MARLBORO, N.Y. — New York City is starting to dig a new water tunnel 600 feet (183 meters) under the Hudson River as it repairs its aging upstate supply system.

The city’s Department of Environmental Protection is starting work Friday on a 2.5-mile (4-kilometer) bypass tunnel under the river near Newburgh. The tunnel is part of a project to fix leaks in the 85-mile-long (137-kilometer-long) Delaware Aqueduct, which helps deliver 500 million gallons (1,893 million liters) of water a day from four reservoirs in the Catskill Mountains.

The bypass under the Hudson will replace the leakiest portion of the tunnel. Crews are using a 470-foot (143-meter) long tunnel boring machine that weighs 2.7 million pounds (1.2 million kilograms).

Crews have spent years digging two vertical shafts on either side of the river about 60 miles (96 kilometers) north of the city.