#Snowpack/#Runoff news: Mid-March forecasted inflow to Blue Mesa Reservoir = 950,000 acre-feet (142% of 30 year average)

Click on a thumbnail graphic to view a gallery of snowpack data from the NRCS.

From The Greeley Tribune (Adam Poulisse):

Recreation officials in northern Colorado say the above-average snowpack — one of our primary sources of water to drink, water our fields and play in — will be a boon for outdoor activities like whitewater rafting, paddleboarding, agriculture and fishing.

As long as the snowpack melts slowly and [doesn’t] cause flooding…

Each of the state’s eight basins have a snowpack greater than 100% of their historical average, Kuhn said. The South Platte (116 percent of normal) and Arkansas (135 percent) river basins are the primary rivers in the Front Range, with the other six flowing to the west…

Even snowpack in the upper Colorado River basin, with all of its water concerns, is sitting at 131 percent of normal, according to data maintained by the USDA Natural Resources Conservation Service Colorado.

Blue Mesa Reservoir

From email from Reclamation (Erik Knight):

The mid-March forecast for the April – July unregulated inflow volume to Blue Mesa Reservoir is 960,000 acre-feet. This is 142% of the 30 year average. Snowpack in the upper Gunnison River basin is currently 150% of average. Blue Mesa Reservoir current content is 247,600 acre-feet which is 30% of full. Current elevation is 7437.6 feet. Maximum content at Blue Mesa Reservoir is 829,500 acre-feet at an elevation of 7519.4 feet.

Black Canyon Water Right
The peak flow and shoulder flow components of the Black Canyon Water Right will be determined by the May 1 forecast of the April – July unregulated inflow volume to Blue Mesa Reservoir. If the May 1 forecast is equal to the current forecast of 960,000 acre-feet of runoff volume, the peak flow target will be 7,012 cfs for a duration of 24 hours. The shoulder flow target will be 955 cfs, for the period between May 1 and July 25. The point of measurement of flows to satisfy the Black Canyon Water Right is at the Gunnison River below Gunnison Tunnel streamgage at the upstream boundary of Black Canyon of the Gunnison National Park.

Aspinall Unit Operations ROD
Pursuant to the Aspinall Unit Operations Record of Decision (ROD), the peak flow and duration flow targets in the lower Gunnison River, as measured at the Whitewater gage, will be determined by the forecast of the April – July unregulated inflow volume to Blue Mesa Reservoir and the hydrologic year type. At the time of the spring operation, if the forecast is equal to the current forecast of 960,000 acre-feet of runoff volume, the hydrologic year type will be set as Moderately Wet. Under a Moderately Wet year the peak flow target will be 14,350 cfs and the duration target at this flow will be 10 days. The duration target for the half-bankfull flow of 8,070 cfs will be 20 days. The criteria for the drought rule that allows half-bankfull flows to be reduced from 40 days to 20 days have been met.

Projected Spring Operations
During spring operations, releases from the Aspinall Unit will be made in an attempt to match the peak flow of the North Fork of the Gunnison River to maximize the potential of meeting the desired peak at the Whitewater gage, while simultaneously meeting the Black Canyon Water Right peak flow amount. The magnitude of release necessary to meet the desired peak at the Whitewater gage will be dependent on the flow contribution from the North Fork of the Gunnison River and other tributaries downstream from the Aspinall Unit. Current projections for spring peak operations show that flows in the Gunnison River through the Black Canyon could be near 7,000 cfs for 10 days in order to achieve the desired peak flow and duration at Whitewater. With this runoff forecast and corresponding downstream targets, Blue Mesa Reservoir is currently projected to fill to an elevation of around 7508 feet with an approximate peak content of 728,000 acre-feet.

From The Cortez Journal (Jim Mimiaga):

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.”

[…]

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.

Here’s the Westwide SNOTEL basin-filled map for April 24, 2019 from the NRCS.

Westwide SNOTEL basin-filled map April 24, 2019 via the NRCS.

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