The Lewis River Hydroelectric Projects - PacifiCorp

Saddle Dam Park—Interpretive sign #1 (Hydroelectric Story)
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Saddle Dam
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Dams Along the Lewis
The millions of kilowatt-hours of electricity produced by PacifiCorp’s
Lewis River projects are made possible by dams, which hold and control
the flow of water, using it to generate power. But take a tour of the Lewis’
four major dams and you’ll see that they are quite different from each
other in size, shape, and materials. Why is this so
In general, dams are built to “suit the site”: what a dam looks like and
what it’s made of depend on where it is built and the materials available
to build it.
At sites where the river narrows and bedrock is close to the surface, such
as canyons, concrete arch dams are the best fit. Their distinctive
arch/wedge shapes take advantage of concrete’s enormous compression
strength (ability to withstand the pressure of large forces without
breaking). Concrete arch dams direct the force of water outward onto the
walls of the canyon and downward to the rock of the riverbed.
Where rock abutments and exposed bedrock streambeds aren’t available
to support concrete arch dams, earthen embankment dams are built.
Embankment dams are usually made primarily from materials found on
site, with great care taken to sort and apply the layers to provide
maximum strength and impermeability.
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Captions:
Photo of Merwin Dam construction Merwin Dam, which forms Lake
Merwin, is a concrete arch dam built at the top of narrow Shirt-Tail
Canyon. It was created in stages, using an enormous wooden form that
was moved across the span as each section was poured.
Photos of Yale and/or Swift Dam construction Yale Dam, Saddle Dam, and
Swift Dam are all earthen embankment dams, built from riverbed
boulders and gravels, rocks, clay, and soil excavated during site
preparation, and rocks quarried from the surrounding hillsides.
Sidebar:
Saddle Dam looks like a grassy hill—you might think it was built by
simply piling up earth in a straight line. But like all Lewis River dams, it
is actually a complex and specialized structure, engineered and
constructed to withstand heavy water pressure.
If you could slice the top from Saddle Dam, you would see distinct bands
or layers of material: large boulders, coarse gravels, fine clay, and mixed
dirt. These materials are arranged to provide strength, drainage, and
impermeability to the structure: big boulders and compacted earth
Appendix 1: panel profiles Lewis River Draft I&E Plan page 29