U.S. Geological Survey Open File Report 99-173
Online Version 1.0

The purpose of this report is to compile and analyze existing data which lend support to the development of a sediment budget for the Columbia River, coastal, and offshore regions of southwest Washington. This will contribute to the construction of a sediment budget model which will reflect sediment sources, depocenters, and the sediment contribution to each region. The Columbia River is the source of modern sediment for the beaches of southwest Washington. Development of the sediment budget is necessary to understand the long term effects that reduction in sediment supply has on present day areas of sediment erosion and accumulation in the region. Figure 1 describes the origin, distribution, and thickness of the Mid-Shelf Silt Deposit (MSSD) based on analysis of seismic data acquired between 1976-1980 (Wolf et al., 1997). Sediment volumes deposited during the past 5000 years were calculated for each of the physiographic areal compartments shown in Figure 2. Table 1 organizes the data from Figures 1 and 2 into tabular form. This table provides a representation of the percent volume and weight of sediment types which contribute to the estimated Columbia River sediment budget. The compartments shown in Figure 2 are color coordinated with Table 1.

Nittrouer (1978), Nittrouer and Sternberg(1981) interpret and describe a sediment unit on the continental shelf as a Mid-Shelf Silt Deposit (MSSD). They observed the MSSD on the continental shelf west of the Columbia River mouth to as far north as the Juan de Fuca Canyon which incises the shelf. We recognize the MSSD unit on seismic profiles southwest, west, and northwest of the Columbia River mouth to north of Grays Harbor. North of Grays Harbor the acoustic signature becomes less obvious and difficult to trace.

The thickness of the Mid-Shelf Silt Deposit (MSSD) was contoured at 5 m intervals to 10 meters thickness and at 10 m intervals thereafter. A maximum sediment thickness of 35 meters was observed 10-15 km northwest of the Columbia River mouth. The volume of the total MSSD unit (48.5 Km'), as shown in Figure 1, was determined to facilitate calculations of the Columbia River sediment budget.

Sediments transported directly westward from the Columbia River mouth form two thick lobes bisected by the Astoria Canyon. The northwest lobe has the greater sediment accumulation. The southern lobe, thins to the southwest, suggesting that it formed from sediments transported southward from the Columbia River mouth along the Oregon continental shelf. The bifurcation of the lobes may reflect seasonal control of sediment transport by surface currents flowing north during winter and south during the summer and autumn. The winter phase is the period of high river discharge and high sediment load, consistent with greater sediment accumulation in the northern lobe.

Based on sediment volumes deposited during the past 5,000 years, we estimate that 65.9% of the late Holocene Columbia River sediment were deposited in the MSSD, 5.7% were deposited in Astoria Canyon, 6.3% in Washington Canyons, 4.7% on the Washington-0regon slope excluding canyons, and 17.3% were deposited in the Cascadia abyssal basin floor and channel systems. We calculate that the minimum average sediment load of the Columbia River is 20 million metric tons each year in the late Holocene. Our budget does not include the paralic deposits (inner shelf, shoreline, and estuarine) of the southern Washington and northern Oregon margin that also appear to be mainly derived from the Columbia River sediment source. Because the best estimate of the present-day sediment load of the Columbia River is 5 million tons/year (Sherwood et al., 1990), our data suggest that there has been a minimum of 75% reduction in late Holocene sediment load of the Columbia River.