Tuesday, November 15, 2011

When Winter Met Summer

Posted By on Tue, Nov 15, 2011 at 11:25 AM

click to enlarge fish-kissing-by-joel-lueders.jpg

"Kissing fish" by Joel Lueders

We always knew there was something peculiar about our Eel River steelhead -- those two "standoffish fish clans," as writer Sid Perkins refers to the Eel's winter run and summer run steelhead in a story this week in Science Magazine's online edition.

They don't mingle, they hang out in different parts of the Eel, they come and go at different times of the year. Yet, uncommonly, they're oddly close kin.

Now scientists have a pretty good theory as to why: Apparently, about 25,000 years ago, the two runs were thrown together by geologic calamity and for the next several hundreds of years the two runs, gack, consorted with each other.

And what was this earthy calamity? No surprise: a humongous collapse-and-slither of hillside that blocked the river. Scientists "estimate that 36 million cubic meters of rock -- enough to fill the Superdome in New Orleans more than 10 times -- broke loose and rumbled down to form a 140-meter-tall natural dam."

A huge lake formed behind the dam, goes the theory, and the steelhead runs were stuck together on the downstream side of the dam. A team of geomorphologists recently pieced together this puzzle by using high-resolution digital topography to study a band of thick, layered-sediment terraces about 140 meters or so above the river in the Eel River Valley. They weren't ocean deposits or river deposits, but lake deposits.

Eventually the dam eroded, the lake disappeared, and the two trout types went their separate ways again -- the whole big saga presenting "an exciting snapshot of evolution in action," says University of Washington, Seattle, geomorpohologist David Montgomery in the article.

The family stories those trout must tell...

 

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Heidi Walters

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Heidi Walters has been a staff writer with the North Coast Journal since 2005.

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