SHOSHONE • An extraordinary world lies hidden inside the narrow, deep incision snaking across a basalt field north of Shoshone.
Thousands of years of erosion have sculpted wildly fantastic shapes in the walls of the Big Wood River’s native channel, as water-borne stones scoured and ground and polished the softer basalt. Those who’ve peered over the edge — or hiked into the depths — speak of the spectacle in awe.
“It’s just amazing the power the water has when it gets flowing like that, when it picks up the grits and fines, and what it can do to rock,” said Lynn Harmon, general manager of the Big Wood Canal Co.
Informally dubbed Black Magic Canyon, the sculpted riverbed downstream of Magic Dam is an internationally unique resource that the Bureau of Land Management fought a long court battle to protect from mining claims in the 1990s.
“These outstanding specimens may well be the best examples of their kind in the nation, if not in the world,” wrote BLM geologists Terry S. Maley and Peter Oberlindacher in a 1994 publication for the Idaho Geological Survey.
The Big Wood’s waters are all captured in Magic Reservoir at certain times of year, leaving the riverbed dry for hikers. And a roadside kiosk just off Idaho Highway 75 provides easy access to one stretch of the channel. But the most astonishing formations lie east and southeast of Kinzie Butte — in a remote section of sagebrush-and-lava desert that’s sometimes too muddy to access and always dangerous.
“It’s seldom visited, actually,” said David Freiberg, outdoor recreation planner for the BLM’s Shoshone Field Office, who gets only four or five inquiries a year about that stretch of riverbed. “It’s not for the weak, really. You want to be pretty sure-footed before you embark through here.”
And you absolutely must call ahead to the Big Wood Canal Co. in Shoshone, to ask about water releases from Magic Reservoir. Those incredible channel walls could become a death trap for any hiker caught in the riverbed when the dam opens.
When the water level in Magic Reservoir is high, the canal company might release water into the Big Wood River channel anytime from mid-February to early May for flood prevention, Harmon said. The normal start of those water releases is early March, he said, but doublecheck before hiking at any time. The company has done some unusually timed releases during some winters.
“We probably get, oh, half a dozen calls a year. It’s not as extensive as you would think,” Harmon said. “There may be a lot of people that really aren’t aware of that feature.”
To see it yourself, look for an opportunity within the next couple of months.
The reservoir’s normal releases for irrigation usually run from the first week of May through the first of October, Harmon said. At some points in the summer — when the Big Wood’s waters are diverted into irrigation canals — certain stretches of the river channel can again be hiked.
“Part of it dries out in the July and August timeframe,” Harmon said. At that time of year, however, the channel’s population of rattlesnakes will be awake and active.
I’d rather brave the cold and the ice patches than worry about rattlesnakes.
On Jan. 13, Johnny Garth, geologist for BLM’s Shoshone Field Office, led a hiking group of four — Freiberg, me, husband Mark Hutchins and Times-News photographer Ashley Smith — to a shallow point of the Big Wood River channel near Kinzie Butte.
Basalt polished like dunes lined the dry riverbed, silent but dynamic.
“You can practically see the water flowing that direction over the top of it,” Garth said.
Garth didn’t intend to make the one-mile hike with us, but he waited for us to descend into the first deep bathtub. He wanted to hear the exclamations of surprised delight that he’s come to expect.
He has good reason for that pride. In the 1990s, Garth was involved in the BLM’s effort to preserve these rocks from mining claims. Boise-based United Mining Corp. filed 14 separate 20-acre claims, intending to haul away the sculpted lava near Kinzie Butte and sell it in Japan and elsewhere, Garth said. After a lengthy court process, then-Secretary of the Interior Bruce Babbitt in 1997 withdrew the Big Wood River rocks from mineral entry under mining laws for 50 years — that is, until 2047.
“We fought a long battle ... and we were able to save that for the public to be able to enjoy,” Garth said.
Inside this eerie work of art, a volcanologist might wax enthusiastic about lava blisters, columnar joints and flow foliation. A photographer might exclaim over striking lines and textures.
“It’s so unique,” said landscape photographer Rick Otto of Twin Falls, who offers his artistic lava shots for sale. Last year he came upon a stretch of the channel — almost hidden by sagebrush — while wandering in the desert near Magic Dam looking for subjects to photograph.
“I almost fell in because I didn’t know it was there,” Otto said last week, still excited by the discovery. “It was just an awesome place.”
Judy Brossy of Lincoln County, who has hiked and photographed near the kiosk off Highway 75, says a light snow on the dark lava makes for beautiful pictures. “In the early spring, when the desert is starting to come back alive, that would be a good time too,” Brossy said, because colors on the bank above contrast nicely with the basalt.
But in any season, sunlight inside this narrow, deep gash can challenge even skillful photographers. “The lighting is really tricky, and you have to be there at just the right time,” Brossy said.
On our Jan. 13 hike, Smith acted like a kid in a candy shop, lighting shot after shot with strobes until extreme cold drained his batteries.
All that bold and elegant beauty had only 10,000 years to form.
The Big Wood River channel cuts through 800,000-year-old basalt, in places eroding through to a depth of more than 50 feet and exposing at least three lava flows. But that’s not the river’s ancient route.
Lava that flowed from Black Butte Crater 10,000 years ago — now dubbed the Shoshone lava field — displaced the river, forcing it to carve a new channel southeast of Black Butte. So today’s sculpted features in the rerouted channel are younger than the 10,000-year-old lava flow whose perimeter it follows.
While Smith photographed those carvings, my husband scanned the canyon-bottom beds of polished pebbles, dropping a few gorgeously colored stones into his pockets. Their variety was stunning, and many had fine fractures filled with veins of white quartz.
So why, in the middle of a huge lava field, could he engage in a southern-Idaho version of beachcombing?
The Big Wood River is fed by streams draining the Boulder, Pioneer and Smoky mountains to the north. So those mountains — before the 1907-10 building of Magic Dam — were the source of the pebbles and cobbles that sculpted the Big Wood channel. During an ice age some 7,000 to 10,000 years ago, fast-flowing water from melting glaciers moved large loads of that sediment, a powerful sculpting force.
Masses of those small, polished rocks are collected along the channel bottom or trapped in the very potholes they helped to grind out. Quartzite and granitic rocks account for the biggest percentages, but geologists list at least seven other rock types among them.
You don’t have to remember your high school geology lessons, though, to appreciate their diversity — or their honeycomb artwork on the lava walls.
As waters drove and swirled those pebbles century after century, new potholes in the basalt overlapped older ones. Undercut by erosion, boulders fell from the rim to be resculpted on the channel bottom. Cut through by bigger holes, pothole relicts were left hanging or suspended on the channel walls.
The result is a complex wonderland of whimsical shapes — and an intriguing cutaway view of lava flows.
Each footstep reveals another breathtaking swirl of rock, another moment in the story of the Snake River Plain’s violent creation.
A source for much of this story’s geological information is “Rock & Potholes of the Big Wood River,” by Terry S. Maley and Peter Oberlindacher, Idaho Geological Survey, 1994.