Dolores River: Balancing streamflow forecast and boating releases from McPhee

Dolores River near Bedrock
Dolores River near Bedrock

From The Cortez Journal (Jim Mimiaga):

A sporadic 12-day boating release from McPhee dam into the Dolores River in June was hampered by uncertain runoff forecasts after a late-season snowfall, reservoir managers said at community meeting Tuesday in Dolores.

Boaters faced on-again, off-again announcements of whitewater releases from the dam, which complicated their plans for trips down the river. It was the dam’s first whitewater release since 2011.

A 22-day rafting season was forecast as possible in March when snowpack registered at 130 percent of its median normal. A two-month dry spell erased the advantage, and the release was adjusted to five to 10 days of boating for late May. The forecast then dropped to a three-day release in early June, and after it was confirmed days later, hundreds of boaters flocked to the Dolores as it filled below the dam.

“Small spills are the most difficult and tricky to manage,” said Mike Preston, general manager of the Dolores Water Conservancy District, which manages the reservoir.

But on the fourth day, managers said they realized the volume of river inflow was more than the reservoir could handle, and the dam release was extended nine additional days.

“The second spill was highly under-utilized,” said boater Kent Ford, who added that the lack of notice “killed a lot of multi-day trips.”

Vern Harrell, of the Bureau of Reclamation’s office in Cortez, attributed the uncertainty to the narrow margin of runoff expected to exceed reservoir capacity.

The runoff forecast has a margin of error of 10 percent, “and this year, the spill was within that 10 percent,” Harrell said.

Decisions about dam releases rely on forecasts from the Colorado River Basin Forecast Center, which depends on Snotels that measure snowpack in the Dolores Basin.

When there is possibility for a small spill, managers don’t have the tools to give a lot of notice, Harrell said, so decisions are made day-to-day based on river inflow and reservoir levels.

“By May, all the Snotels are melted out, and we are in the blind,” he said.

In small spill years, managers said they err on the side of caution when announcing the number of days available for boaters. They want to ensure that the reservoir remains full, but they don’t want to end a dam release prematurely.

“We have to be careful we don’t leave boaters stranded on the river,” Harrell said.

Ken Curtis, an engineer with Dolores Water Conservancy District, said the priority is to fill the reservoir, and if there is excess water, it is managed for a boating release.

It was especially difficult to forecast runoff into the reservoir this year, he said, because much of the late-season precipitation came as rainfall.

“In May, we called off the spill because we were not reaching our reservoir elevation,” he said. “Then the forecasters bumped us up by 30,000 acre-feet,” enough for a small spill.

At the end of a five-day release, the forecast center showed a dip in river inflow, “so we started to shut the gates, but the river inflow was hanging in there,” and the spill was extended several days.

Managers acknowledged that they were rusty managing the release. They’d faced many dry winters that hadn’t filled the reservoir, and the unusual winter of 2015-16 complicated the matter.

Sam Carter, president of the Dolores River Boating Advocates, said boaters and the reservoir managers cooperate on potential spills, and this year was a learning experience.

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