Image 1 - Ancient Landslide Explains Genetic Change In Trout
November 15, 2011

Ancient Landslide Explains Genetic Change In Trout

Scientists at two West Coast universities reports that a catastrophic landslide 22,500 years ago may explain genetic changes in steelhead trout.

The team said the landslide dammed the upper reaches of northern California's Eel River, which is 200 miles long today.  The landslide blocked the river with a 400-foot wall of loose rock and debris.

The river is carved into the ground from high in the California Coast Ranges to its mouth in the Pacific Ocean in Humboldt County.

Lead author Benjamin H. Mackey said the study helps explain 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.

"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 said in a press release.

The researchers found a relationship between two types of ocean-going steelhead in the river.  An interbreeding of the two fish may have occurred while the river was dammed, Mackey said.

"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," he said in a press release.

Mackey also said that once the dam burst, the fish would have reoccupied their preferred spawning grounds and resumed different genetic trajectories.

"The damming of the river was a dramatic, punctuated affair that greatly altered the landscape," co-author Joshua J. Roering, a professor of geological sciences at the University of Oregon, said in a press release. "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."

Researchers believe the lake's surface formed by the landslide covered about 12 square miles.  They said that after the dam was breached, the flow of water would have generated one of North America's largest landslide-dam outburst floods.

They say that landslide activity and erosion erased much of the evidence for the lake, and without the acquisition of LiDAR mapping, the lake may have never been found.

The area affected by the landslide caused dam accounts for about 58 percent of the modern Eel River watershed.

The team believes based on today's general erosion rates, that 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 said in a press release.

The researchers said they were drawn to Eel River 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," Mackey said in a press release. "This was the first sign of something unusual, and it clued us into the possibility of an ancient lake."

The research was published in the Proceedings of the National Academy of Sciences.


Image 1: The Eel River at the dam site. Much of the evidence for the dam has been eroded over time. Credit: Ben Mackey, Caltech

Image 2: View down the Eel River, with the reconstructed ancient lake surface in blue. Credit: Ben Mackey, Caltech

Image 3: Fine silt and mud were found upstream, indicating sediments in the still waters of a lake. Credit: Ben Mackey, Caltech


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