Dillon Reservoir water levels hold on despite statewide drought

Low water levels Aug. 18 at Dillon Reservoir expose sand rings around the lake's islands.
Low water levels Aug. 18 at Dillon Reservoir expose sand rings around the lake’s islands. The reservoir, which is the largest in the system supplying Denver Water’s customers, is about 94% full. (Lindsay Fendt/Aspen Journalism)

By Lindsay Fendt

FRISCO — Amid one of the hottest summers on record for Colorado, Dillon Reservoir is 94% full, nearly 5 feet below its capacity. The reason is a complex combination of past weather patterns, current water-use habits and recent changes to the lakebed.

For most of the summer, Dillon Reservoir has been down about 4 1/2 feet. This low elevation is noticeable from the shore, but the drop in water level is less pronounced than it has been in other dry years. Around this time in 2018, Dillon Reservoir’s elevation was dropping an inch daily and was down about 11 feet by Labor Day.

Dillon Reservoir is no normal mountain lake. The man-made reservoir is one of the largest sources of drinking water for Denver. Usually in late June, Denver Water holds back water that flows into Dillon Reservoir from the Blue River basin and stores the water until it’s needed along the Front Range. In late summer, Denver Water typically begins piping water out of Dillon Reservoir via the Roberts Tunnel, a 23-mile pipe that runs under the Continental Divide and into the North Fork of the South Platte River. From there, the water flows down toward Strontia Springs Reservoir, where it’s delivered to Denver Water’s customers.

In most normal water years, managers at Denver Water are able to fill the reservoir to its 257,000 acre-foot capacity in the spring, and recreation along the reservoir is usually best when it’s full. This year, unseasonably warm spring weather created dry soil that absorbed much of the moisture from melting snow before it reached rivers. Wind and low precipitation in May also contributed to a lackluster runoff season. Denver Water was able to fill Dillon Reservoir to 244,000 acre-feet of water, about 95% of its capacity. The reservoir levels have hovered around that number ever since late June.

“You know, 95% seems like it would be pretty full, but in the past, at this point, we would be moving docks and boat ramps would be unusable,” Frisco Bay Marina General Manager Tom Hogeman said. “But other than tightening cables on docks to adjust for different water levels, we haven’t had to move anything.”

Sailboats on Dillon Reservoir
Sailboats anchor offshore Aug. 18 at Dillon Reservoir. It has been a busy summer at the lake for recreation, and the Frisco Bay Marina already has brought in 18% more revenue than last year, with a month left to go before boating season is over. (Lindsay Fendt/Aspen Journalism)

Big dig

The operational changes for the marina are due to an excavation of the lakebed in 2019. That spring, the lake was at historic low levels after the 2018 drought. The town of Frisco and Denver Water took advantage of the dry lakebed and rolled out heavy digging machines to excavate areas near the shore. The $4 million project moved more than 85,000 cubic yards of dirt, deepening the area around the marina and lengthening the beach.

The “Big Dig,” as the project was dubbed by the town of Frisco, was designed to improve navigation for boaters and lengthen the boating season by making the parts of Dillon Reservoir that are more desirable for recreation less prone to elevation fluctuations. The project is one of the main pillars of the Frisco Bay Marina Master Plan, a long-term blueprint for projects to expand recreation and tourism on Dillon Reservoir.

The reservoir, already a significant source of tourism for Summit County, has seen a bump in visitors this year. The increase is likely the result of the COVID-19 pandemic, which has increased demand for outdoor recreation across the High Country. The marina this year has already brought in 18% more revenue than last year, and there is still a month left before boating season is over.

Last summer, the changes from the lakebed excavation were less noticeable because healthy snowpack from the previous winter filled the reservoir. With water levels down again, Hogeman said it’s clear that the project was a success.

“That has really paid off,” he said. “We are in a better position to deal with these smaller fluctuations. Before, our slip holders would have to adapt to their boats being in different places at different times of the year depending on water levels. Now we’ve just got an improved level of consistency.”

While the lake excavation helped to ward off problems from small water-elevation drops, a more severe drop would still threaten recreation at Dillon Reservoir. According to the U.S. Drought Monitor, the entire state is currently at some level of drought for the first time in eight years. Both Summit County, where Dillon Reservoir is located, and Denver County, where the lake’s water is used, have a mix of moderate and severe drought within their borders.

This level of drought has been manageable this year for Denver Water partly because of the 2018-19 winter. Snowstorms that winter left snowpack levels at about 104% of normal all the way through April 2019, and the reservoir filled to capacity last summer.

According to Nathan Elder, the manager of water supply for Denver Water, that extra water was a big help when this spring-runoff season produced less water than normal.

“We had a really great water supply year last year, and we came into this year roughly 5% above normal (storage at Dillon),” he said. “We pretty much maintained that until late June.”

The storage boost was also helped along somewhat by water use — or lack thereof — in Denver. The city is experiencing one of its hottest years on record, with 65 days seeing temperatures hit at least 90 degrees, a number that is second only to 2012. Despite the heat, water use is only 11% above the five-year average, and Denver Water has not had to implement any restrictions beyond its normal summer watering guidelines.

According to Elder, residential water use has gone up, but with many businesses closed due to the pandemic, commercial water use has dropped significantly.

“Our customers, despite it being hot and dry, (have) been pretty good with usage this year,” Elder said. “We haven’t seen the use that we would expect for these types of temperatures.”

Drying mud at Dillon Reservoir
Inflows Aug. 18 into Dillon Reservoir have slowed as drought expands through Colorado. However, storage in the reservoir was above average following the 2018-19 winter. (Lindsay Fendt/Aspen Journalism)

Tunnel maintenance

Unusually, Dillon Reservoir will have another chance to fill this year. Typically, Denver Water pulls water from the lake using the Roberts Tunnel through the end of the year, but the tunnel will be undergoing about two months of maintenance this fall. That project will cut off Denver from Dillon Reservoir and require Denver Water to rely heavily on Cheesman Reservoir, which draws water primarily from the South Platte River basin, on the Front Range.

This will give Dillon Reservoir an extra chunk of time to bolster its reserves, but only if it rains. According to Elder, forecasters are not predicting a very rainy September. Without a large amount of carryover storage going forward, next year’s levels at Dillon Reservoir will depend on snow from this winter. Although the lake avoided a drought disaster this year, a prolonged dry period could change that.

“The worst-case scenario is that the reservoir doesn’t fill again next year,” Elder said. “So hope for rain.”

Aspen Journalism is a local, nonprofit, investigative news organization covering water and rivers in collaboration with the Summit Daily News and other Swift Communications newspapers. This article appeared in the Aug. 28 edition of the Summit Daily News.

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