The amount of snowpack on the Front Range and rate of melting due to high temperatures or rain events also impacts when Denver Water turns on the Roberts Tunnel and how much water it takes out of Dillo
Denver Water officials late last week increased the release of water from Dillon Reservoir into the Blue River to about 400 cubic feet per second as inflow held steady at about 500 cfs through Monday. The latter number is expected to steadily rise this week as spring runoff picks up.
The current forecast from the National Oceanic and Atmospheric Administration’s Colorado River Basin Forecast Center estimates there is, as of Monday, 146,000 acre-feet of water — in the form of snowmelt — that will flow into Dillon Reservoir through July 31. There’s currently 17,500 acre-feet of space in the reservoir, according to Denver Water, so about 128,500 acre-feet will flow out of the reservoir either to the Blue River or Roberts Tunnel by July 31, with an estimated 13,000 acre-feet through the tunnel.
All of these complex calculations are the first steps in a delicate dance Denver Water performs each spring-runoff season to balance public safety with Denver’s water needs, recreation, hydroelectric demands and obligations to downstream senior water-rights holders.
“Dillon is our biggest reservoir and one of our more complicated to operate,” said Nathan Elder, water resources manager for Denver Water. “Most of our other reservoirs only have one outlet, but Dillon’s got both the outlet to the Blue and the outlet to the Roberts Tunnel, which provides water to the East Slope and down the North Fork (of the South Platte River) to Strontia Springs Reservoir and then to our customers.”
The Roberts Tunnel, finished in 1962 about the same time the old town of Dillon was relocated to its current spot and the Dillon Dam was built to supply its current number of more than 1.4 million Denver Water customers, is a 23-mile concrete conduit that diverts water from the Blue River Basin on the Western Slope to the South Platte Basin on the Front Range.
This system is what’s known as a transmountain diversion — one of many that bring water from the Colorado River basin on the west side of the continental divide to the state’s population center on the Front Range. What it’s not, Elder said, is a way to avoid dangerous spring-runoff flooding.
“We can’t use Roberts Tunnel as a flood-control option,” he said. “So we’re very careful about the amount of water we take from the West Slope over to the East Slope. And when we use the Roberts Tunnel, we can only take it over to the East Slope if it’s put towards the demand. We can’t just dump it over there to prevent flooding or high flows below Dillon.”
The 2014 Colorado River Cooperative Agreement places a 400,000 acre-foot limit on Blue River water stored in existing or future Denver Water storage facilities on the Front Range.
There are more than 1,000 properties in regulatory floodplains in Summit County, according to the Federal Emergency Management Agency, and quite a few of them are along the Blue as it makes its way northwest through Silverthorne and toward its confluence with the Colorado River near Kremmling.
