Early travelers to the area that became Yellowstone National Park found fish were abundant in the Yellowstone River and Lake and their tributaries, but many other lakes and streams were devoid of fish. At first, people thought heat and chemicals from geothermal features killed fish in some places. Then geologists offered another explanation.

The Yellowstone plateau, geologists said, was a huge sheet of volcanic rock left by a super volcano. Across eons, a giant glacier formed over the volcanic rock. When the ice age ended, the glacier melted washing away soft material but leaving hard volcanic rock. This formed a circle of waterfalls and cascades that fish couldn’t get over to populate the plateau.

The geologists’ theory explained the fishless waters, but it left a deeper mystery: How did fish get into upper Yellowstone and its tributaries? Certainly, they didn’t do it by climbing the 300-foot lower fall of the Yellowstone.

Then, people remembered Mountain Man Jim Bridger’s tale of the “Two Ocean Pass,” a place on the headwaters of the Yellowstone where creeks crossed the continental divide. Explorers had documented the existence of the pass, but it wasn’t until 1891 that the U.S. Fish Commission sent an ichthyologist to the area.

Here’s how Dr. Barton Warren Evermann described what he found at the Two-Ocean Pass.

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We stood upon the bank of either fork of Atlantic Creek, just above the place of the ”parting of the waters,” and watched the stream pursue its rapid but dangerous and uncertain course along the very crest of the “Great Continental Divide.” A creek flowing along the ridgepole of a continent is unusual and strange, and well worth watching and experimenting with.

We waded to the middle of the North Fork, and, lying down upon the rocks in its bed. We drank the pure icy water that was hurrying to the Pacific, and, without rising, but by simply bending a little to the left, we took a draught from that portion of the stream which was just deciding to go east, via the Missouri-Mississippi route, to the Gulf of Mexico.

And then we tossed chips, two at a time, into the stream. Though they would strike the water within an inch or so of each other, not infrequently one would be carried by the current to the left, keeping in Atlantic Creek, while the other might be carried a little to the right and enter the branch running across the meadow to Pacific Creek; the one beginning a journey which will finally bring it to the Great Gulf, the other entering upon a long voyage in the opposite direction to Balboa’s ocean.

Pacific Creek is a stream of good size long before it enters the Pass, and its course through the meadow is in a definite channel; but not so with Atlantic Creek. The west bank of each fork is low, and the water is liable to break through anywhere, and thus send a part of its water across to Pacific Creek. It is probably true that one or two branches always connect the two creeks under ordinary conditions, and that, following heavy rains, or when the snows are melting, a much greater portion of the water of Atlantic Creek finds its way across the meadow to the other.

It is certain that there is, under ordinary circumstances, a continuous waterway through Two-Ocean Pass of such a character as to permit fishes to pass easily and readily from Snake River over to the Yellowstone, or in the opposite direction. Indeed, it is possible, barring certain falls in Snake River, for a fish so inclined to start at the mouth of the Columbia, travel up that great river to its principal tributary, the Snake, thence on through the long, tortuous course of that stream, and, under the shadows of the Grand Tetons, enter the cold waters of Pacific Creek, by which it could journey on up to the very crest of the Great Continental Divide to Two Ocean Pass; through this Pass it may have a choice of two routes to Atlantic Creek, in which the down-stream journey is begun. Soon it reaches the Yellowstone, down which it continues to Yellowstone Lake, then through the lower Yellowstone out into the turbid waters of the Missouri. For many hundred miles, it may continue down this mighty river before reaching the Father of Waters, which will finally carry it to the Gulf of Mexico—a wonderful journey of nearly six thousand miles, by far the longest possible fresh-water journey in the world.

We found trout in Pacific Creek at every point where we examined it. In Two-Ocean Pass, we obtained specimens from each of the streams, and in such positions as would have permitted them to pass easily from one side of the divide to the other. We also caught trout in Atlantic Creek below the Pass, and in the upper Yellowstone, where they were abundant.

Thus it is certain that there is no obstruction even in dry weather to prevent the passage of trout from the Snake River to Yellowstone Lake; it is quite evident that trout do pass over in this way; and it is almost absolutely certain that Yellowstone Lake was stocked with trout from the west, via Two-Ocean Pass.

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— Excerpt adapted from Barton Warren Evermann, “Two Ocean Pass,” Inland Educator 2(6): 299-306 (July 1896).

— U.S. Forest Service Photo.

— To learn more about Two Ocean Pass and fishing in Yellowstone in the 1870’s, check out my Big Sky Journal Article, “When All the Fish Were Natives.”