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A 147-foot high concrete gravity arch dam nestled between two rock walls is an awe-inspiring sight. Just ask anyone who’s driven up Eleven Mile Canyon.

At the end of a winding, dirt road along the South Platte River in Park County, this impressive structure plays a vital role in supplying water to more than 1 million Denver Water customers.

May 31 is National Dam Safety Awareness Day, designated to build awareness of the thousands of dams across the U.S. that, like Eleven Mile Canyon Dam, do their jobs and go basically unnoticed every day.

The day commemorates the worst dam failure in U.S. history. On May 31, 1889, the South Fork Dam in Johnstown, Pennsylvania, broke after days of heavy rain. The floodwaters killed more 2,200 people in the valley below.

Denver Water uses dams to store drinking water, but the dams also provide boating and fishing opportunities for the public and produce hydropower. Other dams across the country are used for flood control.

Denver Water has 20 dams, and some are more than 100 years old. The organization conducts its own internal dam evaluations and also takes part in state and federal inspections.

Built in 1932, Eleven Mile had its annual state and internal inspection on May 24. “The inspections are necessary to make sure the dam is safe and functioning properly,” said Darren Brinker, Denver Water’s chief dam safety engineer. “We look for any mechanical issues, cracks in the concrete, and changes in seepage over time.”

John Hunyadi, a dam safety engineer with the Colorado Division of Water Resources, said the state inspects dams to make sure people and property downstream are safe. “We go out with dam owners to inspect their facilities and identify problems before they become major issues,” he said.

Hunyadi said 2013 and 2015 proved to be major tests of dams along the Front Range. “We saw near historic levels of high water and flooding and the dams where we have the most stringent safety requirements performed exceedingly well,” he said.

Denver Water’s on-site reservoir managers look for problems every day. “We do maintenance at the dam and look at past work we’ve done to see how it’s holding up and whether something needs immediate attention,” said Mike Kelly, Eleven Mile Dam caretaker. “I’ve been here for 21 years and this dam is holding up well considering it’s 84 years old.”

Hunyadi found no serious problems during his inspection in May and gave Eleven Mile a standard “satisfactory” rating.

Two of Denver Water’s oldest earthen dams (Antero, 1909 and Marston, 1902) are currently undergoing safety upgrades. “Overall, our dams are in good health,” Brinker said. “Concrete dams are expected to last 200 years and with proper maintenance, our goal is to make them last even longer.”