The Washington and Oregon Mid-Shelf Silt Deposit and Its Relation to the late Holocene Colombia River Sediment Budget

Overview

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.

First posted August 3, 1999