A two- to four-week whitewater release from McPhee Dam into the Dolores River is “very likely” and is expected to begin in late May or early June, reservoir managers told a gathering of 40 boaters Thursday at the Dolores Community Center.
Snowpack in the Dolores Basin is 144 percent of average, enough to fill the nearly empty reservoir and provide recreational and ecological downstream flows.
“Our first priority is to fill the reservoir and provide water for our users,” said Robert Stump, of the Bureau of Reclamation in Cortez. “The exciting part for boaters is the predicted runoff provides an opportunity for a downstream release.”
But how long the dam release will last, and the exact date it will begin is unclear as most of the snowpack is still in the mountains, said Greg Smith, a hydrologist with the Colorado River Basin Forecast Center.
Runoff quantity and timing also depend on temperature and how much will be absorbed into the soil. When soil moisture is below 50 percent average, it knocks 5% to 15% off the runoff forecast.
Snowpack totals and runoff forecasts are extrapolated with modeling that relies on a series of snowpack measuring devices, called Snotels, in the Dolores Basin. The units have historical data from 1981 to 2015.
This year, there is the additional “wild card” of having a record dry winter last year and a wet winter this year that is expected to fill it back up, plus excess.
“This is not something we have experienced, and we’re not 100 percent sure how that will impact the runoff this year,” he said.
Reservoir and river managers use probabilities to inform boaters on the likelihood of a dam release.
According to the River Forecast Center, there is a 70% probability that Dolores River runoff will produce 40,000 acre-feet beyond what the reservoir can hold. That equals an approximate two-week whitewater release. Flows would be around 1,000 cubic feet per second, and peak flows would be would between 2,000 and 2,500 cfs.
A 50% runoff probability shows 130,000 acre-feet beyond what the reservoir can hold, according to the Forecast Center, enough for a approximate four-week whitewater release beginning in early May.
“A spill looks very likely,” said Ken Curtis, a reservoir engineer. “I’d plan for an early June raft trip, subject to change.”
When there is excess water, temperatures that influence snowmelt rate and irrigation demand are big variables dictating when the dam release will happen.
Boaters asked why at the end of a recreation dam release, there is sometimes a sudden spike in flows, and why the release is sometimes stopped then started up again like the release in 2017.
The reason, said Curtis, is part of the margin of error of runoff modeling. The high-elevation snowpack above 13,000 feet elevation is the last to come down, he said, and the exact volume is the most difficult to determine late in the season. When it finally melts off, and there is no room in the reservoir, it must be released downstream, often with not much advance notice for boaters.
A new wrinkle in whitewater dam release is how to manage the ramp-down criteria. The system is designed to gradually reduce the release over many days, with adequate notice, to allow everyone to get off the river without being stranded. However, there is a concern that the ramp-down might release water that could have been stored in the reservoir.
Improving the timing of the ramp-down to ensure the reservoir is topped off while safely accommodating boaters when recreational flows are available, are challenges reservoir managers and boating groups are discussing, officials said.
Even without a dam release, melt-off from above-average low-elevation snows have charged the popular Slick Rock to Bedrock section of the Dolores River with boatable flows.