Record Runoff Expected From Melting Snowpack

Despite a relatively dry February, extremely heavy snowpack still in the mountains means many Idaho watersheds are expected to see their largest spring and summer runoff on record.

That has the U.S. Bureau of Reclamation and other agencies scrambling to make room in reservoirs and officials warning low-lying communities to prepare for some of the worst spring flooding in years, experts said Friday at a meeting of the Idaho Water Supply Committee.

The panel of state and federal agency representatives was formed in 1988 by then-Gov. Cecil Andrus to consider what could be done to cope with what became a seven-year drought. But after severe flooding a year ago and again last month in northern and western Idaho, the problems are different now.

As of Feb. 1, the snow water content in most Idaho river basins was more than 150 percent of average, with some approaching 200 percent. Snowpacks reported in the Boise, Big Wood, Big Lost, Upper Snake and Bear river basins broke records at least 35 years old. And April-through-July runoff levels for the Salmon, Boise, Payette, Big Lost, Big Wood and Upper Snake rivers are expected in some cases to be the highest in at least a century.

Even though February precipitation in many areas of Idaho has been as little as 25 percent of normal, the National Weather Service forecasts March through May will be slightly wetter and warmer than normal.

“It’s been an excellent month for making the space” by releasing water from reservoirs so they will be able to catch the heavy spring runoff, said Ted Day of the Bureau of Reclamation. However, he cautioned that a change from February’s relatively cool, dry weather could seriously short-circuit that effort.

“The major concern regarding the temperatures is how soon it (the snow) comes off” the mountains, said Hal Anderson of the Idaho Department of Water Resources.

Ron Abramovich, a water supply specialist for the U.S. Department of Agriculture’s Natural Resources Conservation Service, compared the runoff scenario this year to 1965. Despite heavy snowpack, precipitation then also was below normal in February and relatively dry conditions that continued into March helped prevent extensive flooding.

Some of the biggest problems caused by this year’s flooding were landslides. Steve Weiser of the state Bureau of Disaster Services said a task force is studying what can be done to reduce the future threat of slides. A report will be prepared for Gov. Phil Batt by early May.