Both the Dillon Reservoir and the Green Mountain Reservoir are expected to reach capacity this summer, Colorado Division of Water Resources division engineer James Heath said at the State of the River in Silverthorne on Thursday, May 22.

The Dillon Reservoir, Denver Water’s largest reservoir, supplies water to Denver and the Front Range through the Roberts Tunnel, which crosses beneath the Continental Divide. The Green Mountain Reservoir, located north of Silverthorne, includes a hydroelectric power plant and delivers water to Colorado’s Western Slope.

An about-normal snowpack in Summit County this winter means both reservoirs are expected to “fill and potentially spill,” Heath said. While the snowpack levels were close to normal, the runoff has been slightly below normal because the county went into last winter with dry soils, he said.

Summit County was one of the few places in the state where the snowpack was at- or above-normal, Heath said. He noted that further to the west of Summit County, the snowpack peaked between about 50% to 70% of the 30-year-median, and was even worse in places further south, where the snowpack has “probably already melted out.”

The snowpack in the Colorado River Headwaters Basin peaked April 7, about a week earlier than normal, Heath said. At 89% of the 30-year-median, the peak snowpack was “not bad but not good,” he said.

The Blue River Basin — a subbasin of the Colorado River Headwaters Basin — fared “a little bit better,” peaking April 8, at 108% of the 30-year-median, Heath said. The peak was also about a week earlier than the median peak, he said.

The Dillon Reservoir is expected to fill around the Fourth of July, Heath said. As of Friday, May 23, the Dillon Reservoir was about 86% full, according to Denver Water. But Heath said that the reservoir levels could rise quickly “with the warm conditions that we’ve got the last couple days and over the next couple weeks.”

Denver Water manager of water supply Nathan Elder said Friday, May 23, that the Dillon Reservoir expects to have an average inflow of runoff this year and slightly lower-than-average outflow.

The Dillon Reservoir should reach an elevation of 9,012 feet by June 18, allowing both the Dillon and Frisco marinas to be fully operational by that time. Outflows from the Dillon Dam should exceed 500 cubic feet per second — the level ideal for rafting the Blue River — around the third week in June and continue until around the Fourth of July weekend, he said.

Diversions from the Dillon Reservoir to the Denver metro area could be “a little above normal” this year due to conditions in the South Platte Basin, Elder said. But he noted that, “It’s pretty early in the season, so there is a lot of uncertainty in what will happen this summer. So it’s always subject to change.”

The Green Mountain Reservoir is also anticipated to fill this summer with dam releases ramping up to generate power at the power plant as the reservoir nears capacity in July, Heath said. Then, from late July through October, releases from the Green Mountain Reservoir are expected to increase as the demand for water in the Grand Valley increases following the runoff season, he said.

Heath noted that forecasts call for precipitation within the next two weeks. With the potential for precipitation and warm weather, he said “we’ll start seeing stream flows increase and a lot of snow melt.”

Looking ahead to the rest of the summer, Heath noted that forecasters are “starting to see a little bit of a monsoon signal.” Since Colorado relies on the snowmelt to fill up reservoirs in the spring, a strong monsoon season can help reduce fire danger and keep streams flowing later in the season as the runoff dries up.

“So that is a good sign,” Heath said. “Forecasters are predicting out into the future that we might have some monsoon conditions, which might elevate some of the low-flow conditions later in the season.”