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Afterbay Dam

Afterbay Dam, seen here discharging a flow of about 11,000 cubic feet per second in April 2017, spills water into the Bighorn River, which saw record runoff last spring.


Last year set records for runoff into the Bighorn River Basin, yet the February forecast for runoff this April through July is even higher than a year ago.

The forecast calls for 1.77 million acre feet of inflow between April and July. Last year the flow was pegged at 1.65 million acre feet.

The average is about 1.2 million acre feet, said Steve Davies, Montana area manager for the Bureau of Reclamation. The agency is responsible for operation of Yellowtail Dam, which creates Bighorn Reservoir. The reservoir straddles the Montana-Wyoming border.

Runoff forecasts in March are much more accurate, Davies pointed out, but snowpack in the Bighorn Basin is on a track similar to what it was last year at this time. The spike in 2017 came late in the season, in April and May, when the snow water equivalent in the mountains that feed the basin jumped to about 21 inches. The average is closer to 13 inches.

The result of all of that snow melting was a record runoff of almost 3 million acre feet of water, or 270 percent of average, between April and July of 2017.

“With the February forecast we recognize there’s still time for more snowpack to accumulate,” said Mahonri Williams, chief of the Resource Management Division of the Wyoming Area Office of the Bureau of Reclamation.

Some of last year’s water is still in the Bighorn Basin’s reservoir system, Davies said. Boysen and Buffalo Bill reservoirs, which feed water into Bighorn Reservoir via the Bighorn and Shoshone rivers, respectively, are both holding slightly more water than last year at this time. Bighorn Reservoir, on the other hand, is slightly lower and is planning to increase water releases on Wednesday to 4,500 cubic feet per second. Bighorn Reservoir managers are also planning to keep the outflows higher than last year for longer, peaking at about 7,000 cfs in May before dropping to 4,500 cfs in July.

“There are still high inflows throughout the basin,” Davies said. “There’s a lot of water moving through the system.”

“At Buffalo Bill Reservoir, because of the expected inflows, we made a small increase in our releases to move more water through before runoff,” Williams said.

The same can’t be done at Boysen Reservoir, he said, because increasing releases now when there’s ice on the river could cause ice jams and flooding. Instead, the Wyoming office waits until spring runoff to boost flows out of Boysen.

To compensate, the Bureau is proposing to drop Bighorn Reservoir lower this spring — an elevation of about 3,607 feet — and keeping it low about two weeks longer. That would mean the Horseshoe Bend boat launch wouldn’t be usable until early June.

“Our challenge is to manage the water that comes our way whether that’s rain or snow,” Williams said. “Our tools are mostly focused on snowpack and that’s what has been more reliable.”

The Bureau of Reclamation’s Montana office has been accused by the Bighorn River Alliance of poor management of Bighorn Reservoir’s water. The group released a scathing report in January. Davies said he doesn’t think the media has been fair in reporting on the issue.

In the last eight years, since the Bureau of Reclamation set up new operating criteria, Davies said the Bighorn Basin has had only one water year that was “close to normal.”

He said the Bureau is internally reviewing its operating criteria while also taking comments from river and reservoir users.

“There was a big effort developing the criteria,” he said. “We don’t want to arbitrarily change it without understanding how it will impact any particular user.”