Resolution No. 4814 - Laserfiche WebLink

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Santiam Pass, north of the Three Sisters. Mafic volcanoes typically erupt less explosively than do composite volcanoes, so that impacts of eruptions are less widespread. Most mafic eruptions in the Three Sisters areas have produced tephra deposits and lava flows that typically traveled 3 to 9 miles from the vents and rarely 9 to 12 miles from the vents. Tephra deposits rarely exceed 4 inches in thickness at distances 6 miles from the vent. Belknap Crater, about 1,500 years old, is one of the youngest mafic volcanoes in the Cascades. The Sand Mountain field, a cluster of cones and lava flows west of Santiam Pass, was formed during three eruptive periods between about 2,000 and 4,000 years ago. The USGS study of Volcano Hazards in the Three Sisters Region includes three hazard zones: proximal hazards, distal hazards, and a regional lava flow hazard zone. The proximal hazard zone is limited to the immediate area around the Three Sisters and is an oval area about 8 miles (east-west) by 10 miles (north-south). The proximal hazard area is the area subject to the most intense volcanic hazards including lava flows, tephra flows, pyroclastic flows, landslides and debris flows and lahars. Fortunately, this area is predominantly wilderness with very Iow population. The distal hazard zones are river valleys extending away from the proximal hazard zone that are subject to landslides, debris flows and lahars. The distal hazard zone has three levels for areas subjected to lahars (and other flows) of varying sizes. Areas subjected to lahars include Squaw Creek into Sisters, Tumalo Creek into Bend, the valley between Sparks Lake and Crane Prairie Reservoir, and the McKenzie River (and tributaries) west of the Three Sisters. The regional lava flow hazard zone includes a band about 30 to 40 miles wide covering the entire crest of the Cascades. Locations throughout this zone, which includes Sisters, Bend, and the Santiam Pass, are subject to lava flows from mafic volcanism which could occur anywhere in this entire zone. Of these Three Sisters volcanic hazards zones, only the distal hazard zone potentially affects the Eugene/Springfield Metro Area. The proximal hazard zone, the lava flow hazard zone, and the direct effects of the distal hazard zone do not extend to the Eugene/Springfield Metro Area. However, substantial landslides, debris flows, lahars, or ash falls affecting the McKenzie River or it tributaries would affect the water system in the Eugene/Springfield Metro Area. Water drawn from the McKenzie River would likely have high turbidity and potentially cause operational problems at water treatment plants and degradation of water quality. Current proximal hazard zone maps indicate that minimal impact would occur in the upper Willamette tributaries, presenting Iow risk to SUB's treatment plant on the middle fork of the Willamette. Furthermore, substantial landslides, debris flows, lahars and snowmelt runoff from an eruption of one of the Three Sisters volcanoes could have significant impacts on the Pubhc Review Draft: October 11,2004 11-5