Dry tunnels, overflowing reservoirs

Posted 4/11/22

UPPER DELAWARE RIVER — The approaching shutdown of the Delaware Aqueduct is an issue of clear importance to the New York City region.

The aqueduct takes 500 to 600 million gallons of water …

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Dry tunnels, overflowing reservoirs


UPPER DELAWARE RIVER — The approaching shutdown of the Delaware Aqueduct is an issue of clear importance to the New York City region.

The aqueduct takes 500 to 600 million gallons of water per day from the Rondout reservoir to communities in and around the city. It will be shut down on or around October 1 of this year for the repair of a pair of leaks—a larger leak in Newburgh and a smaller leak in the hamlet of Wawarsing. From five to eight months after the shutdown, the city will receive no water from that reservoir.

The decision has a less obvious but equally crucial impact on the communities of the Upper Delaware River. Jeff Skelding, the executive director of the the Friends of the Upper Delaware River, an environmental advocacy organization, appeared at a meeting of the Upper Delaware Council on April 7 to explain the impact that shutdown may have on those communities.

Of particular concern are the three reservoirs upriver that feed the Rondout reservoir: the Cannonville, Pepacton and Neversink reservoirs, known collectively (together with the Rondout) as the Delaware system. Because the Rondout reservoir will not send any water through the Delaware Aqueduct during the shutdown, the Delaware system of reservoirs will not send the water they typically do into the Rondout reservoir.

That backup could lead to problems with flooding along the Upper Delaware, said Skelding. There are four ways that water can get released from reservoirs: diversions to other reservoirs, releases into the Delaware River, spillage over the top of the dams and evapotranspiration (the process by which water evaporates and moves into the atmosphere). Take one of those options away, and more water will build up in the reservoir; if enough builds up, it could spill over the top of the reservoirs’ dams and cause flooding along the river.

The New York City Department of Environmental Protection (NYCDEP), the organization in charge of the Delaware aqueduct and its shutdown, has accounted for that possibility in its plans for the shutdown. Jennifer Garigliano, chief of staff for the NYCDEP Bureau of Water Supply, attended the UDC meeting along with Skelding to help explain the organization’s plans.

The NYCDEP plans to divert more water than usual out of the Delaware system of reservoirs ahead of the shutdown, said Garigliano. That will empty out the Delaware system to a certain extent—a planned 30 percent or more, depending on rainfall—and will fill up the other two systems that fed the city’s water supply, the Catskill and the Croton systems.

Once the Delaware Aqueduct shuts down, the NYCDEP will switch to drawing water out of the Catskill and the Croton systems. Then, when the aqueduct resumes operations, the NYCDEP will switch back to using the Delaware System, drawing excess water away.

Even with that plan in place, there were concerns that the shutdown could lead to flooding, especially if the region experiences a particularly wet year.

“We’re worried that below the dams… depending on weather, that we might see conditions that… will make folks nervous,” said Skelding. The river has seen flooding even when diversions are operating as normal, he said. And once the project gets underway, the NYCDEP won’t be able to reopen the Delaware aqueduct to respond to changing conditions.

The NYCDEP is trying to account for those factors in its planning, said Garigliano. It has a bailout plan if conditions look unfavorable leading to the shutdown. It took a look at the different percentiles of possibilities: the 10th percentile covers dry conditions, the 50th percentile covers normal conditions, and the 90th percentile covers wet conditions. And while the Delaware aqueduct is shut down, the NYCDEP will monitor the hydrologic conditions of the river to determine what level of releases to maintain.

The possibility of flooding is still there, however, even with the best modeling.

“It certainly is unsettling,” said Cochecton UDC representative Larry Richadrson, especially considering the extreme weather patterns that the region was seeing.

The unpredictability of the situation made communication between local groups and the NYCDEP crucial throughout the project. Garigliano was active in that effort—she hadn’t planned to appear at the meeting, but had chosen to of her own accord after seeing that it was taking place—and all parties at the meeting discussed ways of getting news about the shutdown out into the community.

Delaware Aqueduct, Upper Delaware River, Rondout reservoir, Delaware system, flooding, repairs, reservoirs, Delaware river


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