diadromous fish like river herring to come home to spawn.
Riparian owners decide when this is done. Island riparian associations are allowed to drain the great ponds as they see fit, under state legislation passed in 1904.
“I’ve had calls from a lot of homeowners with water in their basement,” Kent Healy, West Tisbury selectman, civil engineer, and one of three commissioners of the Riparian Owners of Tisbury Great Pond told The Times on Monday, as the third nor’easter of the month bore down on the Island. “The thing to remember, we’ve had almost 20 inches of rain this year, and the water table in Tisbury Great Pond is much higher than usual.”
Healy said he didn’t expect the water level to rise any more. “The pond level is not coming up; it’s been quite stable,” he said. “The water’s going out through the beach as fast as it’s coming in. The water has started to drop in one of the houses. We plan to open the pond on Saturday. It looks like we’ll get a good opening because it’s supposed to be calm.”
Healy literally wrote the book on the Tisbury Great Pond, with a 64-page study “The Hydrology of Tisbury Great Pond,” which is available on the West Tisbury town website. He said he’s measured the water levels in the pond twice a week for 30 years, and has only seen it this high once before.
“The weather forecast a month ago predicted it would be at the normal pond-opening elevation, which is about plus-five feet, using the National Geodetic Vertical Datum of 1929, which [translates to] about three to four feet above mean sea level,” he said. “The pond now is at an elevation of plus-seven. Back in the hurricane of 1938, the pond level was at plus 12; in 1991 during Hurricane Bob, the water level was at plus-nine. This is the highest level recorded since then.”
Edgartown shellfish constable Paul Bagnall said Edgartown Great Pond is also at historic levels, peaking over five feet above sea level. “It’s as high as it’s been since 1991,” he said. “All ponds work on different heights above sea level. Oyster Pond, for instance, will normally go to five feet above sea level, and it was so high they had to have a February opening. The Edgartown Ponds Foundation usually acts to open the pond when the level is around three and a half feet, but that was not possible due to this year’s storms. We usually target March 15, but depending on weather, it may be a few weeks before or a few weeks after.”
Bagnall said Edgartown Great Pond was opened this past Sunday, but not before a significant number of properties near the pond experienced flooding. “Anyone that lives close to the pond has been in distress,” he said. “We started getting calls after the first storm.”
This week’s storm could add to water levels. “When snow falls on ground that’s already frozen, the ground stays frozen, and for the first few days of melting, all the water flows into the brooks and the ponds,” he said. “Eventually the ground thaws and starts soaking up the water, but even then it goes into the groundwater. Edgartown Great Pond gets millions and millions of gallons a day in groundwater flow.”
The herring spawn is a major factor in the spring great pond openings. Healy said openings are managed under the aegis of the Division of Marine Fisheries. Brad Chase, diadromous fish specialist with the DMF, advises the riparian owners when to open the pond. “John Hoy is the herring warden in Tisbury Great Pond. He and I have been talking for a month, trying to decide the best time to open it,” he said. “He likes to open it in April. The old-timers used to say you open it by March 15. You don’t want to do it too early, because it’ll close up before the herring come back.”
Healy said the pond has to be two to three feet above sea level to drain properly, so if it is opened too soon and closes up before the herring come back to spawn, there might not be enough mass behind the water to drain the pond by the time the herring arrive. “There are a lot of things to consider,” he said.
Bagnall said if the opening that was cut in Edgartown Great Pond closes up soon, another opening could be dug toward the end of the herring run in early June: “I’m thinking it’s going to stay open for a while because the pond was so high, but every time it blows from the southwest, it closes the pond a little more.”
Edgartown Great Pond also has a sluiceway that can provide passage for spawning herring. “You get the fish running into Crackatuxet Cove, and then you adjust the boards so they can make the jump into Edgartown Great Pond,” he said. “But I’d much rather have them swimming through a breachway.”