A little-known police agency is the first line of defense in protecting the city from any terrorist plot to poison the water or blow apart the infrastructure that brings it here from upstate.

Equipped with a nine-boat fleet, a helicopter, an airplane, a Scuba unit, a heavily armed Emergency Services Unit, bomb-sniffing dogs, ATVs, motorcycles, and bicycles, the city’s Department of Environmental Protection police force, which has tripled in size since the September 11, 2001, terror attacks, guards the rural watersheds.

“Since September 11,there has been no higher priority for the DEP than water-supply protection,” said Chief Ed Welch, a retired city police officer who heads the DEP police force, which stands at 200 officers strong, up from 60 three years ago.

The officers patrol the 19 dams and 23 reservoirs that make up the vast watershed that serves 8 million city residents and a million people in Westchester County.

“In a 2,000-mile watershed, 200 officers don’t go a long way, so we try to be as smart about it as we can,” Chief Welch said.

The watershed is divided into the Delaware system, consisting of four reservoirs totaling 300.4 billion gallons, and the Catskill system, with two reservoirs totaling 140.5 billion gallons, according to a DEP spokesman, Ian Michaels. These systems flow into Kensico, formerly a village but now a dammed reservoir of 30 billion gallons near the town of Mount Pleasant.

These systems account for 90% of the water supply. The older and more locally situated Croton system consists of 12 reservoirs totaling 88.6 billion gallons and accounts for 10% of the water supply. The water supply totals about 580 billion gallons, meeting the city’s average consumption of 1.1 billion gallons a day.

In addition to ramping up its police force, the city’s watershed infrastructure has undergone $70 million worth of security improvements since the terrorist attacks, based on the results of a study by the U.S. Army Corps of Engineers, said DEP spokesman Ian Michaels.

“We did virtually everything they suggested, and we made some additional improvements to the police force,” Mr. Michaels said.

Chief Welch said the security improvements include closing down Westlake Road over Kensico Dam to motor vehicle traffic and installing a guard post at each end with armed police, vehicle-blocking lift gates, and security cameras with close-circuit television and video recorders. New buildings are being put up, along with solar-powered telephones that will let joggers and bicyclists report suspicious activity.

“Without the community involved, you’re missing 90% of the information you need to protect the assets,” Chief Welch said.

Security cameras, motion detectors, and enhanced lighting are being implemented at dams and gatehouses throughout the watershed, with extra attention being paid to facilities close to the city.

Chief Welch said the police force has recently added a Emergency Services Unit dubbed Triton, the DEP’s aquatic counterpart to the NYPD’s Hercules unit. Triton includes a Scuba unit with divers conducting underwater inspections of the dams, as well as the Strategic Patrol: a unit that combs the watershed’s woodlands with off-road bicycles and ATVs.

The DEP police force has also added a canine unit with five bomb-sniffing dogs to sweep the dams. A bomb squad from a separate agency would be called in to defuse explosives, though none have been discovered yet.

Chief Welch said his police force coordinates with the NYPD, state police, and Westchester Country law enforcement in conducting patrols and investigations. In addition to its anti-terrorism security efforts, the DEP police are trained in the laws of environmental protection and know how to recognize faulty sewer systems and petroleum leaks.

Chief Welch said the primary lesson of the September 11 attacks is that in order to beat the terrorists, you have to think like them.

“I wouldn’t want to err on the side of lack of imagination,” he said. “We have to assume that they know a lot more than we give them credit for.”

A recent study by the Environmental Protection Agency of water supply vulnerability nationwide identified a wide range of potential adversaries, from untrained vandals armed with hand tools, firearms, and commercially available chemicals, to international terrorist organizations staffed by skilled individuals with tools, pumps, firearms, explosives, chemical and biological warfare agents, and possible insider collusion.

New Yorkers often assume that terrorists targeting the water supply would attempt to contaminate it with deadly toxins, but the threat of contamination is lessened by the sheer volume of the watershed.

“We have so much water, that you need a significant amount, and I don’t know what that amount would be, to impact the water quality,” Chief Welch said. “The biggest concern is threat to the infrastructure.”

Chief Welch declined to discuss in any detail the workings and vulnerabilities of the gatehouses, which regulate the flow of water, and dams, but he said the system is designed to continue functioning in the event of an attack,

Standing on the 1,358-foot long Kensico Dam, Chief Welch described the structure as “formidable,” and it’s easy to see why. Completed in 1917, the dam was fashioned from solid stone blocks, some weighing up to 10 tons.

On the wet side of the dam is 30 billion gallons of water, serving as the city’s immediate and most accessible supply. On the dry side is the city of White Plains and surrounding communities, with at least 200,000 residents.

Westchester County Executive Andrew Spano wants to have Westlake Road reopened to motor vehicle traffic during rush hour, contending than its closure unnecessarily inconveniences the thousands of commuters who used to drive it every day and blocks access to emergency vehicles.

Mr. Spano’s intentions have raised the ire of New York City politicians, who plan to hold hearings in September to discuss the security of the Kensico Dam as well as the overall security of the water supply.

“Security needs to be the no. 1 priority,” said James Gennaro, a Democrat of Fresh Meadows who is chairman of the Environmental Protection Committee. He wants Mr. Spano to reconsider his efforts. “Guarding this road from terrorist attack will not only protect the water supply of 9 million New Yorkers, but it will also protect the quarter-million people living below the dam from a devastating flood,” he said.

The chief adviser to Mr. Spano, Susan Tolchin, said that the DEP has not proven that the dam is effectively protected and has not justified the need for closing the road.

“We’re just as concerned about the security of the dam,” said Ms. Tolchin. “We don’t want anything to happen to that dam and we’re going to do whatever we need to do in order to protect it.”