The New York City Department of Environmental Protection (DEP) is continuing to investigate and repair an ongoing turbid discharge from a rock embankment below Cannonsville Dam. As this work continues, DEP has begun to draw down the reservoir by increasing drinking water diversions and downstream releases. The turbid condition does not represent any imminent threat to the safety of the dam, but DEP believes it is prudent to act out of an abundance of caution and prioritize public safety while the repairs are made. Actions being taken at Cannonsville Reservoir do not pose a risk to New York City’s water supply.
|July 29, 2015
|Change from previous day
|Current downstream release
|Current drinking water diversion
|Previous 24-hour runoff into reservoir
The turbid flow below the dam was discovered on July 8, while workers were drilling borings in preparation for design and construction of the future hydroelectric facility to be built there. When they noticed water coming out of a rock embankment near the release chamber, workers immediately called DEP engineers and ceased all work. A preliminary investigation indicated that the drilling released ground water under natural pressure, known as an artesian condition, several dozen feet below ground, causing an upward flow of water and sediment that is reaching the West Branch Delaware River.
DEP continues to meet with its regulators, consulting engineers and other experts to investigate the condition, and identify appropriate steps for monitoring and repair. In addition to reducing storage at Cannonsville, DEP is taking several risk-reduction steps that include 24-hour monitoring, regular analysis of dam-safety instrumentation, and testing of the turbid water to identify and understand the origin of the sediment.
As DEP reaches key milestones in this work, regular updates will be posted to this web page and our watershed Facebook page at facebook.com/NYCWatershed.
While there is not an imminent threat to the safety of Cannonsville Dam, DEP is encouraging local officials, emergency responders and downstream residents to familiarize themselves with the inundation maps that are found in the dam’s emergency action plan. These maps denote areas (highlighted in blue) that would be inundated under a worst-case-scenario breach of the dam, if Cannonsville Reservoir was 100 percent full at the time of the breach.
This is not the condition that exists today, as DEP is continuing to draw down reservoir storage. However, we believe it’s prudent for the public to have this information in case conditions at Cannonsville Dam change and DEP activates the emergency action plan.
Please note that these maps were developed in 2013 and are subject to future revisions.
Located at the western edge of Delaware County, southwest of the Village of Walton and about 120 miles northwest of New York City. Formed by damming the West Branch of the Delaware River, which continues south and becomes part of the lower Delaware River, the border between New York and Pennsylvania. Placed into service in 1964, it holds 95.7 billion gallons at full capacity.
The Cannonsville is one of four reservoirs in the City’s Delaware system and the newest in New York City’s water supply. Water drawn from the Cannonsville enters the West Delaware Tunnel and travels 44 miles to the upper end of the Rondout Reservoir. From there, it’s carried in the 85 mile long Delaware Aqueduct under the Hudson River and ordinarily makes its way to the West Branch and Kensico Reservoirs for further settling. Leaving Kensico, where it also mixes with Catskill system water, it passes through two aqueducts to the Hillview Reservoir in Yonkers, where it enters New York City’s water supply distribution system.
The Cannonsville watershed’s drainage basin is 455 square miles, the largest basin in the City’s system, and includes parts of 17 towns, all in Delaware County: Andes, Bovina, Delhi, Deposit, Franklin, Hamden, Harpersfield, Jefferson, Kortright, Masonville, Meredith, Middletown, Roxbury, Sidney, Stamford, Tompkins and Walton.
Water not taken for the City’s supply is released from Cannonsville Dam at the reservoir’s west end and flows into the lower West Branch of the Delaware River. Under a 1954 U.S. Supreme Court ruling, New York City can take up to 800 million gallons a day from the Delaware River, provided it releases enough water to insure adequate flow in the lower Delaware for New Jersey and other downstream users. This process is overseen by the Delaware River Master. The City also, in conjunction with the New York State Department of Environmental Conservation (DEC), releases water from Cannonsville to help maintain the fisheries of the lower West Branch River.