Dillon Reservoir’s frigid waters and the toll it takes on the human body are at the forefront of a discussion on whether to allow swimming in the high alpine man-made lake.

In February, Denver Water lifted its ban on human contact with the water, saying concerns dating from the 1980s of water treatment capabilities had been resolved.

Denver Water officials didn’t expand on what prevented human contact with the city’s largest singular water storage facility, but long-time (more than three decades) Dillon Marina manager Bob Evans speculated that it had to do with viruses, which are minuscule and tougher than bacteria to treat. Oil from boats is easy to separate, Evans said, and fuel evaporates before it becomes much of a problem.

Now that human contact with the water has officially been deemed harmless, the Dillon Reservoir Recreation Area Committee – a group comprised of representatives from all vested agencies and organizations – needs to decide whether swimming will be allowed. A few years ago, stand up paddleboarding was permitted, but outright swimming still isn’t allowed. A decision was supposed to be made by July, but committee members appear to be leaning toward a delayed decision.

Most of those interviewed for this story said Dillon Reservoir is too cold for them to consider a swimming hole. However, none said it shouldn’t be allowed, many said there should be designated swimming areas, and some felt it could be an attraction.

According to Doug Silver, a private researcher who teamed with Colorado State University to study Dillon Reservoir, surface water temperatures are currently about 46 to 48 degrees. In the hottest part of August last year, he measured surface water temperatures at 63 degrees.

Evans is a tough swimmer. His parents tell him he wanted to be in the water as soon as he could walk. He is even willing to soak himself in water that’s about 50 degrees just to toughen himself up.

But, “I’ve been in 60-degree water in the ocean, and I was probably the only one in from the beach,” he said. The point being that his tolerance for cold water isn’t common.

Evans was able to sit down in the 50-degree water for about six minutes until he couldn’t stand it anymore.

When humans are exposed to extreme cold, the extremities – feet, legs, hands, arms – shut down and stop accepting blood, so the core can stay warm and keep the heart beating and the brain working. It makes it difficult to swim, so the best survival reaction is to float and wait for rescue.

It’s called the “mammalian dive reflex,” Evans said. It’s not a strong reflex in humans, but it is there, and it’s triggered by the face hitting water colder than 70 degrees. Immediately, the heart rate slows 10 to 25 percent, requiring less bloodstream oxygen, which is also a reason the extremities shut down.

Silver also measures temperatures in the water column to see how temperatures change at depth. He said at 15 feet below the surface, the water temperature is sub-40 degrees. Those temperature changes make him wary of any activities that involve depth – particularly moving from warm air temperatures to frigid water and down into even colder water. Installing a dive platform, for instance, might not be a great idea for Dillon.

“We are really sensitive to changes in temperature,” he said.

Summit County Sheriff John Minor isn’t planning to go in the water anytime soon, but he’s not opposed to the recreation committee allowing swimming. The policy decision is separate from the purview of the Sheriff’s Office, although the sheriff is in charge of policing the reservoir and making rescues as necessary.

In part, Minor isn’t keen on over-regulation, but he also sees Dillon Reservoir as an untapped resource for certain tourist and money-making activities. For example, a triathlon could be a draw, Minor said, and he looks forward to seeing that happen. The Town of Frisco wanted to hold a triathlon this summer, but turned it into a bike-run duathlon instead as they waited for the decision on swimming in the reservoir.

“As far as response goes, we’re not opposed to water contact,” Minor said. “We’ll have to see, once it becomes the norm, what the issues are.”

The reservoir’s winter ice layer typically melts off in May, just in time for the three-and-a-half months of summer in the High Country. That’s not much time for the water to warm up.

He added that, with stand up paddleboard and boating in general, people are falling or jumping into the water. It’s difficult to enforce the rules as they currently exist, Minor said. Increased patrol manpower and associated costs will happen, he speculated, but any measurements on those won’t come until after a decision is made and an informational year passes.

“I’m not here to regulate common sense. That’s not what we do. … Use your common sense. If you think it’s cold, put on a wetsuit,” he said. “We’re not here to monitor your idiotic behavior unless your idiotic behavior crosses into the criminal realm.”

The most common swimmers in Lake Dillon are dogs.

But even some of those furry friends will limit how cold they’re willing to go.

Evans’ German shepherd wouldn’t wade about three weeks ago. Now, she’s swimming.

“Anything below 50 degrees, it takes a lot of coaxing to get her in,” Evans said.

People who visit Summit County from around the world are typically used to swimming in 75- to 80-degree water, which makes him wary of allowing swimming without plenty of education, in and designated areas. Evans is also concerned about the winds on the reservoir. If a boater decides to take a dip and his boat drifts away faster than he can swim, he’s in trouble.

“We need more facts,” Evans said. “I’d like to do more research into the cold water aspect of it. I’d like to know the temperature of Dillon in the middle of the summer. And the temperature down 6 feet. I think we need to look into it a little bit further and take our time.”