Abstract

The coastal river basins of the northern California Coast Ranges have the highest sediment discharge of basins of comparable size with the United States (Judson and Ritter, 1964; Brown and Ritter, 1971; Knott, 1971; Janda, 1975; Brown, 1975). The high erosion rates in this region are caused by the unique combination of tectonic setting, geology, land use, and climate. This rapidly eroding region is located adjacent to the seismically active triple junction of three crustal plates near Cape Mendocino, California; late Tertiary and Quaternary underthrusting along the continental margin has caused recent (post-mid-Miocene) uplift (Silver, 1971). The area is underlain by the Mesozoic and early Cenozoic Franciscan assemblage, which is a deformed, faulted, locally sheared, and in part metamorphosed accumulation of continental-margin deposits. These deposits consist of intermixed units of melange and more coherent sandstones. The melange consists of an intensely sheared matrix of siltstone or sandy siltstone in which are dispersed large blocks and smaller boulders of sandstone, metasandstone, greenstone, chert, blueschist, and greenschist. Before European settlement, the area was vegetated with redwood (Sequoia sempervirens), Douglas fir (Pseudotsuga menziesii), and interior expanses of grassland and grass-oak woodland. Since 1870, the region has undergone progressively more intensive land use. Cattle and sheep grazing on the prairies became widespread by the 1870's, and harvesting of redwood in the lower basin started in the late 1880's. Extensive harvesting of Douglas fir commenced on private lands in the middle and upper portions of the basin after World War II and continued until the mid-sixties. The region has high annual rainfall (125 to 300 cm) that occurs almost entirely between October and April. Between two and six intense rainstorms each winter furnish the streamflows that transport the majority of the annual sediment yield. An important climatic feature is the occurrence at intervals of 100 to 500 yr of storms of extreme intensity and duration that may be responsible for much of the erosion in the region. One such storm occurred in December of 1964.

All of the above features, the climate, tectonic setting, geology, vegetation, and land-use history, probably contribute to the high erosion rate of the region. In order to reach some understanding of the relative contributions of these factors and of how sediment moves through the watershed in this region, a detailed analysis was undertaken of the processes and distribution of erosion and deposition in the upper 575 km2 of the 1,111-km2 Van Duzen River basin (Kelsey, 1977). The Van Duzen is the northernmost tributary of the Eel River in north coastal California, and flows into the Eel 22 km upstream of its mouth (Fig. 1). Field work for the study took place in 1973–1976. In addition, six sets of aerial photos covering the basin from 1941 through 1975 are available, and stream-gaging records for a U.S. Geological Survey gaging station at the downstream end of the study area near Bridgeville, California, have been available since 1951. A study of changes recognizable in the aerial photographs and interpretation of stream-gaging data for 1940–1975 and sediment discharge data for the periods 1956–1967 and 1975–1976 make possible an analysis of erosion and sedimentation for the 35-yr period, 1941–1975. Surveying records going back to 1869, plus datable trees buried by flood deposits, make it possible to extend some aspects of the record to the last century.

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