Characteristics of Columbia River Sediment and Sediment Transport

Abstract

ABSTRACT The mean particle size of sediment carried on the bed of the Columbia River increases downstream. The Snake River and other upstream tributaries contribute mostly fine sediment carried in suspension. Many downstream tributaries produce coarser sediment, which is carried largely in the bedload. Bedload is appreciable only in the lower portions of the river, and is a small percentage of the suspended load Large migrating sand waves form the top surface of the bed. The distribution of sediment on some of the reservoir floors changes throughout the year. Sediment deposited during low discharges may be scoured from the reservoirs during periods of high discharge and transported downstream through the dams. The amount of sediment transported by the Columbia River system during a single flood may exceed that which is transported during an entire average year. The mineral and chemical composition of Columbia River sediment obtained from reservoir floors suggests that the sediment particles have undergone relatively little chemical weathering. Chemically mineral grains and lithic fragments abound. Quartz and elements considered relatively immobile during chemical weathering show no obvious enrichment in the sediment. The ratio of quartz to all unstable constituents (feldspars, mafic minerals, and lithic fragments) decreases downstream, whether the latter are taken individually or as a group. Columbia River sediment is apparently derived from two principal sources: the upstream sediment from surficial deposits is derived ultimately from metamorphic, plutonic, and sedimentary rocks; and downstream sediment is derived largely from the erosion of andesitic volcanic rocks and sediments. The upstream sediment is fine grained and is transported by the Columbia in suspension. The volcanic sediment is coarser and generally forms the bedload. It is likely that these two sediment types remain more or less physically separated during transport and deposition.