The Columbia River is the dominant sediment source for the continental shelf near the northwestern United States. The Washington continental shelf is nearly covered by modern sediment derived from the Columbia River. This sediment moves generally northward away from the river mouth. Sand generally occurs at depths less than 90 m. Coarse silt occurs in deeper water. The modern sediments generally contain less than 1.5 percent CaCO 3 and less than 1 percent organic carbon. Relict sediment covers the continental shelf off southern Vancouver Island and occurs along the seaward margin of the continental shelf off southern Washington and northern Oregon. These relict sediments exhibit complicated patterns of grain-size distributions and, in general, contain more organic carbon and calcium carbonate than the modern sediment.