Photo courtesy of NYCDEP

The New York City Department of Environmental Protection (DEP) is set to begin essential repairs on Oct. 1 to the Delaware Aqueduct in the Hudson River Valley by temporarily shutting down the Rondout to West Branch Tunnel (RWBT) until June 2025.

The tunnel, located north of the city and running underneath the Hudson River, stretches from Newburgh to the town of Wappinger.

In preparation for the effort, which has been in the planning stages for more than 20 years, the DEP began modified pre-shutdown water supply management operations earlier this summer.

Beginning at the Rondout Reservoir in Ulster County in the Hudson River Valley, the 85-mi.-long Delaware Aqueduct is the longest tunnel in the world, with depths up to about 1,500 ft. It supplies roughly half of the city's roughly 3.1 billion gal. water supply every single day.

The aqueduct began flowing in the mid-1940s, but inspections in the early 1990s revealed something troubling: there were leaks in the tunnel that contributed to a daily loss of more than 30 million gal. of drinking water.

Further research revealed that the majority of the water loss was happening at a specific location of the tunnel underneath Newburgh, N.Y. A second leak was discovered in the nearby Ulster County town of Wawarsing.

To tackle the issue, in 2021, the DEP completed the massive $1 billion, 2.5-mi. bypass tunnel 600 ft. under the Hudson River just north of Newburgh to replace the most compromised segment of the aqueduct. The project was designed to skirt the affected areas and restore more efficient water delivery to New York City.

The new project beginning this fall will be devoted to making additional repairs to the RWBT, including fixing the smaller leak in Wawarsing. DEP noted on its aqueduct project page that the extensive process requires the complete shutdown and drainage of the Hudson River tunnel to integrate it with the new bypass section.

The planned eight-month shutdown was initially scheduled for 2023 but was postponed allowing for further testing and to mitigate impacts in the areas of the leaks.

Once the RWBT stops flowing, DEP anticipates it will remain offline for eight months. However, as with any major infrastructure project of this scope and magnitude, slight variations may lessen or slightly increase that timeframe.

In anticipation of the tunnel's closure, DEP began modifying water management strategies to increasingly rely on the northern and eastern Catskill and Croton Systems. According to the state agency, those adjustments will help control the water levels across New York City's 19 reservoirs, as well as accommodate seasonal usage patterns, and ensure adequate water supply while minimizing any risk of overflow or spills.

During the stoppage, fluctuations in reservoir water levels may differ from typical seasonal patterns, with some reservoirs east of the Hudson River likely experiencing notable temporary reductions in capacity.

New York City constructed the Delaware Aqueduct in the 1930s and 1940s to connect the Rondout Reservoir and the Delaware water supply system in the southern Catskill mountains to the city's five boroughs.

By June 2025, the system will move into its post-outage phase, which will include rebalancing the water supply system across the three water supplies.

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