Twenty-two thousand years ago, a lake was formed, burst its dam, and washed away. Recently, two sciences came together to re-discover this short-lived chunk of water. Find out how even a brief rush of water and rock left genetic and geological traces that were picked up millenia later.

Sometimes the sciences get all in a tangle. Things as different as genetics and geology influence each other. A 1999 survey of fish, published by the journal Ecology of Freshwater Fish, found that two kinds of steelhead trout had converged, genetically, in the Eel River in California. These fish traveled from the ocean into the rivers to spawn, and did so without genetic crossover in every other river in the region. Even in the Eel River, they spawned in very different sections of the river.


When one plant releases a cloud of pollen, or a fish just leaves its eggs in a crag along a river bank, the effort is wasted, evolution-wise, if that genetic material simply hooks up with a different kind of organism. So one group will only spawn at a certain time, or at a certain altitude, keeping clear of its near-relatives. This isn't a strategic effort on the part of the plants and animals. It's just that, if they didn't do just this kind of separation, they would have already converged by the time scientists took it into their heads to study them. The fish in the Eel River kept separate, and seemed always to have done so, so why did their genes cross over?

The answer didn't come from genetics, or biology at all. Instead, geologists using LiDAR found evidence of a massive landslide near the river. In a paper to be published in the Proceedings of the National Academy of Science, they explain that this would have dumped huge amounts of sandstone into the bed of the river and blocked most of it, causing a 12 mile long lake to form. It also would have blocked one set of steelhead trout from heading up to its preferred breeding ground. Forced to make do, it would have spawned much lower down the river than it usually did, in the spawning grounds of the other variety of trout. Their genes would have been mixed, and the two converged.

There's no definite lifespan given for the lake, but scientists believe it would have been filled with sediment in 600 years. Sometime before that happened, the 'dam' created by the landslide burst, and the river returned to its natural state. The fish returned to their natural breeding grounds. The two types of fish didn't noticeably converge, and the sandy, fast-eroding soil gave way enough that there was no visible evidence for the lake having ever existed. (It was this sandy soil that kept such a thing from happening in any other rivers in the region.) No one would have known about either without advanced genetics studies and LiDAR being used for geology. Two very different advanced sciences came to confirm the brief existence of a lake that was washed away 22,000 years before anyone today was alive.


And that? Is why science is awesome. (Also the smallpox vaccine, but that's a different article.)

Via University of Oregon and Ecology of Freshwater Fish.

Top Image of Dried Lake: Wing-Chi Poon

Top Image of Eel River: Humbolt Head