Denver Water and the DWR reach agreement for Dillon Reservoir to mitigate flood risk along the Blue River should the need arise

Denver Water employees Rick Geise and Nate Hurlbut assisted in setting the plug, which helps prevent chunks of ice and snow from falling into the spillway. Photo credit: Denver Water

From the Summit Daily News (Joe Moylan):

The Colorado Division of Water Resources recently signed off on a first-of-its-kind proposal that could significantly reduce the risk of catastrophic flooding events in Summit County.

The plan, proposed by Bob Steger, manager of Raw Water Supply for Denver Water, would allow the state’s largest water utility to divert excess flows from Lake Dillon to the Front Range by way of the Roberts Tunnel in order to prevent a destructive water event in Summit County, most notably in Silverthorne.

Summit County Emergency Management director Joel Cochran said earlier this month during a Summit County Commission workshop that record snowpack combined with unseasonably warm spring and early summer temperatures could cause flooding on a magnitude not seen in two decades in the Blue River Watershed.

According to data Cochran presented during the commission’s first meeting in May, this season’s total snowpack consists of the equivalent of 17 to 20 inches of rainwater. It’s the highest concentration of snowpack in Summit County since 1995, the last year there was significant flooding in Summit County, Cochran said.

In addition to record snowpack, Cochran said spring and early summer temperatures are hovering between 6 and 10 degrees above normal throughout the state. Although Summit County last week caught a break from unseasonably warm temperatures, the return of spring has local officials concerned that the runoff could be triggered earlier than usual.

Historically, runoff in Summit County begins the first week of June, peaks about the middle of the month and ends before early July, Cochran said.

However, floods aren’t triggered by mountain runoff or even an accelerated runoff, Cochran said.

“A lot of people remember 2011 when we lost Coyne Valley (Road), but you can’t have (extreme) flooding due solely to spring runoff,” Cochran said. “We lost Coyne Valley because we had a major rain event when the Blue River was at peak water.”

With this season’s snowpack, it’s almost a certainty the Blue River will reach its capacity of 1,800 cubic feet per second of water at some point in the coming weeks, said Summit County assistant manager Thad Noll. If Summit County receives a significant rain event while the Blue is peaking, the damage could be extensive all over the county, but particularly in Silverthorne.

“Silverthorne got by relatively unscathed once in the past when the Blue reached 2,100 cfs, but anything higher than that and we’re trying to keep Silverthorne from getting washed down to the Sea of Cortez,” Noll said. “Denver Water’s proposal would relieve that pressure on the Blue by sending excess water to Denver in the event of a flood.”

That water would be transported by way of an underground aqueduct known as the Roberts Tunnel, which was constructed to carry water more than 23 miles from Lake Dillon to the North Fork of the South Platte River, where it is then distributed to several other reservoirs in and around Denver. Each year, water from the Blue River and Lake Dillon accounts for about 40 percent of the water annually collected and stored on the Front Range.

The South Platte’s capacity is about 680 cfs, according to a letter by Steger, which means up to that much water could be sent through the tunnel to the Front Range. Depending on South Platte flows, the water diverted downtown could relieve a significant amount of strain on the Blue River should it reach critical mass.

However, prior to receiving approval, Noll said the idea sparked an interesting debate among West Slope water advocates who opposed the proposal. Although Lake Dillon is owned and operated by Denver Water, it was previously prohibited from sending water to the South Platte if Front Range reservoirs were full.

Opponents were particularly critical of the idea to divert water to Denver considering Front Range reservoirs are expected to reach capacity this year.

“It raises an interesting question because the Blue River’s natural flow is toward the Colorado River,” Noll said. “The debate was whether saving the tiny town of Silverthorne, Colorado supersedes the rights of stakeholders down the line.”

The Colorado Office of the State Engineer thinks that it does, so long as Denver Water doesn’t cause flooding on the Front Range in trying to prevent the same in Summit County.

More Blue River watershed coverage here.

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