The Lewis River Hydroelectric Projects - PacifiCorp

Lewis River Interpretive & Education Plan
times the Lewis River’s normal flow.
The cause of the flood was discovered within two days. In a high mountain
depression in the headwaters of Dry Creek (which enters the Lewis River
almost within sight of the Beaver Bay boat launch), rain and snowmelt had
accumulated, forming a lake. The lake rested in a bowl of basalt formed by a
100-year-old lava flow, which lay atop soft sediments, which in turn lay atop
another basalt flow. Both lava flows were riddled with lava tubes that had been
packed full of sediment. The water had worked its way through the lava, forcing
sediment out of some of the tubes and causing faster and faster lake drainage as
the blockages eroded away. Downstream, the increasing torrent ripped deep into
the ground and surged into the Lewis River.
Before it abated, the flood scoured a 600-foot-deep canyon (Christmas Canyon)
adjoining the Lewis River. The surge carried trees and other debris downstream,
dumping it by the ton into the newly-formed Lake Merwin. Ariel Dam operators
allowed the maximum amount of water to surge through the dam’s structure,
but water still rose, eventually overtopping the spillway. Flooding downstream at
Woodland was extensive with flows exceeding 133,000 cfs. The island between
Swift No. 2 powerhouse and Beaver Bay was created by this event.
The high lake continued to drain, creating a muddy flood through Christmas
Canyon, for over a month after the initial breach.
Sea Reach Ltd • 146 NE yamhill Street • Sheridan, OR draft 3 • November 2008 • page 87