Dillon Reservoir water level dropping due to less rain, higher temperatures

Water storage in the Dillon Reservoir has started to drop due to less rain, higher temperatures and increased need for water on the Front Range.
Lindsey Toomer/Summit Daily News

Moving into the month of September, rainfall in Summit County is starting to slow down and temperatures have remained high, meaning the Dillon Reservoir is starting to look a little dryer.

The reservoir is currently 91% full after reaching full capacity earlier this summer.

Nathan Elder, manager of water supply for Denver Water, said this is typical of the reservoir’s fill and drawdown cycle but added that the water level took a slightly sharper decline at the start of the month due to increased water needs, as well.

“So far this month, hot weather and limited precipitation in our service area has required we move more water from Dillon through the Roberts Tunnel to the South Platte River,” Elder wrote in an email. “Additionally, we have several planned maintenance projects in our north end that require more water from our south system (Dillon, Roberts Tunnel and the South Platte) this fall and winter.”

Compared to recent years, Elder said the reservoir has stayed full longer because of above-average precipitation in the Blue River watershed and less demand in the area the reservoir serves. He said the levels did not set any records, though.

“We were able to keep water levels high this summer due to a wet spring on the Front Range, as well as good moisture this summer, including some helpful monsoons in the Dillon watershed,” Elder said.

Elder said the drawdown of the reservoir typically starts in early to mid-July, but this year’s July was very wet in the Blue River watershed. Dillon Reservoir started to slightly lose storage in mid-July, but in late July and early August, precipitation brought the reservoir back up, keeping it full longer than normal.

“Later in August and so far in September, we have not had the precipitation that we would normally see at Dillon Reservoir or in our service area, and temperatures have been above average, as well,” Elder said.

Elder also said the Roberts Tunnel, which moves water from the Western Slope to the Front Range, was turned off for a part of August when it would typically be running at 215 cubic feet per second. He said Denver water has not started releasing more water down the Blue River but has been consistently releasing 105 cubic feet per second for the past month.

A chart shows the average water storage in Dillon Reservoir compared with 2021's storage.
Denver Water/Courtesy graphic

Treste Huse, a senior hydrologist at the National Weather Service in Boulder, said while September typically isn’t a big month for rainfall in Summit County, it has had even less than normal so far this year.

From Sept. 1-13, Huse said Dillon typically gets an average of 0.68 inches of rainfall. This year, it has seen only 0.23 inches of rainfall in the same time period.

“The good thing is it’s not a big precipitation month for Summit County,” Huse said. “So wait for the big snow melts to come.”

Huse said streamflows were generally above normal throughout the summer, but they have started to come back down again. She said at least half of the streams in Summit County are running normally, but six of them have now dipped below normal.

Looking at the whole summer, Huse said Dillon had more rainfall than it typically does. From May through August, Dillon received 9.08 inches of rainfall, which is 145% of the average amount of 6.25 inches.

“What really helped was that monsoon moisture that came in,” Huse said. “We had pretty good monsoon moisture, and it really helped us this year.”

Water storage in Dillon Reservoir has started to drop due to less rain, higher temperatures and increased need for water on the Front Range.
Lindsey Toomer/Summit Daily News

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