Friday, May 16, 2008

A surge runs through it

Weekend temperatures should bring first river-level spikes

Express Staff Writer

A group of mostly Colorado boaters lifts a cataraft over a guard rail and onto a snow ramp at Cape Horn Creek, a tributary of Marsh Creek, which is the early-season put-in for the Middle Fork of the Salmon River. Photo by Greg Stahl

With 3 to 4 feet of heavy snow icing the edges of Cape Horn Creek west of Stanley on Wednesday afternoon, Emylie Patten watched as the last of five rafts was lifted over a guardrail bordering Highway 21 and deposited on a steep, snowy slope.

Patten was part of a group of rafters and kayakers, most from Colorado, departing for a seven-day trip on the Middle Fork of the Salmon River. The heavy snow lining the creek near the headwaters of the Middle Fork was a dramatic change from the previous spring's trip, she said. In May 2007 there was no snow at all at the put-in.

With weekend weather forecasts calling for temperatures in the 70s in the mountains and as high as the 90s at Idaho's lower elevations, Patten's group should float through the first high-water spike of the 2008 runoff season.

For whitewater boaters, the coming week's rising water should warrant some caution.

"The dangerous part of being on the river now is that it's on the rising part of the hydrograph, and that's when the river has more potential to pick up downed trees," said Ron Abramovich, a hydrologist with the Natural Resources Conservation Service. "You have to be careful and cautious, especially after the forest fires of last year. Things can change overnight."

Throughout central Idaho, spring temperatures have retained snowpacks at near-average or above-average levels into mid-May, and that means warming temperatures could bring water levels up fast. The Salmon River basin as a whole has 124 percent of its average snowpack for this time of year. The Banner Summit snow monitoring site at the headwaters of Cape Horn Creek reads 110 percent of average, down dramatically from 160 percent of average a couple of weeks ago.

Abramovich said that means this weekend and into early next week should bring the first high-water spike of the runoff season. He qualified, however, that long-range weather forecasts are calling for another cool-down, and that could mean a year when rivers peak more than once.

"We'll get through this weekend and see what the peak looks like," he said. "Whether the future peak exceeds this weekend's peak will depend on the temperature. We're pushing record high temperatures this weekend, so that will put the snowmelt in full swing."

For the sake of comparison, Conservation Service hydrologists are using the 2003 water year as an indicator of what this year's runoff could become. The 2003 snowpack was, like this year, near average throughout much of the state, but cool spring temperatures retained the snow until about June 1. In 2003 the sudden and late warming trend produced river level spikes throughout the state that far exceeded average peak flows.

But because of the anticipated cooling next week, it looks like 2008 could produce at least two high-water spikes, and that means both spikes would be lower than they would be should sudden, sustained warm temperatures descend on the Gem State.

"Usually, multiple-peak years are better than one peak because you have longer sustained flows, and it produces water later in the year," Abramovich said. "Plus, it's easier to manage for the water managers.

"Basically, what it comes down to is we can do a good job managing water and predicting stream flows with normal weather, but we have a difficult time managing water with extreme weather and really hot temperatures."

The Big Wood River basin snowpack stands at 94 percent of average, and the Big Wood River at Hailey flowed at 1,000 cubic feet per second Thursday morning. That's the river's long-term average flow for the date.

"We've got to go up to 4,000 cfs to get to flood stage," Abramovich said.

The Big Wood's average peak is about 1,600 cfs, and this year's Conservation Service models are calling for a "near max" peak flow of about 3,200 cfs, far below the spring 2006 peak of 7,800 cfs, but still far above the long-term average.

In the northern reaches of the state, there is much more snow this spring. The Clearwater River basin snowpack stands at about 160 percent of average for mid-May, and Abramovich said that means more potential for sudden flooding and multiple water-level peaks.

"The farther north you go the more potential you have for sustained flows after this weekend, and the more potential for multiple peaks, too," he said. "There's more snow in those mountains waiting to melt compared with the southern third of Idaho."

Back at Cape Horn Creek in the headwaters of the Middle Fork on Wednesday, the Colorado group loaded the last of its gear onto rafts, and the mood was decidedly relaxed.

"Sometimes you've got to lower your standard of living to increase your quality of life, and that's why I live in Crested Butte, Colorado," Patten said.

They shoved off onto the clear, shallow waters. About 100 yards downstream, they would join with Marsh Creek. Ten miles further they would join with Bear Valley Creek, a confluence that officially constitutes the named segment of the famed Middle Fork.

The Middle Fork jumped 1,000 cfs overnight between Wednesday and Thursday, and this weekend's forecasted temperatures certainly indicate an entertaining trip could be under way for the Colorado boaters.

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