Tuesday, September 21, 2010

Down in the flood

A moment of chaos...

Imagine this:

It's 17,000 years ago and you are a wooly mammoth grazing in a region which, someday, will be called Eastern Washington. Things are looking up. The climate is getting warmer and the melting ice sheets have uncovered large new tracts of land.

The same melting ice has pooled in a giant lake, one that is now half the size of modern Lake Michigan. This melted ice water is trapped behind an ice dam.

But, on this bright Spring day, the dam begins to leak, and then, suddenly fails, unleashing a wall of water hundreds of meters high.

In the distance you hear a roar, and then, raising your matted head to face the oncoming deluge, you are swept up in the roiling waves, and moments later your consciousness is extinguished...

It happened many times...
Geologists believe floods of this variety occurred around 40 times during the time period from 18.6-15.9 thousand years ago. This evidence comes from multiple high water marks in the Glacial Lake Missoula basin, and stacked layers of sediments from high-energy flooding events on land and in deep-sea sediment cores off the coast of Washington.

The land
Eastern Washington is dominated by features from the Missoula Floods. During late August, I spent two days in Douglas County looking at erosional and depositional features from the flood. Here are some pictures and short descriptions:

This picture shows mega-ripples along the banks of the Columbia River. Ripples form from the agitation of particles in a current-driven or wave-driven fluid. For example, you can witness structures like those depicted in this picture forming in wave-modulated currents along the ocean coast. However, those ripples are typically a few centimeters in height and spaced a few tens of centimeters apart. The sheer dimensions of these mega-ripples indicates a flow strength many orders of magnitude greater than any ocean current.

Dry Falls. During the floods this waterfall would have been greater than any waterfall which exists in the modern world. Water flowed 300 feet above the upper surface of the falls.

A classic formation of the Channeled Scablands. The floods carved deep incisions into the basalt which covers much of Eastern Washington. The majority of these heavily eroded sites is now dry. The shaping of this landscape occurred during a few catastrophic events.

Here is a layer of unconsolidated sediment which was deposited by some major flooding event. The geometry indicates a rapid flow with a lot of entrained sand and silt.

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