Saturday, December 7, 2013

Washington's Largest Dry Watershed

Cold Creek is the largest dry watershed in Washington based on my meanderings and map reviews. The Cold Creek watershed drains off of the eastern south slope of Umtanum Ridge and the north slopes of the Rattlesnake Hills and Rattlesnake Mountain.
Cold Creek drainage area outlined in blue
The drainage path is from west to east

The watershed happens to be located in the driest area of Washington State with annual rainfall at Hanford less than 7 inches. Most of the drainages in this area are dry most of the time so it is not a surprise that Cold Creek is dry. However, it becomes rather impressive when the size of the drainage is considered; its a big area not to have surface water flow.

The upper part of the drainage does show evidence of periodic flow.

Base of slope has been under cut by periodic flows on upper Cold Creek drainage

Concrete over flow structure at drainage crossing near St Michelle Vineyards

With such a large drainage area with steep ridges an occasional just right storm can get water flowing in the upper half of the drainage. The most likely scenario is rare heavy summer rain storms. Cloud burst floods do happen in eastern Washington, but are very rare at any given location and much less frequent than other desert areas to our south. However, in the case of Cold Creek a road engineer must have thought it prudent to design the road for occasional flooding.

While cloud burst induced stream flows may take place in the western half of the drainage, getting the flow of water as surface water all the way to the Yakima is very unlikely and by appearance of the lower Cold Valley exceedingly rare. The lower half of Cold Creek follows a broad ice-age flood channel that underlain by sand and gravel and hence water flows that reach this area readily infiltrate into the ground versus surface water flow.

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