This is the massive new tunnel that will replace the old leaking tunnel that supplies NYC water through the Delaware Aqueduct
This is the massive new tunnel that will replace the old leaking tunnel that supplies NYC water through the Delaware Aqueduct. PHOTO PROVIDED

NYC water tunnel shut down to repair Delaware Aqueduct
Flood concerns incorporated into plan

| March 25, 2022

With two workers dwarfed by the tunnel for the Delaware Aqueduct, you can get some sense of its size. PHOTO PROVIDED

New York City’s Department of Environmental Protection is preparing for a high-wire act to balance the city’s need for water (lots of water); the need to repair a vital tunnel that fulfills that need and the concern of up river communities about the risk of flooding.

Soon this 2.5 mile tunnel — part of the 85-mile Delaware Aqueduct —  will be shut down to repair a  20-year-old significant leak under the Hudson River. 

The New York City DEP and its Bureau of Water Supply know the value of clean fresh water and the leak — estimated to be about 20 million gallons per day — is not water they want to lose, especially as it’s been working for many years on conserving water throughout the system.

There’s a lot of water that travels through this tunnel. The Delaware System (Cannonsville, Pepacton and Neversink reservoirs) supplies somewhere between 50% and 60% of New York City’s drinking water.

That water usually drains off of the three reservoirs through separate tunnels and into the Rondout Reservoir and then via this aqueduct to the West Branch Reservoir in the Croton System north of New York City, then it goes to Kensico Reservoir before the treatment process begins, eventually arriving at Hillview Reservoir.

So if that much water is always draining off the reservoirs, and that gets shut off when the new tunnel is being fitted to the old, which will take from five to eight months, where’s all that water going to go?

And how is New York City going to get its drinking water?

Those are big questions. It’s not surprising that it took 20 years to get to this finish line. It’s the largest and most complex repair in the 180-year history of NYC’s municipal water supply, at a cost of $1 billion.

New York City’s plans were the topic of a presentation that Jennifer Garigliano, chief of staff with the Bureau of Water Supply, gave to the Regulated Flow Advisory Committee of the Delaware River Basin Commission on Wednesday. The RFAC is charged with advising the Commission about regulated flows and potential changes to the Water Code; providing a forum for public discussion; disseminating accurate scientific information and increasing understanding of operational and legal constraints and opportunities.

Shutting down the Delaware Aqueduct from October 2022 for five to eight months may affect how much water is in the river even though part of the plan is to minimize those effects. 

The water in the river (and the branches below the reservoirs) will be running higher and faster than usual through the summer months as the shutdown draws near, to allow those reservoirs to empty, though they won’t actually empty.

Here’s a slide that Garigliano presented at the meeting showing the likely levels in the three reservoirs affected. At their lowest, Pepacton could draw down to about 74% (that’s the measure of “void” or emptiness); Cannonsville to about 60% and Neversink to about 70%.

2020 update

Slide presented at Regulated Flow Advisory Committee meeting 3/23/22 by NYCDEP show planned levels of the three affected reservoirs.

That drawdown is to reduce the likelihood they will get full and pass water through spillways, which could cause flooding in the branches just below the dams for each reservoir and possibly in the upper main stem Delaware. During these shut down months, the reservoirs will still get water from rain and from tributary streams, so how much water to leave in was a big part of the planning.

In an email after the meeting Garigliano wrote: “It really depends on the hydrologic conditions at the time, and while I have state of the art tools in helping me to predict the hydrologic conditions, they are still not a crystal ball and there’s still a chance those forecasts can be wrong and NYCDEP will adjust its operations to the best of its ability to maintain the CSSO voids at a minimum. ”   

CSSO stands for Combined Seasonal Storage Objectives, which is the gold standard for how the reservoirs operate — aiming for a 15% void. 

Garigliano pointed out that “the reservoirs are not a flood control system.” Their primary purpose is to supply drinking water to New York City.

During the shutdown, water from the Catskill and Croton systems will be used exclusively, allowing the Delaware reservoirs to refill.

As Garigliano put it, when the Delaware system is offline, “All we have left is the Catskill and Croton systems, and they’ll be drawn down to almost zero.”

  • Those systems, older than the Delaware system, have been getting some extra attention so that they will be able to take up the slack and provide all the water New York City needs while the Delaware Aqueduct is offline. 

    Then, the Delaware reservoirs will refill as the shutdown proceeds so they are ready to be put back into service when the final connections are made.

    So there’s a balance sought between how full to keep the Delaware reservoirs — so they can take over as soon as the shutdown is over and how much they should be emptied to mitigate any chance of flooding.

    Another presenter on this topic was Garth Pettinger, from Trout Unlimited, who has spent time analyzing New York City’s releases and suggesting ways that those releases might improve the cold-water habitat needed for trout.

    This time, he never mentioned fish. He was concerned about the risk from flooding. He insisted that all the affected reservoirs (including the Rondout) should be kept at a 15% void. Garigliano did say that the 15% void was the goal of the Bureau of Water Supply

He examined the “inflow” to the reservoirs over the past 10 years and predicted that there was a likelihood that each reservoir would max out at times and that would lead to spillage at different times. Essentially, the water coming into the reservoirs would exceed the ability of the reservoirs’ release capacity.

Here’s how he put it (from his slides):

“From the point of view of residents and communities below the dams, when a storm is forecast, the first thing they do is look at the levels in the reservoirs.

If the reservoirs are full, they consider moving everything out of their basements to the first or second floor; and for some, potentially abandoning their home for a couple of days: not to mention the concerns of Town Supervisors.

However, If there’s a 15% void in the reservoirs, which would provide at least a 3-4 day attenuation of spilling from a potential storm surge; they can breathe a sigh of relief, as the likelihood of imminent flooding would be greatly reduced.”

Before his presentation, Garigliano had recognized that flooding was a concern expressed by people living “below” the dams — an area also known as the tailwaters — but insisted that those concerns had been taken into consideration in the development of the plan.

The New York State Environmental Quality Review Act demanded that the DEP develop an environmental impact statement and hold a public review process for the project. It was completed in December 2017, and Garigliano noted that the document is available to read on its website:

She suggested reading Chapter 10 to find all the details of the process.

During a relatively short Q and A after the presentations, Jeff Skelding, executive director of the Friends of the Upper Delaware, wearing another hat — the Tailwaters Coalition — asked what was being done by NYCDEP/Bureau of Water Supply to give all the supervisors of towns below the dams the information they need to reassure their communities and maybe also, to prepare them if the worst happens.

“They don’t know this is coming,” he said.

Garigliano said that a communication plan was in the making but said she wasn’t the final decision maker on the plan, which isn’t ready to share yet.

Also in the Q and A, Karl Schmidt asked Pettinger what allowance did he make in his review of the floods that happened in 2005/6 and 2011.

Pettinger said that he deliberately did not include those, rather he used data from the past 10 years.

There are strong opinions held by some throughout the Delaware Basin that the reservoirs should have been used to mitigate those flooding events, but studies have shown that they largely attenuate the heavy flow of water simply by their size and in almost all cases increasing the void would not have helped that flooding. That was the conclusion reached in a 2009 study jointly produced by the U.S. Army Corp of Engineers; the National Oceanic and Atmospheric Administration; the National Weather Service; the United States Geological Survey and the Delaware River Basin Commission. More here:

Pettinger said at the beginning of his presentation that “everyone benefits from a successful conclusion to the project,” acknowledging that the work needs to be done.

Garigliano offered her email, especially for people who were waiting to ask questions at the end of the session but who did not get a chance to speak:

Meg McGuire

Meg McGuire

Meg McGuire has been a journalist for 30 years in New York and Connecticut. She started in weekly newspapers and moved to full-time work in dailies 25 years ago. She knows about the tectonic changes in journalism firsthand, having been part of what was euphemistically called a "reduction in force" six years ago. Now she's working to find new ways to "do" the news as an independent online publisher of news about the Delaware River, its watershed and its people.


  1. Jennifer Canfield on March 8, 2023 at 2:25 pm

    Thanks for a greatly informative article. It really brought me up to speed again as I had forgotten so much over the last year due to health issues. Would you know of any updates to the proposed project. I plan to help my local community stay informed with regard to flood safety and mitigation. My farm is on the main stem of the Delaware River.
    Jennifer Canfield
    Damascus, PA

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