August flash floods lead to murky Boise River, which will stay muddy awhile

rphillips@idahostatesman.comAugust 29, 2014 

A man fishes on the banks of the Boise River near the diversion dam at Ann Morrison Park on Thursday August 28, 2014.

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You know the old saying about what rolls downhill? We're seeing proof of it in the Boise River now.

Flash floods and mudslides poured thousands of tons of sediment into the Middle Fork and South Fork of the Boise rivers, as well as numerous tributaries.

That mud has now made its way through the Arrowrock and Lucky Peak reservoirs and can be seen in the Boise River in town.

The river could stay muddy for weeks, according to Brian Sauer, operations manager for the Bureau of Reclamation, which operates Lucky Peak Dam.

What happens next depends largely on the weather. If more rain comes, more sediment will be coming downstream.

If weather stays dry, or freezes, the river will start to clear up sooner.

Sauer said it’s likely we will see murky water to some degree throughout fall.

The murky water doesn’t pose an immediate threat to anything, but sediment can eventually cover spawning beds of fish, which rely on gravel to deposit their eggs.

Silt can also fill reservoirs over time and reduce water storage capacity, but Sauer said that’s not a concern for those large reservoirs, even in the foreseeable future.

Mud in the Boise River is not uncommon. It typically happens when there’s a major blowout upstream, such as the one that happened in the South Fork of the Boise River last September. That sent plumes of mud through three reservoirs: Anderson Ranch, Arrowrock and Lucky Peak, and into the Boise River.

But what makes this one a little different is those past rainstorms were fairly isolated and caused a tributary stream or several to spew mud for a short period of time.

Storms earlier this month were more widespread and dumped massive amounts of rain as multiple rainstorms hit throughout a week.

According to National Weather Service meteorologist Josh Smith, Cascade got an inch of rain, Fairfield three-quarters of an inch, Jerome 3.65 inches and Twin Falls a whopping 5.5 inches. Boise received one-tenth of an inch of rain during that same time.

When intense rain hit slopes denuded by wildfires, it can cause entire hillsides to slide into rivers in multiple places.

Road crews are still working on the Middle Fork of the Boise River to clear a mudslide there and reopen the road to Atlanta.

Sediment from slides is not always visible in reservoirs. It travels downstream underwater and then come out the bottom of Lucky Peak Dam.

“That fine stuff could stay suspended for a while,” Sauer said. “It could be into September” before the Boise clears up.

Roger Phillips: 377-6215, Twitter: @rogeroutdoors

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