In coastal southwest Washington, there is an unusually large (largest along the Pacific coast) and thick Holocene sand barrier 38 km long by 2.0 to 3.5 km wide by 17 to 25 m thick. The barrier is 2 to 8 times thicker than previously described Holocene barriers along the North American coast, all of which have lower-magnitude tidal regimes. The Willapa barrier is influenced by (1) a 3.7 m upper-mesotidal regime, (2) high 4-7 m winter storm waves, and (3) a high sediment influx via northward longshore transport in winter from the nearby Columbia River mouth. From the perspective of oil reservoir modeling, assuming 20% porosity and 40% recovery, the barrier has a potential reservoir of recoverable fluids of one billion barrels.

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