Denver Water officials will close Dillon Dam Road to public car traffic as of midnight Tuesday, July 8, out of concern that the dam might be "targeted."

Denver Water is the state's largest water provider, serving more than 1 million customers in Denver and some surrounding suburbs.

The utility said the dam's public recreation trail will remain open to bicycles and pedestrians, and the utility also will work with local officials to allow emergency vehicles to access the road.

"We have decided to close the road as a proactive measure to maintain public safety," said Penfield Tate, first vice president of the utility's board of commissioners, in a statement. "We are not aware of an imminent threat, but in the last several months we've grown far more concerned about the vulnerability of the dam and the potential for catastrophic consequences downstream if the dam were targeted."

Denver Water owns and operates the Summit County reservoir, which is the primary source for Denver's drinking water supply. Dillon is the utility's largest and most accessible reservoir.

Since Sept. 11, 2001, the security of water supplies and facilities has been the subject of increasing scrutiny, Denver Water said in a statement.

The utility decided to close the road to public access after conducting several vulnerability assessments and consulting with federal and state agencies. The findings indicate the need to close the dam road to public vehicular traffic to help ensure the safety of those who live and work below the dam, the utility said.

The one-mile public-access road crosses the top of the dam from Dillon to Frisco. Dillon Reservoir holds 254,000 acre-feet of water, is of earth-fill construction and is immediately upstream of a large retail area, a recreation facility, Interstate 70 and many homes.

If the dam were compromised, thousands of lives would be affected, the utility said.

"Good sense dictates closure of the road," Tate said in the statement.

Some public access roads across dams in other parts of the country -- New York and California, for example -- also have been closed due to vulnerability concerns, the utility said.

"We understand that the closure will inconvenience motorists in Summit County," Tate said. "However, we have concluded that the grave danger presented by a dam failure outweighs the disruption to travelers."

In the last seven years, Denver Water has spent more than $10 million to enhance the safety and security of its facilities. At Dillon, changes include fencing, barriers, camera surveillance and 24-hour security guards.

"Having exhausted other options and still being uncomfortable with the level of risk, we consider road closure the action necessary to best protect the residents of Summit County as well as the water supply," Tate said. "The best long-term solution to this issue may be an alternative road alignment that connects one side of the reservoir to the other without traversing the dam. This type of solution would require a cooperative effort among many different parties."

Denver Water said it's hired engineering firm HDR Inc. to consider potential alternative alignments, and preliminary meetings with affected jurisdictions have begun. HDR is based in Omaha, and has offices in Denver, Colorado Springs and Fort Collins.