Barrier Systems of California, Oregon, and Washington

Summary

West Coast barriers have formed under a different geologic setting than the more familiar barriers of the Gulf and East coasts and, correspondingly, have a somewhat different appearance. All the West Coast barriers are spits, generally short because of the ruggedness of the coast, and have only one inlet because of the relation between their outflow (tidal prism and/or river discharge) and the local wave climate. Although West Coast barriers may have been larger during periods of lower sea level when the shoreline was more regular, variations in the present coastline morphology and sediment supply, which are due to the high stand of sea level and to the active continental margin, control their present shape.

The two examples of barrier systems from California illustrate the difference between barriers at the mouths of large, sediment-laden rivers and barriers fronting large coastal bays. They also show that tectonic activity is a minor factor in determining barrier type or size bacause the Tijuana River-Silver Strand barrier system is in a tectonically stable area and the Eel River-Humboldt Bay system is in a tectonically active one. The Tijuana and Eel Rivers both deliver large amounts of sand to the ocean, and spits have formed at their mouths. The interaction between stream flow, which is seasonally quite variable, and waves leads to a less stable spit than does the interaction between tidal flow and waves. The Tijuana River has a short period of high discharge followed by a long period of low discharge. During high discharge, the river controls the shape of the spit; during low discharge, the waves control it. Over a year, the balance between waves and tides is more constant than that between waves and stream flow, and neither force is able to shape the spit for long periods of time. The resulting river-mouth spits are more dynamic and the bay spits are more stable; the two barriers of each type are similar to each other even though in regions of very different tectonic activity.

The Oregon and Washington barriers further illustrate the importance of coastal morphology on barrier size. The barriers at Willapa Bay, Washington, and at Gray’s Harbor to the north have formed along coastal lowlands between widely separated headlands. Their large size is also due to their close, down-drift proximity to the Columbia River, one of the largest sediment sources on the West Coast. Conversely, the Oregon coast has neither the long stretches of coastal lowlands nor the large sediment source of the Columbia River. The Oregon barriers are smaller because they front estuaries at the mouths of coastal rivers that are confined by prominent, closely spaced headlands.