OLIVE, N.Y. (AP) — A century-old reservoir in the Catskill Mountains that provides New York City with about 40 percent of its drinking water will get $750 million in upgrades to its dam, dikes, bridge and buildings, city officials announced Thursday.

The Ashokan Reservoir is a crucial piece of the sprawling but aging system that supplies water to 8.5 million city residents plus another million people north of the city. The series of reservoirs and gravity-fed tunnels delivers more than 1 billion gallons a day and is considered an engineering marvel.

Vincent Sapienza, acting commissioner for the New York Department of Environmental Protection, said the upgrades to the scenic reservoir located about 90 miles (145 kilometers) north of the city will help keep the infrastructure in good repair for generations.

Crews flooded out entire communities to create the reservoir in the early 20th century, which began supplying water to all five boroughs in 1917.

The Ashokan Century Program will stretch over 10 years with construction expected to begin in 2023. Work will be done on about 5.5 miles of dikes and a dam, as well as a spillway and a bridge across the reservoir.

City officials say it will be the largest public works project in the lightly populated area in more than 50 years.

"The Ashokan Century Program heeds the advice of those who built the water supply, and it makes good on our mission to protect public health and safety long into the future," Sapienza said.