Experts: Heavy Snowfall Increases Flood Risk in Idaho

February 15, 2008

The persistent snows of January and early February have increased the risk of flooding in low-lying areas of north and central Idaho this winter and spring, even while water shortages remain possible this summer in the southern part of the state.

The National Weather Service has highlighted 11 areas with an increased risk of flooding, including the Weiser River in Adams and Washington counties, streams and main stem rivers in the Idaho panhandle, tributaries of the Big Wood River near Ketchum and small streams and creeks in Ada, Boise, Valley and Clearwater counties. Mudslides and debris flows are also possible in the Payette, Big Wood and Salmon river basins, which all experienced significant destruction from wildfires in 2007.

“Get flood coverage and get it now,” Dave Jackson, mitigation program manager with the state Bureau of Homeland Security, said at a meeting of the interagency Idaho Water Supply Committee.

Hydrologists at the meeting said the snowpack has climbed to above-average levels in each of the state’s 19 river basins after a wet start to 2008. The Weather Service reported 100 percent snow cover across the state on Feb. 8 and between four and 11 feet of snow across north Idaho. In the northern resort town of Cascade, the 50 inches recorded there is the second-highest since recording began in 1948. The highest was 58 inches in 1949.

At three low elevation sites in the Weiser River Basin, the average water content in the snow was recorded on Feb. 8 at 5.6 inches, which is 224 percent of the 2.5-inch average and the highest measurement since recording began in 1996, according to the Natural Resources Conservation Service. Overall, snowpack in the entire Weiser basin is 134 percent of average, greater than any of the state’s other river basins.

Water experts say that lower elevation snow is more of a risk for flooding because it can rapidly boost the levels of streams and rivers when it melts, whereas the higher elevation snow in north Idaho will likely melt later in the spring and get partially soaked up by reservoirs.

Yet even with all the worries about too much precipitation in north and central Idaho, forecasters say the southern part of the state still could face water shortages this summer.

The U.S. Drought Monitor still considers southern Idaho to be in severe drought, mostly because of depleted reservoir levels, said Weather Service hydrologist Jay Breidenbach. The Upper Snake River Reservoir System in eastern Idaho remains about 46 percent full.

Conditions in the Upper Snake River Basin last summer forced the state to threaten farmers, ranchers and other groundwater users with water curtailment. That doesn’t seem likely this year, though the basin could face a shortfall of as much as 30,000 acre-feet of water by July, based on the latest forecast, state hydrologist Steve Burrell said.

An acre-foot is the amount of water needed to cover an acre to a depth of one foot.

“We’re not going to be looking at shutting down any pumps this year,” Burrell said. “At this point, it looks like the system should be filling, but we’ll see what happens in the next couple months.”

The Weather Service is predicting the continuation of cool, wet temperatures for the rest of the winter because of the persistence of cooling waters in the Pacific Ocean, a periodic phenomenon known as La Nina, Breidenbach said.

“There is no reason to believe we’re going to see a change in the pattern of above-average precipitation,” he said. “It doesn’t rule out a drier pattern, but I wouldn’t bet on it.”

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