Modern Sediments of Willapa Bay, Washington: A Coastal Plain Estuary

摘要

Willapa Bay, a coastal plain estuary, contains extensive tidal flats cut by channels and distributaries. Well-sorted fine sand occurs throughout the bay, with increasing amounts of silt- and clay-size sediment present on the tidal flats near the sourht end of the bay and adjacent to the mouths of rivers flowing into the bay. The channels are generally scored of this fine sediment by strong bottom currents. Organic carbon and organic nitrogen contents increased with decreasing mean grain-size, and the ratio of carbon to nitrogen revealed two types of organic matter in the bay. One type, associated with the fine-grained sediment on the tidal flats, had an average ratio of 13.8; the other type, associated with the coarser channel sediments, had an average ratio of 6.1. At the bay entrance a littoral drift to the north results in extensive shoaling north of Leadbetter Point and erosion on Cape Shoalwater. A southward migration of the channel through the tidal delta off the bay entrance indicates a littoral drift to the south in this area. Spits and shoals forming inside the bay entrance are a result of a net influx of fine sand into the bay from the eroding cape. The distribution of brackish-water to marine suites of Foraminifera is closely related to the sedimentary environments in the bay. Heavy minerals indicate source areas for the sediment from the Columbia River drainage basin as well as the Willapa Bay drainage basin. Fine sand on the tidal flats east of Long Beach Peninsula is a relict of the growth of the spit. (Author)