|
MWMC FACILITIES PL~,N
<br />
<br />because most water was held on land in the form of ice. As the ice started to melt, however,
<br />both coastal and inland areas were inundated (Thieman, 2000; Allen et al., 1986).
<br />
<br />The most recent significant geologic events that have shaped the Wigamette Valley as we
<br />see it today are the Lake Missoula Floods, which occurred from 12,000-15,000 years ago. The
<br />most recent of these flood events is the Bretz Flood (Allen et al., 1986). Prior to the Bretz
<br />Flood, the Willamette Valley was likely much as it is now, although the valley was likely
<br />deeper and the Willamette and McKenzie Rivers larger, roaring with glacial melt from the
<br />ice-capped Cascades. Flooding from the Bretz Flood began far up the Columbia River
<br />Watershed in Montana and Idaho at Lake Missoula. Lake Missoula was mn enormous lake
<br />formed behind large ice dams created by a glacial finger of the continental ice sheet that
<br />extended into northern Idaho. The ice dams broke suddenly and rapidly, allowing 500 cubic
<br />miles of lake water to rush out at 60 miles per hour in volumes greater than ten times the
<br />crtrrent volume of all the rivers on earth (Parfit, 1995). This flooding may have occurred a
<br />number of times starting 600,000 years ago. The most recent flood event, the Bretz Floods,
<br />occurred 12,000 years ago (Allen et al., 1986).
<br />
<br />Flood water roared through Idaho and down the Columbia River, carrying boulders,
<br />icebergs, glacial wash, loess, and other materials from as far away as Idaho and eastern
<br />Washington down through the Columbia River Valley and into the Willamette Valley.
<br />Water was directed through two gaps at Lake Oswego and Oregon City when a hydraulic
<br />dam was created between Kalama Gap and Crown Point. Approximately a third of the flow
<br />in the Bretz Flood sluiced down the Willamette Valley. h-t effect, the Willamette Valley was a
<br />backwater alcove for the floods. Each flood inundated the Willamette Valley from the
<br />Columbia River as far south as Eugene under nearly 400 feet of water. This lake, named
<br />Lake Allison, was one of the four temporary major lakes formed by flooding, glacial melt,
<br />and impoundment, and extended as far south as Eugene..As water flowed farther down the
<br />valley, it slowed, leaving larger bedload materials lower in the valley and depositing silts
<br />and smaller materials farther south. The Eugene area, at the far end of Lake All/son's reach,
<br />experienced the finest deposition of silts and clays. Most of these depositions reach to the
<br />west of Eugene. These Mits form the lower parts of the Willamette Silt soil type (Alien et al.,
<br />1986).
<br />
<br /> Seismic Hazards
<br /> The seismic hazards in the region result from three seismic sources: interplate (subduction)
<br /> events, intraslab events, and crustal events. Each of these events has different causes, and
<br /> therefore produces earthquakes with different characteristics (that is, peak ground
<br /> accelerations, response spectra, and duration of strong shaking). All three types of
<br /> earthquakes threaten the Eugene-Springfield area. However, because the strength of
<br /> shaking decreases with increasing distance from the earthquake source, the most severe
<br /> shaking will result from either shallow crustal earthquakes or great subducfion zone
<br /> earthquakes (Mabey and others, 1993).
<br />
<br /> Two of the potential seismic sources, subduction and intraslab events, are related to the
<br /> subduction of the Juan de Fuca plate beneath the North American plate. Subduction events
<br /> occur as a result of movement at the interface of these two tectonic plates. Lntraslab events
<br /> originate in the subducting tectonic plate, away from its edges, when built-up stresses in the
<br /> sUbducting plate are released. These source mechardsms are referred to as the Cascadia
<br />
<br /> 2-.6 MWMC_ZO_REV23 DOC
<br />
<br />
<br />
|