The Lewis River Hydroelectric Projects - PacifiCorp

Eagle Cliff Park—Interpretive Sign #1 (Natural History)
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Eagle Cliff Park
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Watershed
The western slope of the Cascade Range is a land of rain. Here,
westward-facing valleys nestle like open baseball gloves in the path of
Pacific rainstorms, scooping rain and snow from the sky in staggering
amounts. Tumbling through waterfalls, trickling down countless tiny
tributaries, or oozing steadily downhill through the soil, all of that water
gradually makes its way downhill, funneled by slopes and valleys into
rivers such as the Lewis.
The “funnel” of land that directs water into the North Fork of the Lewis
River is called its watershed. It covers some 847 square miles, extending
from Woodland to the peak of Mt. Adams. Its boundaries snake their way
along ridgetops including Mt. St. Helens, tracing high ground to the
banks of the Columbia. A map of streams in this watershed, from the
highest ridgetop streamlets to bustling torrents such as Swift Creek, looks
like a map of blood vessels, branching to supply the body.
Scientists gage the health of a watershed by the health of its
interconnecting streams and the habitats through which they flow. In a
watershed, effects move along stream pathways: a poorly-built road can
wash out, choking the downstream reach with sediment. A blocked
channel can close off access to an upper stream, depriving that area of
fish.
For the most part, the tributaries of the Lewis flow through forested land
(much of the watershed is part of the Gifford Pinchot National Forest)
and are quite healthy. However, increasing development along the Lewis
River, poor timber harvest practices, and other human activities will
always pose challenges to the health of the Lewis watershed.
Sidebar:
The watershed of the Lewis River’s north fork is dominated by volcanic
activity. Millions of years of lava flows and violent eruptions have
formed the ridges and peaks, as the Lewis has tumbled through the
valley below. Repeated lahars, or volcanic mudslides, have changed the
course of the Lewis and its tributaries.
The most recent volcanic episode occurred in 1980, with the eruption of
Mt. St. Helens. This huge explosion denuded 30 square miles of the North
Fork’s watershed, sending millions of tons of ash, mud, and shattered
trees down into the river. Sediments pouring into Swift Lake reduced the
water’s depth at the upper end of the reservoir, and for over ten years
afterwards, the reservoirs were kept at lower levels in anticipation of
more mudslides.
Appendix 1: panel profiles Lewis River Draft I&E Plan page 52