Dillon Reservoir expected to fill in June; average spring snowfall could bring long rafting season in the Lower Blue
*This story has been updated to clarify a couple of points — the Lower Blue flow of concern for flooding 1,800 cfs, not 1,400 cfs; and, the operation of the Roberts Tunnel is not used for flood control since that’s not a decreed use.
By Bob Berwyn
SUMMIT COUNTY — With average snowfall the next couple of months, Denver Water expects Dillon Reservoir to fill up completely by June, with decent flows for rafting in the Lower Blue from the end of May through sometime in July.
If spring ends up being an abnormally dry, however, that scenario could change, with flows in the Lower Blue predicted to range only between 93 and 336 cubic feet per second during the runoff season, not quite enough for boating.
With a wet spring, the Lower Blue could peak at flows close to 1,400 cubic feet per second in June, well below the critical 1,800 cfs flow that brings potential flooding issues. In the wet-spring scenario, flows in the Lower Blue could ramp up to more than 800 cfs by sometime in May and stay above 1,000 cfs through sometime in July.
Snowpack in the mountains continues to be above average through mid-March in most basins, with a 120 percent of average reading in the Colorado River Basin and 116 percent in the South Platte Basin. But conditions across the Front Range have been dry all winter, so Denver Water will need to take some water from Dillon Reservoir to replenish reservoirs in the South Platte drainage, said water resource engineer Bob Peters.
“It’s pretty dry over here. We’ll need some tunnel water,” Peters said.
As they do each spring, Denver Water officials release monthly updates with projected flows and reservoir levels.
A winter-long maintenance project on the Roberts Tunnel should be complete by mid-April, he added, giving the utility some additional flexibility in balance the water level in Dillon Reservoir with storage the South Platte Basin reservoirs.
With average moisture in the spring, Denver Water is forecasting that average flows in the Lower Blue will hit close to 600 cfs sometime in May, climb to about 1,100 cfs in June, then stay near 1,000 cfs until sometime in July before dropping down to about 470 cfs in August. All the figures are monthly averages and all the estimates are subject to change depending on weather conditions during the runoff season.
Last year, for example, the snowpack in the Blue River Basin experienced an extremely sudden meltdown in June, surprising water managers with higher-than-expected inflows into Dillon Reservoir. Then, flows dropped dramatically in late spring, to near-record lows before monsoon moisture in July brought the water levels back up.
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