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The finding may help explain the genetic differences between two populations of steelhead trout.

Using remote-sensing technology known as airborne Light Detection and Ranging (LiDAR) and hand-held global-positioning-systems (GPS) units, researchers found evidence of the late Pleistocene, landslide-dammed lake along the river, about 60 miles southeast of Eureka, California.

The evidence for the ancient landslide, which, scientists say, blocked the river with a 400-foot wall of loose rock and debris, is detailed in a paper published online in Proceedings of the National Academy of Sciences.

Above, Ben Mackey in the field. Below, the location of the ancient lake bed in the Eel River. (Credit: University of Oregon)

The new research explains emerging evidence from other studies that show a dramatic decrease in the amount of sediment deposited from the river in the ocean just off shore at about the same time period, says lead author Benjamin H. Mackey, who began the research while pursuing a doctorate at the University of Oregon and is now a postdoctoral researcher at California Institute of Technology (Caltech).

“Perhaps of most interest, the presence of this landslide dam also provides an explanation for the results of previous research on the genetics of steelhead trout in the Eel River,” Mackey says, referring to a 1999 study by U.S. Forest Service researchers J.L. Nielson and M.C. Fountain.

In that study, published in the journal Ecology of Freshwater Fish, they found a striking relationship in two types of ocean-going steelhead in the river—a genetic similarity not seen among summer-run and winter-run steelhead in other nearby rivers.

An interbreeding of the two fish, known as genetic introgression, may have occurred among the fish brought together while the river was dammed, Mackey says. “The dam likely would have been impassable to the fish migrating upstream, meaning both ecotypes would have been forced to spawn and inadvertently breed downstream of the dam. This period of gene flow between the two types of steelhead can explain the genetic similarity observed today.”

Once the dam burst, the fish would have reoccupied their preferred spawning grounds and resumed different genetic trajectories, he says.

“The damming of the river was a dramatic, punctuated affair that greatly altered the landscape,” says co-author Joshua J. Roering, professor of geological sciences at the University of Oregon.

“Although current physical evidence for the landslide dam and paleo-lake is subtle, its effects are recorded in the Pacific Ocean and persist in the genetic make-up of today’s Eel River steelhead. It’s rare for scientists to be able to connect the dots between such diverse and widely-felt phenomena.”

The lake’s surface formed by the landslide, researchers theorize, covered about 12 square miles. After the damn was breached, the flow of water would have generated one of North America’s largest landslide-dam outburst floods. Landslide activity and erosion have erased much of the evidence for the now-gone lake. Without the acquisition of LiDAR mapping, the lake’s existence may have never been discovered, researchers say.

The area affected by the landslide-caused dam accounts for about 58 percent of the modern Eel River watershed. The river today is 200 miles long, carved into the ground from high in the California Coast Ranges to its mouth in the Pacific Ocean in Humboldt County.

Based on today’s general erosion rates, researchers theorize the lake could have been filled in with sediment within about 600 years.

“The presence of a dam of this size was highly unexpected in the Eel River environment given the abundance of easily eroded sandstone and mudstone, which are generally not considered strong enough to form long-lived dams,” Mackey says.

He and his colleagues were drawn to the Eel River—among the most-studied erosion systems in the world—to study large, slow-moving landslides.

“While analyzing the elevation of terraces along the river, we discovered they clustered at a common elevation rather than decrease in elevation downstream, paralleling the river profile, as would be expected for river terraces. This was the first sign of something unusual, and it clued us into the possibility of an ancient lake.”

Michael P. Lamb, professor of geological and planetary sciences at Caltech is also an author on the paper. The National Center for Airborne Laser Mapping provided LiDAR data used in the project. Additional funding support came from the Keck Institute for Space Studies and the National Science Foundation.

More news from University of Oregon: http://uonews.uoregon.edu/