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.