Wednesday, July 21, 2004

Storms transform river

Summer rains represent a return to normal

---

By GREG STAHL
Express Staff Writer

The Big Wood River turned to chocolate milk Friday and Monday following heavy thunderstorms in the mountains of Central Idaho, but the heavy, periodic rains don’t represent anything all that unusual. Rather, they represented a return to average following a series of consecutive hot, dry summers.

With torrential rains in the mountains over the past week, the Big Wood River changed color. The likely explanation is that a land slide fell into a tributary in the Boulder Mountains and into Trail Creek near Trail Creek Road, said U.S. Forest Service biologists. Express photo by Willy Cook

According to a gauge in Hailey, the river received a healthy boost along with the color change on Friday. The water rose from 280 to 360 cubic feet per second in the span of 12 hours. Both the Big Wood River and Trail Creek were heavy and brown with silt.

The river crested again Monday morning, again about 360 cubic feet per second.

In the last several years, thunderstorms have not contributed significantly to increased flows in the Big Wood River during summer months. As mountain snowpacks waned, the river gradually settled into its low, clear late-summer routine.

According to fisheries biologists for the Sawtooth National Forest and Sawtooth National Recreation Area, the muddy water was caused either by high mountain landslides or by overflowing feeder streams. Locally heavy rains would have caused both events.

"When intense thunderstorms settle on these streams, they’re going to move some material," said Mark Moulton, SNRA fisheries and water program leader. "That’s how mountains become valleys and how natural systems work."

Moulton said the source of the muddy water in the upper Big Wood came from the high slopes of Easley and Silver peaks in the Boulder Mountains.

"Those are naturally, highly erosive slopes," he said.

The localized source of the silted water in Trail Creek is, well, murkier.

"The most likely scenario is that a slide fell into the river or one of its tributaries," said Dan Kenney, north zone fisheries biologist for the Sawtooth National Forest. "I wouldn’t be surprised if most of our land slides and mass slough movements around here are caused by thunder storms."

Kenney said other causes of murky summer water could be human disturbance or high water that breaches an alpine beaver dam.

Fish in the Big Wood drainage should not have been impacted by the events, the biologist agreed.

"Certainly long-term turbidity can clog up the gills, but these fish have been around a long time and have built up a resistance to this sort of thing," Kenney said.

Moulton said most fish egg incubation should be completed by now. Fry might have been affected to a small degree. Adult fish might have experienced detrimental effects locally, where turbidity was high. Downstream, the silt was probably good for the system.

"It’s probably good because it moves nutrients around," Moulton said.

According to meteorologists with the National Weather Service in Pocatello, this summer’s precipitation represents a return to near-normal levels. The rain is hardly putting a dent in the overall water year, however.

Prior to the summer rains, the Ketchum Ranger Station gauge was at 65.34 percent of average for the year, which begins in October and ends in September. Following the rains, the score jumped to 66.8 percent of average.

June garnered precipitation that was 75 percent of average. Central Idaho has not had an above average month since February, when precipitation was 157 percent of average.

The Idaho Mountain Express is distributed free to residents and guests throughout the Sun Valley, Idaho resort area community. Subscribers to the Idaho Mountain Express will read these stories and others in this week's issue.