Above: A rendering of the drilling incident that caused muddy discharge at the base of the Cannonsville Dam, according to the NYC DEP. Source: NYC DEP.
The Cannonsville Dam isn’t leaking, an investigation by the New York City Department of Environmental Protection has found.
After a mysterious muddy discharge appeared below the dam on July 8, fears that the dam was compromised prompted the DEP to draw down the reservoir and warm communities downstream.
Now the DEP knows that the sediment leaking into the West Branch of the Delaware River downstream of the dam isn’t coming from the earthen dam itself, DEP spokesman Adam Bosch said on Friday, July 24.
Instead, the sediment is coming from a silt layer in a rock embankment about 50 yards away downstream where contractors were drilling on July 8, Bosch said.
“The testing at a very high tech lab confirmed that for us yesterday,” he said. “The dam is absolutely safe. We feel really good about that.”
The sediment began gushing into the river when the contractors accidentally pierced a pressurized artesian aquifer while drilling bore holes into the rock embankment to prepare to build a hydroelectric plant, Bosch said.
The lab tested the sediment and found that it matched the silt near the exploratory bore holes. It did not match the rocks, dirt and clay that make up the dam itself, Bosch said.
Repairs race the cold water supply
The DEP will continue to drain water from the Cannonsville Reservoir until the turbid flow is fixed, Bosch said. Repairs could be finished in a few weeks, he said.
This weekend, the DEP plans to drill relief wells to relieve the pressure causing the flow. Next week, it plans to plug the troublesome bore holes, Bosch said.
Much of the water draining from the Cannonsville into the river is cold water from the bottom of the reservoir. Trout require cold water to live, and the large releases are causing usually low temperatures and a great summer fishing season for trout fishermen downstream in the Delaware River.
But the amount of cold water in the Cannonsville is finite, and Bosch said that the reservoir will run out of its supply by August 17. He said that it’s likely that repairs will be finished by then.
If they aren’t, it’s possible that warmer water from the surface of the reservoir will make its way downstream. That could kill the trout, a prospect that worries fisherman and business owners, some of whom expressed their concerns to the Times Herald-Record earlier this week:
Ken Tutalo, owner of the Baxter House in Roscoe - which offers lodging and guided fly fishing tours on the river, says if there is no rainfall between now and when the cold water is gone, the results could be “catastrophic.” “People are nervous because of the potential of complete economic loss and loss of actual fishery,” Tutalo said
If all goes as planned with the repairs, Bosch said, the DEP will get permission from federal regulators to stop emptying the reservoir around the time the cold water runs out on August 17, if not before. At that time, the reservoir will still be 40 to 60 percent full, he said.
In the meantime, "dam safety right now trumps everything," Bosch said.
"DEP drains Cannonsville Reservoir after drilling incident," July 20, 2015