Saturday, June 14, 2008

Double Bluff

The bluff rising above the beach has been eroding for thousands of years and the beach has been moving back with it. The erosion has been gradual, persistent, inevitable. Every few years chunks of the bluff collapse on the beach, forming piles of talus to be gradually removed by the waves. In another 100 years, the bluff will look much the same, but it will be located a few tens of feet farther inland.

Beaches and bluffs retreat as a system. Efforts to protect the toe of the bluff from further erosion fundamentally disrupt this system. They decouple the beach from the bluff: one stops, the other keeps moving. We tend to focus on specific effects of armoring - loss of sediment supply, changes to riparian vegetation patterns, modification of beach substrate that could impact forage fish spawning or infauna abundance. Some of these may be minimized or mitigated with creative design, but I think the real problem is this systems problem. The system has to move to stay the same. For our grandchildren to see the beach as we see it today, it must be allowed to erode.

The views are spectacular from this beach - to the south (at least on a clear day) Mount Rainier looms above the distant Seattle skyline and to the west the Olympics are seen over Foulweather Bluff.

Double Bluff has two parts (hence its name, I suppose). This is the northwestern portion - at the southern end of Mutiny Bay. The layers exposed in this bluff record a complicated history of glaciations. This is the type section of the Double Bluff Drift - the spoils of an ice age that occurred 100,000 years before the most recent one. 1-2 miles to the east lie the high sandy bluffs that mark the eastern portion of Double Bluff (April 2006).

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