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Near the southern boundary of Yellowstone National Park is a curious location—a pass on the Continental Divide that is so gentle, fish can swim across, moving from the Pacific to the Atlantic watershed!

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Annie Carlson, Research Coordinator at the Yellowstone Center for Resources, Yellowstone National Park.

Map of the Continental Divide in North America
Map of North American showing the Continental Divide -- an invisible line that separates the major watersheds flowing to different oceans.

The Continental Divide of North America is a crest that stretches from Alaska south through the Canadian Rockies, the contiguous 48, and down through Mexico. It is an invisible line that separates the major watersheds flowing to different oceans. In general, if a raindrop falls on the west side of the Continental Divide, it will join rivers that wind their way to the Pacific Ocean. If a raindrop falls on the east side of the divide, it will flow to the Atlantic or Arctic Oceans. Of course, there are many more possible fates that could befall a water droplet, but you get the idea.

In the contiguous 48, the Continental Divide follows the spine of the Rocky Mountains for ~3,000 miles. Trails and roads through the Rockies frequently cross the divide. In fact, driving the main highways in Yellowstone National Park, you cross it in three different places! You can even hike the entire length of the divide from the Mexican border to the Canadian border on the Continental Divide National Scenic Trail… if you have five months to spare. In some places, the divide is obvious, like a high mountain pass clearly separating the watersheds. But in other locations, the divide is barely perceptible. One of these unique, indiscernible locations is tucked into a wild corner of the Greater Yellowstone Ecosystem, named Two Ocean Pass.

Located in Wyoming’s Bridger-Teton National Forest, the high alpine meadow at Two Ocean Pass doesn’t look like a pass at all. The profile of the pass is so low that when the meadow is flooded in the spring during a wet year, a fish can swim from the Pacific watershed into the Atlantic watershed! This is how the Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri) is believed to have originally colonized Yellowstone Lake following the end of the Pinedale glaciation about 14,000 years ago. As the glacial ice receded, fish were not able to swim upstream in the Yellowstone River all the way to Yellowstone Lake due to the natural barrier of the Lower Falls. Thus, cutthroat trout likely colonized the lake by swimming across Two Ocean Pass and then were the only trout species living in the lake for thousands of years.

Map of Two Ocean Pass and the Parting of the Waters
Map of Two Ocean Pass and the Parting of the Waters. Pacific Creek flows southwest toward the Snake River, and Atlantic Creek flows northeast toward the Yellowstone River. The green line is the Continental Divide.

Two Ocean Pass is not only relevant to the dispersal of native Yellowstone cutthroat trout, but also to nonnative fish species. A recent study investigated a hypothesis that nonnative lake trout (Salvelinus namaycush) could have invaded Yellowstone Lake by swimming across Two Ocean Pass rather than the original assumption that they were illegally introduced by a person. Researchers used environmental DNA sampling, electrofishing, and angling to determine if lake trout or other nonnative fish were present in the waters near Two Ocean Pass, finding that it was a viable hypothesis. While the origin of lake trout in Yellowstone Lake remains uncertain, the potential for nonnative fish to cross Two Ocean Pass in the future presents a serious problem. To protect the native Yellowstone cutthroat trout in Yellowstone Lake, it may be necessary to continue monitoring these waters and proactively removing nonnative fish.

In addition to being a fish passage, another fascinating feature of Two Ocean Pass is the Parting of the Waters, a National Natural Landmark. Here, North Two Ocean Creek branches into two streams exactly along the Continental Divide, forming the aptly named Pacific Creek and Atlantic Creek. An old wooden sign posted at the Parting of the Waters indicates that it is 3,488 miles to the Atlantic Ocean and 1,353 miles to the Pacific Ocean, quite the “choose your own adventure” for a water droplet.

With thousands of miles to explore, the Continental Divide offers countless fascinating stories of human history, geology, hydrology, ecology, and more!

Further reading: Koel, Todd M., Colleen R. Detjens, and Alexander V. Zale. "Two Ocean Pass: An Alternative Hypothesis for the Invasion of Yellowstone Lake by Lake Trout, and Implications for Future Invasions.Water 12.6 (2020): 1629.

The Parting of the Waters, on the Continental Divide south of Yellowstone National Park
The Parting of the Waters, looking southeast. North Two Ocean Creek splits into Atlantic Creek, flowing to the left in the photo, and Pacific Creek, flowing to the right. The wooden sign indicates that it is 3,488 miles to the Atlantic Ocean and 1,353 miles to the Pacific Ocean.
Alpine meadow near Two Ocean Pass, just south of Yellowstone National Park
Alpine meadow near Two Ocean Pass on the Continental Divide just south of Yellowstone National Park.

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