Water from the Pepacton, Neversink, and Cannonsville Reservoirs flows to the Rondout Reservoir by gravity via three tunnels, all together spanning 75 miles, and all significant infrastructure projects in themselves. The combined waters are then sent to the city in one of the world’s longest continuous underground tunnels, the Delaware Aqueduct, which bridges the span of 85 miles from the Rondout Reservoir, under the Hudson River, to Hillview Reservoir in Yonkers. 

A blue toned monotone illustration with white border and text. The drawing is well or tunnel sticking out from the ground, with a plume of smoke and explosion debris flying in the air from it. There is a mountain ridge in the background.
An artist’s rendering of the initial blast at Shaft 3 of the Delaware Aqueduct. This is a page from the program brochure for the ceremonies initiating the tunnel construction, featuring New York City Mayor Fiorello LaGuardia, which took place on March 24, 1937. Source: Ellenville Public Library & Museum / New York Heritage Digital Collections. Link.
A drawing of a cut-away view of a tunnel. there are tracks and an "agitator" car carrying a box-like structure. The contraption seems to lay down concrete as it travels the track and a screed and other platform follow behind it to smooth the concrete.
Delaware Aqueduct concreting. Inside the Delaware Aqueduct, mixed concrete traveled by train along a temporary steel bridge, then was chuted to the bottom of the tunnel where workers shoveled it into position. This is part of a drawing showing the method of placing tunnel concrete. October 15, 1940. Source: NYC Water on Flickr. Link.

“The Delaware Aqueduct, a circular, concrete-lined tunnel 13.5 to 19.5 in diameter, stretches 83.8 miles connecting the Rondout and Hillview Reservoirs. Built from 31 vertical shafts, the aqueduct is actually three distinct pressure tunnels driven through bedrock at depths ranging from 300 to 1,550 feet below the surface.”

— From Liquid Assets by Diane Galusha

A hand drawn map, mostly black with some color coding, showing the NYC water system including the Delaware/Catskill, and Croton systems, and the aqueducts that connect them to NYC. Also seen is the old Ridgewood watershed, no longer in use. Cut aways of the tunnels showing their shape and comparative size are labeled below the map.
Map of the New York City water supply system showing reservoirs and connecting aqueducts, 1938. Source: NYC Water on Flickr, courtesy DEP NYC via Places Journal. Link.

The Neversink, East Delaware, West Delaware Tunnels carry water to the Rondout Reservoir, where water from the Delaware watershed collects before entering the Delaware Aqueduct.. From there, the Delaware Aqueduct carries water southeast to reservoirs in the Croton region before arriving in New York City. The three sections of the Delaware Aqueduct are the Rondout-West Branch Tunnel, the West Branch-Kensico Tunnel, and the Kensico-Hill View Tunnel.

Geology is really important in planning tunnels, so the Board of Water Supply investigated and documented it carefully. This photograph of a waterfall in Ulster County was taken to show the black shale and sandstone beds of the Hudson River formation between two shafts of the Delaware Aqueduct. September 16, 1938. 

– NYC Water on Flickr

A black and white photo of a waterfall snaking down a rocky cliff
Waterfall in Ulster County. Source: NYC Water on Flickr. Link.
A yellow-toned monotone photo of a tunnel lined with wood planks and concrete. There is a rail track and pipes going down into the far distance. Scaffolding sits off to the side.
Black and white photograph of the Delaware Aqueduct tunnel interior during construction. Source: New York Heritage Digital Collection. Link.
A black and white photo of two men in hard hats peering down into a dark shaft that has a pole and and a contraption with three spokes with wheels attached to it.
Delaware Aqueduct core boring. West Branch-Kensico Tunnel. View showing parts of the Calyx core drilling equipment being assembled at the top of the 48 inch diameter core hole at the exploration site. A 22 foot 11 inch section of driving stem has been attached and is ready to be lowered. Note the three armed guide bracket equipped with rollers. These are attached at about 40 foot intervals. They are for the purpose of centering the driving stem and preventing its whipping when in operation. Copy of Ingersoll-Rand Company Photograph. December, 1938. (Caption by NYC Water) Source: NYC Water. Link.
A black and white photo of a man standing next to a column of rock that is taller than he is. It is about 4 ft wide.
Delaware Aqueduct core boring. West Branch-Kensico Tunnel. A worker poses with piece of the core obtained from the 48 inch Calyx drill boring at the exploration site. This 3 foot 8 3/4 inches diameter and 6 foot long specimen was obtained from the hole at Elevation -10, or 224 feet below the working platform. The rock at this elevation was sound Fordham gneiss and the weight of this piece of core is approximately 5 1/4 tons. Note the circular drill markings made by the steel shot on the face of the core. This photograph was taken for the contractor, the Seaboard Construction Corporation, by H. L. Jackson, photographer, Brewster, New York. December 14, 1938. (Caption by NYC Water) Source: NYC Water. Link.
A black and white rock tunnel with a metal tube that fits into the tunnel with just a couple of feet to spare. A man in a hard hat is looking at the tunnel, which is twice his size.
Inside the West Delaware Tunnel, “spiders” preserved the steel interlining’s roundness during transportation, installation, and placing of concrete in the gap along the rock surface, after which they were removed. 44 miles long and over 11 feet in diameter, this tunnel connects Cannonsville Reservoir to Rondout Reservoir, where water joins up to the Delaware Aqueduct and heads toward the City. December 1961. (Caption by NYC Water) Source: NYC Water on Flickr. Link.
A black and white photo from the bottom of an excavated site. Rock has been blasted and carved away to make a large passage leading into the entrance of a tunnel. There is a couple of trucks at the entrance to the tunnel.
Neversink Dam diversion tunnel. Using a Lorain 104 electric shovel to load muck into a Euclid dump truck. These workers were excavating a diversion tunnel to redirect the Neversink River during construction of Neversink Dam and Reservoir on February 27, 1942. The project was put on hold during WWII and the reservoir was placed into service in 1954. Its water travels to Rondout Reservoir before heading into the Delaware Aqueduct toward NYC. (Caption by NYC Water) Source: NYC Water on Flickr. Link.
A black and white photo of 6 men in hard hats sitting in a metal bucket that is staged on a crane platform. In the background is a hilly landscape.
Rondout-West Branch Tunnel Headframe. These men are about to be lowered 800 feet down a shaft to work on the Delaware Aqueduct in Dutchess County. June 7, 1938. (Caption by NYC Water) Source: NYC Water on Flickr. Link.

“In contrast to earlier aqueduct and reservoir projects, contractors did not maintain work camps for laborers, who were now able to drive to work and so lived or boarded independently in neighboring communities… Another difference between the Delaware Aqueduct and earlier projects was its speed.”

— From Liquid Assets by Diane Galusha

A black and white photo of a long steel tube with two wood sheds straddling it, as if the tube is a railway car going through a tunnel
A welding cradle in Grahamsville holding three 40-foot lengths of steel pipe conduit destined for the Neversink Tunnel. The two sheds protected the pipe joints as well as the welders, who stayed in one position while the pipe was rotated on rollers as welding progressed around the circumference. Steel bracing inside the pipe is visible on the left; this preserved the shape of the pipe during fabrication, transportation, and concreting. Water travels six miles in the Neversink Tunnel from Neversink Reservoir to Rondout Reservoir, where it mixes with water from the other two Delaware system reservoirs, Cannonsville and Pepacton, before heading south towards NYC via the 85-mile-long Delaware Aqueduct. September 10, 1952. (Caption by NYC Water) Source: NYC Water on Flickr. Link.

The Board of Water Supply intended to embark on the construction of the Delaware Aqueduct by 1932, but lost significant funding for the project during the height of the Great Depression. The enormous impact on the City’s finances halted significant work on the project until 1936. Just five years later, there was a new national crisis that halted progress on the waterworks.

“Most of the aqueduct was driven, and much of it concrete, by 1941. But progress on the Rondout Reservoir, and completion of the aqueduct, slowed considerably with the approach of U.S. participation in [WWII]… Users of essential construction materials were given preference ratings according to their relative importance to defense, public safety, and the country’s welfare… The [Board of Water Supply] suspended much of its bidding and construction activity for the duration of the war.”

— From Liquid Assets by Diane Galusha.

A black and white photo of four men in hats in a low-ceilinged room gathered around a horizontal wheel that they all have their hands on and are turning.
Mayor Fiorello LaGuardia (third from left) and other officials opening tunnel sluice gates of the Delaware Aqueduct, 1944. Source: NYC Water on Flickr, courtesy DEP NYC via Places Journal. Link.
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