Denver Water taking 375 cfs through Roberts Tunnel, inflow only at 177 cfs as local streams drop toward historic low flows
By Bob Berwyn
SUMMIT COUNTY — After quickly filling up in late May, Dillon Reservoir is starting to drop as Denver Water draws significant quantities of water through the Roberts Tunnel.
As of July 19, about 375 cubic feet per second were being diverted under the Continental Divide to the South Platte drainage.
“It’s been really hot in Denver,” said water resource project manager Dave Bennett, “Demand is up.”
Bennett said about 800 acre feet of water so far have been withdrawn from Dillon Reservoir, leaving it a few inches shy of its highest elevation. The drop is most noticeable where there are shallow mud flats along the shore, like the grassy cove at the Prospect Point neighborhood in Frisco, near the junction of Dillon Dam Road and Summit Boulevard.
Little said about 109 cfs is flowing out through the dam and into the Lower Blue, still well above the minimum 50 cfs level set by state rules to protect trout.
The total combined inflow for all the Blue River and all its tributaries flowing into the reservoir was only 177 cfs on July 19.
Blue River water commissioner Scott Hummer said most local streams, notably the Snake River, are flowing near historic lows after the snowpack quickly vanished in June.
Making an educated guess, Hummer said some local streams could drop to the levels of the historic drought in 2002, when some streams reached all-time record low flows.
The good news is that all the reservoirs in the state are full or nearly full, giving water managers a buffer to work with.
Bennett said there no downstream calls for water at present. Hummer said he expects there may be a call for irrigation water from the Grand Valley irrigation district in the Grand Junction area within the next few weeks. The orchards and hay ranches in that area own some of the most senior water rights on the river, and when those rights are exercised, it affects stream flows and reservoirs right up into the headwaters.
Even if monsoon rains start to materialize, it won’t make a huge difference to daily stream flows, Hummer said. Heavy rains can send flows to a peak for a few hours in a specific drainage, but don’t really have a significant effect on the daily averages.
Summer rains in the Denver area could reduce the demand for water from Dillon Reservoir, Bennett said, explaining that operations of the Roberts Tunnel are weather-dependent. Cool, wet weather in Denver could lead to a reduction in diversions through the tunnel.