THE SOUTHWEST WASHINGTON SHELF: IS IT A SOURCE FOR BEACH SAND?

The sediment of the continental shelf and barrier coast along northern Oregon and southern Washington was derived mainly from the Columbia River during the Holocene. The barrier and beach deposits of this 150-km section of coast comprise 3-4 km³, while, by contrast, the shelf deposit is approximately 80 km³. Understanding how sand is partitioned throughout this system and if there is exchange between the large shelf deposit and the beaches is important because the beaches, which had accreted for 5,000 years, recently have undergone severe erosion in localized areas. Seismic-reflection, sidescan-sonar, and surface sediment data were used to map the geology of the shelf to determine if it is a potential supply of sediment to the adjacent beaches. These data show that the shelf deposit is not uniform in distribution or composition along this entire section of coast. It is 15-50 m thick off the beaches of the southern part of the study area but is thin to absent in the northern third of the area. Surface sediment texture of the shelf deposit varies as well. Pleistocene-aged gravel covers much of the inner shelf in the northern third of the area and is coarser than sediment on the adjacent beaches. To the south, the Holocene-aged shelf deposit is composed of fine sand near shore that fines offshore. Material coarser than medium sand (>2 phi) comprises on average 10% of beach samples but averages less than 3% of the shelf deposit (4-74 m water depths). Although a huge volume of sediment covers the shelf, its uneven distribution indicates that in places only small amounts are available to the adjacent beaches, and in other places its finer-grained nature indicates that significant winnowing would be necessary to make it compositionally equivalent to sediment on adjacent beaches.