Downstream leak stopped, emergency releases halted at the Cannonsville Dam

Above: A stream of cloudy water, center, flowed from a rock embankment downstream of the Cannonsville Dam for several weeks. Photo via the NYC DEP

A disturbing gush of cloudy water below the Cannonsville Dam was stopped on Saturday, August 1, and the New York City Department of Environmental Protection dialed down the emergency cold-water releases gushing from the Cannonsville Reservoir into the West Branch of the Delaware River on Sunday, August 2.

Fears that the dam was compromised arose on July 8, when a mysterious turbid discharge appeared below the dam, prompting the DEP to draw down the reservoir and warm communities downstream.

On July 23, tests confirmed that the sediment leaking into the river downstream of the dam wasn't coming from the earthen dam itself.

Instead, the sediment came from a silt layer in a rock embankment about 50 yards away downstream at the site of a planned hydroelectric plant where contractors were drilling earlier this month. The sediment began gushing into the river when the contractors accidentally pierced a pressurized artesian aquifer while drilling bore holes into the rock embankment.

Above: Workers pump water from one of four relief wells drilled into an embankment near the Cannonsville Dam. Photo via NYC DEP

Last week, engineers drilled relief wells to reduce the pressure in the aquifer. On Saturday, after the fourth relief well was finished, the cloudy discharge disappeared, DEP spokesman Adam Bosch said today.

"It stopped, and it has continued to stay stopped," Bosch said.

On Sunday morning, the DEP began slowing the emergency water releases from the reservoir. By Wednesday, Aug. 5, the flow into the Delaware River will be back to its normal 500 cubic feet per second, according to an update issued by the DEP on Sunday. (The emergency releases were three times that, around 1,500 cfs, Bosch said.)

At the high emergency release rate, the Cannonsville was slated to run out of cold water by mid-August. After that, it would have begun releasing warm water downstream, which would have harmed the trout, which die when the temperature gets too high.

By shutting off the emergency releases, Bosch said "basically, you just tripled the life of the cold water."

Bosch said that some cold water reserves will likely remain in the reservoir until the fall.