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2. STUDY AREA CHARACTERISTICS SIWF · Salem. The Salem series consists of very deep, well-drained soils that formed gravelly medium and moderately fine-textured mixed alluvium over very gravelly coarse ~ textured alluvium. They are on valley terraces at elevations of 100 to 800 feet. Slope gradients range from 0 to 12 percent. They are well-drained with slow runoff and moderately slow permeability over very rapid permeability. Salem soils are used for production of cereal grain, corn, pole beans, berries, orchards, pasture and hay. The native vegetation is ponderosa pine, Douglas fir, Oregon white oak, bigleaf maple, wild rose, and annual and perennial grasses. · Coburg, Malabon and Awbrig soils also occur at the SIWF. 2.2.3 Geologic Hazards Geologic hazards that reasonably could be expected to occur in the MWMC service area include seismic hazards (earthquakes), volcanic eruptions, and landslides. The following sections include a brief discussion of the geology and geologic hazards in the MWMC service area and a description of historical seismicity and the probability of a seismic event occurring within 60 miles of the Eugene-Springfield metropolitan area. It should be noted that the following discussion on geology was adapted from a geologic description originally published in the Aquatic and Riparian Habitat Assessment for the Eugene-Springfield Area, September 2002, Final Report, by the Eugene-Springfield Metropolitan Endangered Species Act Coordinating Team (MECT). Geology The landforms of the study area were created over millions to thousands of years ago by a combination of influences including ice ages, volcanism, and cataclysmic hydrologic events. The area is comprised of numerous geologic units that fall within three major geologic formations comprised of basaltic rock, Missoula flood deposits, and river alluvium. Basaltic units are found below the steeper slopes and their rock outcroppings form the southem boundary of the study area. Specifically, these hills were formed from andesitic basaltic or pyroclastic bedrock formed 10-25 million years ago (Thieman, 2000; U.S. Army Corps of Engineers, 1953). The Missoula flood deposits consist of that part of the main valley floor buried with silts deposited primarily during the Bretz Floods that filled the Willamette Valley with sediment 12,000-600,000 years ago (Allen et al., 1986). The third geologic formation is the river alluvium. This is the area within and near the rivers that has been scoured of silts left over from the Bretz Floods and is characterized by coarse sediments and gravel deposited by rivers originating in the Cascade Mountains (Figure 2.2.3-1). Prior to the geologically recent series of ice ages (40-50 million years ago), the Willamette Valley was submerged under the Pacific Ocean. Fossil remains of marine mollusks, crabs, and sharks indicate that the climate was tropical (Thieman, 2000). From 25-40 million years ago, the Willamette Valley dried as the Coast Range rose from the ocean floor, blocking marine inundation. Two to three million years ago, a series of ice ages sent glaciers stretching south of Seattle (Kettler ,1995). Glacial melt water flooded the Willamette Valley, leaving behind till and debris (Thieman, 2000). During the Wisconsin ice age, for which there is the best geologic record, sea levels were significantly lower than they are currently MWMC 2.0 REV23. DOC 2-5