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Geology of the National Parks

GEOLOGY OF OLYMPIC NATIONAL PARK:
PART I OLYMPIC GEOLOGY

Running Water: The Persistant Sculptor

Anyone who has stood at the brink of an Olympic gorge and watched the foaming river at the bottom knows that running water is king in this land (fig. 28a). From the very instant that the part of the earth's crust that is now the Olympics pushed up above the sea, rain and melted snow have been carrying bits of it back down again. As the land rose and the growing mountains intercepted more moisture, trickles grew to streams, and streams grew into rivers.

Elwha River at Gobblin's Gate
Fig. 28a. Elwha River in the gorge at Gobblin's Gate

The running water cut deeper and deeper into the rising land mass. Several factors contribute to a river's ability to cut: the amount of water it carries, the steepness of its descent, and, very important, the load of silt, sand, and gravel, which are its cutting tools. For example, an Olympic river that is laden with silt, sand, and gravel supplied by glaciers cuts bedrock faster than does a clear river not fed by glaciers.

The radial drainage pattern of the Olympics results from the domal uplift: as the Olympic land mass bowed up from the sea, water tended to run off in all directions. Rivers that had some advantage over their neighbors in rate of erosion extended headward more rapidly and captured more drainage area. The Elwha, which appears to breach the very center of the uplift (fig. 29), may have captured more drainage than the other rivers because it had a shorter, steeper course to the sea. The eastern rivers are short today, but they must have had a longer trip to the sea before the Cordilleran ice sheet carved out Hood Canal.

 
Development of basic drainage pattern
Fig. 29. Development of the basic drainage pattern of the Olympics

Material in this site has been adapted from Guide to the Geology of Olympic National Park by Rowland W. Tabor, of the USGS. It is published by The Northwest Interpretive Association, Seattle.

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