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Strange Course of Idaho's Rivers

If we could trace the history of a river over time, we might better understand the clutter and chaos of our wild geology. Each bend, each change of direction signifies a challenge that the river had to overcome on its way to the ocean. And Idaho's two main rivers — the Snake and the Salmon — have faced a multitude of challenges!

The Snake currently flows east to west across the entire southern portion of the state, before turning north into Hells Canyon, on its way to the Columbia and the Pacific Ocean. But there's a complicated story waiting to be told about this relatively young river.

"The evolution of the Snake River is a very complex thing, and it changed markedly through time," says Bill Bonnichsen. "The Snake River is a collage of many parts of many pre-existing rivers, and it's been integrated only in the last few hundred thousand years into its present form. You can look at certain parts of it, and you can deduce that the water had run in the opposite direction at times. And there are people who propose that somewhat earlier in eastern Idaho there were rivers that ran from north to south across that direction that have nothing to do with the present Snake River drainage."

Take Hells Canyon, for example. It's hard to imagine a Hells Canyon without the Snake River coursing through it. This is one of the deepest river gorges in the world — much deeper than the Grand Canyon — and every Idahoan has grown up believing that the Snake is the primary reason.

Hells Canyon [Credit: Idaho Public Television]

But the evidence suggests that the Snake is a relative newcomer to this canyon, and that at one time, Hells Canyon waters actually flowed south, not north. "And the evidence for that is, if you look at all the tributaries to the Snake River in the southern part of Hells Canyon," says Bonnichsen, "they actually are flowing toward the south, and the whole thing was going to the south.

"At that time Lake Idaho likely drained in some other direction, rather than through Hells Canyon, because Hells Canyon had yet to be cut to its present depth. In fact, part of the course of what became the Snake River, in the upper reaches of Hells Canyon, likely flowed south into Lake Idaho, as indicated by how many of the Snake's tributaries in that area branch in a way suggesting that flowage was to the south. At that time, those millions of years ago, the outlet of Lake Idaho was in some other direction, not down the present course of Hells Canyon."

In time, the river did find a way to drain massive Lake Idaho through Hells Canyon, and the cutting down through rock commenced in earnest about two million years ago. The Bonneville flood, which occurred about 15,000 years ago, didn't do much to deepen the river, but it certainly widened it. You can see the effects of that one-time deluge in the broad terraces and the gravel bars, hundreds of feet above the present level of the river.

With each passing year, the Snake is deepening Hells Canyon; but since the dams were installed, it's not doing it nearly as quickly. That's because the dams restrict the abrasive sediments that give water such a potent advantage over rock.

"Once the drainage down the course of Hells Canyon was established, erosion took over, cutting the canyon deeper and deeper. This went on for a few million years, eventually leading to the complete draining away of Lake Idaho, lowering of the base level in the southwest Idaho region so that other canyons, such as those of the Bruneau and Owyhee Rivers, could be excavated, and the downcutting of Hells Canyon to its incredible depth as the deepest gorge in North America could take place."

Rafting the Salmon River [Credit: Idaho Public Television]

Every rafter knows how quickly a log jam or a new rapid can alter a river, or a well-planned vacation. Now try to imagine what a lava flow or an uplifted mountain can do. That's why Idaho's undammed masterpiece, the Salmon River, travels in every direction of the compass — east, north, west, and south.

"The Salmon River has a very tortuous course," says Bonnichsen. "When the Columbia river basalt was erupted just a few million years ago, it was flowing from east to west. Here come the basalts, packed in against it. It dammed it up so now the river has a right angle bend at Riggins and goes all around like that and goes into the Snake."

Geologists suggest that, before nature conspired against it, the Salmon was likely Idaho's main river, with the much shorter Snake River a tributary to the River of No Return. "Yes, I think that for a while, millions of years ago, the ancestral Snake River was a tributary to the Salmon. But the Snake of that time was much shorter and less voluminous than the present Snake River," explains Bonnichsen. "Now, because it has gotten larger and larger through time, the Snake is the main river and the Salmon is its principal tributary, even though it started out that the ancestral Snake was a minor tributary of the Salmon."

Text by Bruce Reichert