Mother Nature really has to get this right.”

Diane Tharp

How big are the leaks?

The total leakage data is estimated at around 20 million gallons per day (MGD)—for reference, the aqueduct currently delivers between 500 and 600 MGD—with 95 percent coming from the Newburgh portion.

How does the repair work? (image 3)

The planning for the project has been ongoing for decades

While the tunnel is shut down for repairs a bypass tunnel is connected before and after the leaks.

The New York City Department of Environmental Protection (DEP) has dug that bypass tunnel already and will connect to the bypass tunnel when the aqueduct is shut down.

 NEW YORK STATE — In an unprecedented event, the Delaware Aqueduct will be shut down for five to eight months to complete vital leak repairs on Tuesday, October 1.

The shutdown has local officials concerned about flooding. Six hundred million gallons per day are usually diverted from the Upper Delaware and sent to New York City; instead the water will pool in Upper Delaware reservoirs, including Pepacton, Neversink and Cannonsville. 

If the reservoirs start to overflow, there are few tools to safely empty them.   

According to, John Milgrim, Director, of External Affairs for  NYC Environmental Protection Bureau of Water Supply,  "the reservoirs’ release capabilities throughout the shutdown are vastly higher than the amount of water drawn toward the City."

“We all pray that the aqueduct will close, Mother Nature will be kind to us with no crazy storms, and construction will go as planned and finish in five months rather than eight months,” said Diane Tharp. “Everyone living below the dams must remain vigilant and watch the gages to monitor the capacities of the reservoirs throughout the closure, [and] take action if needed.” 

Tharp is the executive director of North Delaware River Watershed Conservancy Ltd. (NORDEL). For the last 18 years, she has worked with various organizations involved with the shutdown, including New York State, New Jersey, Pennsylvania, and the New York City Department of Environmental Protection.

A particular focus is to protect the people in the Delaware River Basin from catastrophic flooding.

The shutdown has been delayed twice and was previously scheduled for October 2023.

Drip, drip, drip

Despite delays, fixing leaks is crucial. That 600 million gallons of water a day sent to New York city constitutes about half the city’s daily water needs.

The leaks were first discovered in the 1990s—there’s one beneath the Hudson River near Newburgh and another in the Ulster County town of Wawarsing. So the repair job is decades in the making and required a lot of planning. In order to repair the leaks, portions of one tunnel that connects the reservoir system to the rest of New York City’s water system will be shut down. 

The water diversions to the city will stop during the shutdown. Tharp said typical diversions are on average 500 to 600 million gallons per day from Pepacton, Neversink and Cannonsville. This additional water will build up in the reservoirs unless released.

And if there’s a flood?

Once the aqueduct is closed for the five-to eight-month shutdown, it is very difficult to re-open it quickly.

Making sure the repair is done right is all the more important. NORDEL and Jeff Skelding, the executive director of Friends of the Upper Delaware River (FUDR) have requested NYC and the New York State Department of Environmental Conservation take measures to prevent flooding caused by the shutdown.

Skelding told the River Reporter that one of the suggestions was to implement temporary siphons, which can move water from higher to lower elevations, but that the NYCDEP’s data modeling didn’t show that siphons would be needed as a secondary backup for Pepacton, Neversink, and Cannonsville. 

The last 63-year historical data was used to determine that siphons were not needed. “Some weather experts are suggesting that modeling be done for the last 10 to 20 years rather than historical data to better see the influence of climate change on reservoir inflow rates,” Tharp said. 

“I would be a lot less anxious about flooding if temporary siphons had been placed at the Delaware reservoirs, especially Pepacton.”

The NYCDEP is drawing down the reservoirs to create a 30 percent void—or lower—in the reservoirs’ total capacity before October 1. Tharp said that all but Pepacton will reach this goal, and it is now evident that maximum releases from Pepacton should have begun earlier.

 Milgrim told the River Reporter that "all releases throughout the project will be made pursuant to the regulated Flexible Flow Management Program and that includes any additional higher-rate Discharge Mitigation Releases used to keep levels lower than usual until they need to be fully refilled."

“It’s too late” to do anything preventative, Skelding said.

“We must all now rely on the initial voids during the shutdown and the releases during the shutdown to keep us from catastrophic flooding,” Tharp said.

Skelding suggested sandbags for folks close to the river.

Paul Rush, deputy commissioner for the Bureau of Water Supply at the New York City DEP, has worked with stakeholders to address concerns. Tharp said they met a lot of NORDEL’s requests.

Rush has said that on Day 1 of the closure, the DEP will make aggressive and proactive releases from the Delaware reservoirs. Those will help maintain a significant void and prevent spilling until the reservoirs need to refill when the aqueduct reopens.

A successful outcome of this project is based primarily on weather conditions. Tharp said, “Mother Nature really has to get this right.”

It’s an anxious time for the NYCDEP; they are responsible for determining the preparations necessary to minimize the risk of flooding in the Delaware reservoirs as well as their responsibility to supply sufficient water to the city while the Delaware system is down.

Read related reporting here:

https://riverreporter.com/stories/another-aqueduct-amendment,119765?

https://riverreporter.com/stories/liveries-concerned-with-delaware-aqueduct-shutdown,96230?

https://riverreporter.com/stories/fears-shared-about-upper-delaware-flooding,93716?

https://riverreporter.com/stories/water-we-worried-about,53907

https://riverreporter.com/stories/test-shutdown-a-success,90167?

Editors note: This article has been updated as of  2:50 P.M., September 25, to add additional information from John Milgrim, Director, of External Affairs for  NYC Environmental Protection Bureau of Water Supply, and ammend "far fewer tools" to "few tools".