HAGERMAN — If you have a taste for the dramatic, add this to your outdoors to-do list this summer: Watch from above as crews and cranes build a fish ladder in the deep, narrow canyon of a spring-fed river.
Idaho Power this week began construction of a ladder to allow the Malad River’s wild rainbow trout to swim in both directions past the diversion dam for its Upper Malad hydroelectric project.
It’s a project significant to anglers and other Idaho outdoorsmen.
But it’s also an engineering spectacle — one that visitors to the Malad Gorge unit of Thousand Springs State Park can watch from a canyon-rim overlook at the unit’s west end. That overlook — where a new interpretive sign explains the project — offers a view down into the tight river bend where Idaho Power’s upper dam diverts a portion of the Malad River.
The cliffs are practically vertical, and building a fish ladder here requires some serious rockfall prevention: anchors drilled into the canyon wall for a system of cables that hold a chain drape.
In the initial construction phase, Idaho Power and its contractor will open the spill gates and drain the forebay, the artificial pool of water behind the dam. But the river, of course, will keep coming. So they’ll build a cofferdam — a series of huge sandbags and an impermeable liner — so they can pump water out of a portion of the forebay and pour concrete below the waterline.
In August, Idaho Power senior engineer Bill Lynch expects a crane to drive big piles into the canyon floor and install a wall to hold back the rocks and dirt of the canyon’s southwest side. The wall is necessary so the future fish ladder beside it can be freestanding and the springs along the base of the canyon wall can persist.
Lynch expects the upper section of the project to be completed by the end of September; then the tailrace below the dam will be dewatered to start the downstream work. The project’s last phase will be the transport channel itself: a big concrete structure holding a series of 26 pools.
Fish will swim from pool to pool through the ladder’s slots, traveling an 18-foot hydraulic drop and a 300-foot hydraulic length.
The lower three miles of the spring-fed, gin-clear Malad River — from the waterfall in the Malad Gorge park unit to the Malad’s mouth at the Snake River — is excellent habitat for trout spawning and juvenile rearing. The Malad’s rainbow trout are fluvial: spawning in the tributary, migrating to the larger Snake to grow big, then returning to the Malad to spawn and rear young.
During Idaho Power’s 2005 dam relicensing, state and federal wildlife agencies wanted to restore that life history. A fish ladder finished in 2008 — at Idaho Power’s diversion for its Lower Malad hydro plant — reconnected the lower two miles of the Malad River.
On average, about 2,500 trout pass upstream through that ladder each year, said Steve Brink, senior fisheries biologist for Idaho Power. Something between 1,000 and 8,000 trout head downstream through it each year.
But until now, the mile of river between Idaho Power’s upper diversion and the Devil’s Washbowl waterfall at Malad Gorge park has remained isolated.
Planning of the upper fish ladder — another requirement of the same 30-year federal relicensing — has been in the works for years, in consultation with Idaho Department of Fish and Game, U.S. Fish and Wildlife Service and other state and county entities. And Brink has been a key player since Idaho Power began researching Malad River fish passage in 1996.
“I have quite a bit of attachment to these things finally getting built,” Brink said, visiting the Upper Malad project site in late June.
Researchers mapped more trout-spawning habitat above the upper diversion than in either of the lower reaches of the Malad. It’s not easy walking along the bank through the stinging nettle and poison ivy that line the Upper Malad forebay, but some anglers did anyway.
“This has been a good fishery for a select few locals who know how to get in here,” Brink said.
Building fish ladders for upstream passage, Brink said, has been well mastered elsewhere. But Idaho Power spent the extra time and money to figure out downstream passage in the Malad River; its Lower Malad ladder to date has passed at least twice as much downstream fish traffic as upstream.
Now, at the Upper Malad, it wants to recreate that success.
To best intercept trout heading downstream, Idaho Power’s team developed a 3-D flow model for the Upper Malad forebay. The answer: Position the new fish ladder in the southwest corner of the canyon’s tight turn, where 600 to 650 cubic feet per second of spring water enters the river.
“That spring water is excellent trout habitat,” Brink said.
The ladder’s system of slots and pools needs only 14 cfs of water. But to better attract fish, the ladder will pull in about 64 cfs. The extra 50 cfs will be screened, then piped to the bottom to rejoin the water that runs through the ladder.
Through a window in the ladder’s fish-viewing vault, Idaho Power must count the fish that pass through. It’ll use the automated video imaging software it spent years developing at the Lower Malad ladder — able to recognize each fish’s species and record its length and direction of travel — but in hardware that incorporates the camera and computer into something the size of a pop can.
That full automation is a victory that frees up a lot of labor — particularly for a biologist in Brink’s office who has been processing partially automated fish-count data from Lower Malad.
“He’s been looking at fish TV,” Brink said, “for nine years.”