The weirs along the Old Croton Aqueduct were small structures that were built right over or next to the aqueduct, allowing the weir tender to have direct access to the aqueduct tunnel. The weirs had secondary tunnels that could direct water from the main aqueduct channel into nearby streams if the water flow was too high. The weir tenders could control the aqueduct’s water in three ways: 1. Altering the heights of boards to control the aqueduct’s water depth; 2. Dropping stop planks in the aqueduct to completely block its flow; and 3. Opening screw gates to get rid of water in one section of the aqueduct.
Like several other structures that are part of the Old Croton Aqueduct, the weirs are built in an Egyptian Revival style, chosen for its associations with permanence and because its forms could be easily created by stonemasons. All of the weirs have pilasters along their corners, belt coursing, and a projecting cornice. The weir has a single metal-door entrance, no windows, and a ventilating hole in its brick-arched roof. The New Ossining Weir is twenty feet long, ten feet wide, and thirty feet high, including the portion of the weir that is built underground for the waste-water conduit.
The aqueduct’s weirs serve as some of the larger visible elements of the system along its path. While six weirs had been built along the Old Croton Aqueduct when it was first constructed in the late 1830s and early 1840s, it was decided to increase the functionality of four of these weirs in the 1880s, making it possible not only to divert water and control its depth, but to stop it completely at the site of the weirs. This would allow the aqueduct to be shut off in sections, making its maintenance much easier and faster. It seems that the Ossining New Weir was the only one of these later weirs that was an entirely new construction, rather than being a mere alteration to a preexisting weir, as happened in Tarrytown, Yonkers, and Kingsbridge. A new building was required in Ossining because a waste-weir pipe nearly two thousand feet long would have needed to be built if they altered the already-existing weir in Ossining.
The interior is opened only for guided tours. Please see below for upcoming walks and tours of the weir.