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Geomorphic Processes
Based on review of available geologic maps and literature, the modern topography, including the formation of large tributary fans and the valley floor, is the result of erosion and deposition that has occurred since the mid-Pleistocene or roughly within the past one million years (Kunkel and Upson 1960, Johnson 1977). The elevations of surrounding peaks range between less than 1,000 (300 m) to more than 4,000 feet (1,200 m). Many isolated small hills also protrude from the valley floor now and are composed of rock types that are similar to those in the adjacent mountain fronts. The elevation of the valley floor drops from about 340 feet (104 m) near Calistoga to about 50 feet (15 m) near Napa.
The extent and location of geologic units presented in Table 3-3 and Map 3 are derived from the State of California Geologic Map (1:750,000 scale). The Napa Valley makes up about 28 percent of the watershed area, and is underlain by Quaternary alluvial fan and valley fill deposits (Q, Map 3, Table 3-3). The uplands are composed of Jurassic to Tertiary age volcanic and sedimentary rocks. Approximately 27 percent of the Napa River watershed is underlain by Tertiary volcanic flow rocks (Tv, Map 3, Table 3-3). These volcanics are primarily located in the eastern and northwestern portions of the watershed (Map 3). Jurassic and Cretaceous Franciscan rocks and Cretaceous marine sediments form the bedrock in the western, northeastern, and southeastern portions of the watershed. About 9 percent of bedrock geology in the watershed is underlain by the Cretaceous and Jurassic Franciscan complex rocks (KJf, KJfm), while 13 percent of the watershed is made up of the Cretaceous marine sediments (K, Ku, Kl). The uplands in the northern portion of the watershed are mostly composed of soft Tertiary pyroclastic and volcanic mudflow deposits (Tvp, Map 3, Table 3-3), which make up approximately 8.5 percent of the watershed area. The remainder of the bedrock units is shown on Map 3 and Table 3-3.
Stillwater Sciences - Napa River Watershed Limiting Factors Analysis