The Lewis River Hydroelectric Projects - PacifiCorp

Lewis River Interpretive & Education Plan
Natural History
Geology
[note: The Lewis River is divided into the North Fork and East Fork regions,
with the division occurring at Woodland. PacifiCorp’s reservoirs, and all sites
identified in this document, lie on the North Fork of the Lewis. In the interest of
simplicity, for the purposes of this document, “Lewis River” will refer specifically
to the North Fork].
The Lewis River flows from the Cascade Range—a north-south trending range
of mountains that stretches from northern California to southern British
Columbia. These rugged, geologically-rich mountains are part of the “Ring of
Fire”: a great arc of volcanic mountains that rim the Pacific Ocean. Like all Ring
of Fire ranges, the Cascades are the product of the slow collision of the Pacific
tectonic plate against a continental plate—in this case, the North American
plate.
At the juncture between these two plates, the heavier Pacific plate creeps
underneath the North American plate. As this happens, the sinking rocks
of the oceanic plate melt under the intense heat and pressure of Earth’s deep
mantle. The molten rock flows upward through complex layers of volcanic,
sedimentary, and metamorphic bedrock that were formed during previous plate
action. Where the molten rock returns to the surface, it has formed tall, conical
volcanoes that rise prominently above their non-volcanic neighbors. Mt. St.
Helens, Mt. Shasta, Mt. Hood, and Mt. Rainier are just a few of these familiar,
picturesque volcanoes.
The Cascades began their rise over 50 million years ago. During the Miocene
epoch (13 to 25 million years ago), they reached a peak of volcanism; many
of today’s volcanoes emerged during that period. In subsequent millennia,
eruptions throughout the range deposited complex strata of ash, basalt, clay, and
mud. Meanwhile, the action of water—in the form of glaciers and rivers—carved
deeply into the rising range, creating the region’s steep valleys. The Lewis River
flows through—and shapes—such a valley, working its way down through layers
of volcanic rock, mud, and ash, carrying away and depositing this debris in
downstream regions.
Although the Cascades may seem relatively quiet in our human timescale, they
remain a work in progress. Eruptions continue today, with the most recent
major events being the eruption of Mt. Lassen in 1914-1917 and the eruption
of Mt. St. Helens in 1980. The latter eruption had a profound impact on the
Lewis River Basin, as the upper portion of the basin lies on the slopes of Mt. St.
Helens. Although much of the volcano’s explosive power was vented to the north
and west, lahars (mudflows) did extend to the upper waters of the Lewis, and
impacts reached to Swift Reservoir, where lahars descended Pine Creek, Muddy
River and Swift Creek and deposited a large amount of debris in the reservoir.
Sea Reach Ltd • 146 NE yamhill Street • Sheridan, OR draft 3 • November 2008 • page 56