Sunday, April 08, 2007

Alki Point

Flying into Sea Tac last Monday from Sacramento, I got these shots of West Seattle through a smudged window. I'm often asked whether Alki Beach - visible on the northern shore of the point - is a nourished beach. Based on it's setting, and supported by some historical pictures, I think not. Alki is swash-aligned - it is oriented perfectly into the maximum northerly fetch and the geometry of Duwamish Head on the east and Alki Point on the west keep the sand from escaping. This is a recipe for a stable beach in Puget Sound.

Alki's story is a bit more complicated, however, since this whole section of West Seattle was lifted 20' out of the water about 1100 years ago when the Seattle Fault last shrugged. We believe that Alki was previously a double tombolo, with spits tying the little knoll near the tip of the point to the mainland. There may have been a salt marsh behind it. The evidence for this scenario is better on Restoration Point on Bainbridge - same geography, but not as densely developed. The earthquake lifted the entire landform out of the intertidal, resulting in a low point suurrounded by an eroding bluff ten? feet or so in elevation. This may have contributed to the isolation and stability of Alki Beach.

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