The Lake That Shouldn't Have Fish

High in the Rocky Mountains sits Yellowstone Lake — three hundred forty square kilometres of cold water, two thousand three hundred and fifty eight metres above the sea. And it is full of trout. Which is strange, because for most of the last fourteen thousand years, no fish should have been able to get in.

Here is the obvious way in. Water leaves the lake as the Yellowstone River, and runs north toward the Missouri. So a fish from the plains should simply be able to swim up it. Let us measure that climb. Reading elevation for every reach of the river, one after another: over the first hundred and eighteen kilometres downstream of the lake, the Yellowstone falls eight hundred and thirty seven metres.

And it does not fall gently. Fifty seven kilometres below the lake, the profile does this. In a single reach — ninety metres of river — the water drops fifty four metres straight down. That is the Lower Falls of the Yellowstone: a wall of falling water taller than a twenty storey building. No trout climbs that. The front door is locked, and it has been locked since the canyon was cut.

So where did the fish come from? The trout in Yellowstone Lake are cutthroat trout. And their closest relatives do not live downstream at all. They live on the other side of the continental divide, in the Snake River — water that drains to the Pacific Ocean. Which should make them unreachable. Different ocean. Different world.

Unless there is another door. Sixty kilometres south of the lake, at a place called Two Ocean Pass, a single creek arrives at the continental divide and splits in two. One arm becomes Pacific Creek. The other becomes Atlantic Creek. The pass is so flat that in the spring of a wet year, the meadow floods and the two creeks join. For a few weeks, a fish can swim across the spine of the continent.

And follow each arm all the way to the sea, and the scale of that split becomes clear. The left arm runs two thousand two hundred kilometres to the Pacific. The right arm runs five and a half thousand kilometres to the Gulf of Mexico — two and a half times farther, on the opposite side of the continent. Two raindrops, a few steps apart, ending oceans apart. But our trout is not headed for either ocean.

And now trace what waits on the far side. From the pass, down Atlantic Creek, into the upper Yellowstone River, and north into the lake: ninety five kilometres.

We measured the elevation of every reach along it. The largest climb a fish faces on that entire route is zero metres. It is downhill the whole way. Because the back door opens above the falls — inside the locked room.

That is the accepted explanation for the trout. After the glaciers retreated, roughly fourteen thousand years ago, cutthroat trout from the Pacific side crossed at Two Ocean Pass and settled the upper Yellowstone. For thousands of years afterwards, they were the only trout in the lake. An entire fishery, and the grizzlies, eagles and otters that feed on it, resting on one flooded meadow.

But a door does not care which way you walk through it. Today Yellowstone Lake holds lake trout — an invasive predator that has driven the native cutthroat into steep decline. They were long assumed to have been dumped there by people. Then researchers publishing with the United States Geological Survey raised an alternative: that lake trout may have arrived the same way the cutthroat did, over Two Ocean Pass, from the Snake River side. That idea is a hypothesis, not a settled fact. But if it holds, the door that gave Yellowstone its trout is the same door taking them away.

Every measurement of water in this story — the eight hundred and thirty seven metre climb, the fifty four metre wall, the ninety five kilometre route that never once goes uphill — came from one public dataset that maps every stream in the country. The fish were the mystery. The landscape held the answer.