Notes on Shoshone Falls


The Snake River is dammed within an inch of its life. It’s polluted by agricultural runoff. Diverted for irrigation. There are at least 15 dams along its length, most of them in Idaho. But once in a while it can still bust loose and show some serious water. And when it does, there’s no better place to see it than Shoshone Falls, outside of Twin Falls, Idaho.

Shoshone Falls is geologically recent. When Lake Bonneville cut down through the two alluvial fans descending from opposite sides of Red Rock Pass about 14,500 years ago, it let loose the largest known flood event in North American history. The flood promptly eroded away the lava dam at American Falls, releasing American Falls Lake. The combined flow of the Bonneville Flood tore down the Snake River Valley at an estimated peak of an incredible 33 million cubic feet per second. The water deepened and widened the Snake River Canyon. Softer rocks were torn out of the canyon walls and scattered across the Snake River Plain.

At Shoshone Falls, the flood waters eroded down and encountered an older rhyolite flow. Rhyolite is a little harder than basalt. And the canyon widened a bit just below the rhyolite dike. The result was a cliff some 212 feet high, with the Snake River pouring over it. The major flood event lasted only a couple of weeks at most. Shoshone Falls – and the other lesser falls along the Snake River – were created in less than a geological eye blink.

Some 14,500 years later, Shoshone Falls is significantly altered by humankind. There’s a diversion dam that send a substantial amount of water through electric turbines. In late summer, Shoshone Falls is more like Shoshone Trickle. But during early spring runoff, you get an idea what it must have looked like before humanity wrecked the Snake.

Shoshone Falls, Idaho, February 11, 2017

Shoshone Falls, Idaho, February 11, 2017

The total drop is 212 feet — 45 feet higher than Niagara Falls — and the river flows over a rim nearly 1,000 feet wide. You can see the lighter-colored rhyolite dike, a much paler grey against the dark grey-brown basalt.

Shoshone Falls, Idaho, Vertical Composition

Shoshone Falls, Idaho, Vertical Composition

You can see that the water is muddy with snowmelt and floodwaters from upstream. The Snake River drainage has had a lot of snow and rain. There’s local flooding in some of the Snake’s tributaries downstream. But you can also see it’s a thunderous amount of water.

Shoshone Falls was also the natural barrier to migration of anadramous fish. Salmon and steelhead couldn’t make it further upstream. Nowadays, of course, manmade dams bar upstream migration at Hell’s Canyon, much further downstream.

This much water at the Falls is an uncommon event today. Hundreds of folks were out Falls Road to have a look at the spectacle. The road down to the park and primary viewing area is in the shade of the steep canyon walls, and was pretty icy and closed to auto traffic. Maybe one person in four was willing to make their way down the intermittently icy road on foot. Which is kind of pitiful. But as a result the crowds were limited to the upper viewing area, with few folks down below.

Events like the Lake Bonneville Flood, the 20 or so Lake Missoula Floods and Glacial Lake Ahtna give uniformatarianismists a headache. But geology is messy. And, occasionally, spectacular.

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