Open Wide: Examining the Mouth of the Columbia River By

The mouth of the Columbia River was not always where it is today. During the last 10,000 years, the local rate of sea-level rise has decreased substantially. From 10,000 to 8,000 years ago, sea level rose from about 55 to 24 m below present mean sea level, averaging 1.6 cm/year. In the last 5,000 years sea level rose to its present stage at an average rate of only.16 cm/year (Gelfenbaum et al., 1999). Consequently, the mouth of the Columbia River originally extended 11 miles (18 km) further west across the continental shelf, and its waters flowed into what now is Astoria Canyon, a submarine canyon and fan-valley system off the Oregon coast (Niem, 2001). Astoria Canyon, approximately 75 miles (121 km) long, crosses the seaward half of the continental shelf and down the continental slope into Astoria Seachannel, a fan valley. The canyon varies in width from 1.5 to 8.3 miles and is entered by 13 tributaries. The movement of the Columbia River sediment down to the fan valley formed the tributaries (Gelfenbaum et al., 1999). During autumn and winter, when the river levels are low, the ocean water tides reach up to 140 miles upriver, and the ebb tides take the river water miles out in the ocean