Officials prepare for possible high water at Big Horn Lake

The area surrounding north Big Horn County has received plenty of precipitation this year, and much of it is in the form of snow sitting atop the mountains.

As the snowpack continues to build into the cooler than normal spring, many are expecting large amounts of water that will have managers working to prepare rivers and reservoirs for the potential 100-year snowmelt event.

Friends of Bighorn Lake member Bob Croft said the amount of snowpack in areas that feed Big Horn Lake are unprecedented, and estimates for snowpack percent of average are off the charts, because there is no historical data to compare this year’s snowmelt to.

In a normal runoff year, about half the water would be off of the mountain by now, but the 2011 spring has done nothing but add snowpack to the mountaintops.

Croft said the runoff could come fast, possibly over the next 10 days if warm temperature forecasts hold true. Another possible scenario if temperatures remain cool is that the snowpack will stay on the mountain year-round and will probably enter the river system in 2012.

Temperatures through July and August will determine which scenario will occur.

The BuRec is keeping reservoirs low to accommodate the expected flows, Croft said. Moisture in recent weeks has caused damage to homes and property built on the banks of the Big Horn River downstream of Yellowtail Dam.

As of Tuesday, Big Horn Lake was releasing 15,500 cubic feet per second at Yellowtail Dam, while a total of 17,000 cfs was entering the reservoir, including tributaries, rainfall and releases from Buffalo Bill and Boysen Reservoirs.

Croft compares this year’s data with the lake in 1967, when record water levels brought the reservoir within three feet of the top of the dam. In ‘67, 2.2 million acre-feet of water passed through the dam, and Croft said conservative estimates are that 2.6 million AF could be passed though this year. The reservoir can hold about 1 million AF at any one time at capacity.

The lake is full at 3,640 feet, and the top of the dam is at 3,660, but the BuRec tries to keep it below 3,657. Wednesday morning, the reservoir was at an elevation of 3,637.4 feet.

Yellowtail Dam can handle up to 92,000 cfs (enough capacity to take the flows if Buffalo Bill and Boysen breached, Croft said), and the water will not spill over the top. But the flows will test Afterbay Dam, which is designed to take flows of 20,000 cfs and being pushed to 24,000, Croft said.

Currently, Boysen has about 20 feet in elevation left for storage and Buffalo Bill has 45 feet, a total of about 400,000 AF, which will be used to store some of the flows, but the number is insignificant compared to the total 2.6 million AF coming, Croft said.

Local effects

Croft said effects of the water will be evident at Big Horn Canyon, as the water levels will be near the same level as they were in 1967, probably a few feet from the top of the dam, Croft said. The water will shut down boating campgrounds for the summer and will present new water hazards to boaters as the shoreline is changed by rocks, trees and shrubs being submerged, but sitting just below the surface.

“Stay away from the canyon walls,” he said.

Floating restrooms in the park are also being removed because they’re not designed to be anchored in deep water, he said.

Driftwood beached since 1967 could be released into the river system this year, Croft said.

“We could have more driftwood than ever in the system,” he said, noting a huge amount of driftwood was piled near the causeway in ‘67. Dead wood and debris in side canyons will also be swept into the lake. “There will be a lot of places to pick up junk from,” he said.

Croft said boaters should be careful of getting trapped in areas clogged with driftwood, especially at mile markers 20, 32 and 42, where double S curves slow down driftwood as it travels north.

“There will be some spots where boats just won’t go through it and you’ll have to get out your paddles and push through,” Croft said.

Another issue for local boaters will be crossing under the causeway east of Lovell. Croft said lake levels will likely make it impossible to go under the bridge and with the ML ramp under water, there will be no way to launch anything but small boats south of the causeway.

The launch ramp at Horseshoe Bend will remain in use, but the high water level will present challenges for boaters because most of the ramp will be submerged.

Croft participates in weekly conference calls with the BuRec and other stakeholders. In the latest call Tuesday, Croft said plans are being modified to reduce outflows at Boysen next week to help alleviate flooding in the Missouri River system at the request of the Army Corp of Engineers. He said Buffalo Bill outflows could be reduced in the future.

Croft reiterated that the high water depends on warm temperatures. If the forecast for 80-plus degree temperatures in the next 10 days hold true, there will be a lot of water passing by.

By Brad Devereaux

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