The Lewis River Hydroelectric Projects - PacifiCorp

impact of its advance, bowling them over, piling them in heaps, pushing
through roads and railroads, ripping at piers and bursting dikes,
drowning cattle and thousands of chickens, transporting tons of silt and
boulders and trees on its turgid tide.”
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 just upstream 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—more than six times normal
flow.
The high lake continued to drain, creating a muddy flood through
Christmas Canyon, for over a month after the initial breach.
Sidebar
Caption:
The morning before the flood, far up the Lewis, a CCC crew of 200 had
been at work on forest projects when nearby Dry Creek began to rise.
Sensing trouble, the commander had ordered an evacuation. The men
toppled a few large trees across the now-raging torrent and 135 men
teetered across to safety as the flood rose to engulf their makeshift bridge.
Hydro-stats: Hydroelectric power icon
Swift Dam: Year completed—1958; Crest length—2100 feet feet;
Maximum height—512 feet; Annual power generation: 677,555 Mwh
(enough to power over 89,000 homes).
Images:
Graph of flow oscillations, showing how dams can dampen oscillations
but not prevent them. Images of high waters. Diagram of dam and
reservoir, showing how a buffer of storage space is kept. Historic photos
of the Dry Creek flood.
Appendix 1: panel profiles Lewis River Draft I&E Plan page 45