A differing view of sedimentation


I am responding to Al Johnson regarding the truth about sedimentation of the lower Snake River reservoirs.

I reviewed the 1992 Lower Granite Drawdown study and the U.S. Army Corps of Engineers’ latest sedimentation study. I cannot verify his claim that they have silted in at one percent per year.

To the contrary, the reports verify that sediments accumulate in the upper reaches of Lower Granite Reservoir, and where tributaries enter the reservoirs downstream. Larger sediments drop out near the head of the reservoir, and finer ones remain suspended as muddy water even to the ocean.

Lower Granite Reservoir operates differently from the others. When the Lewiston levees were built, they were kept low intentionally and built with landscaping and trails to be more aesthetically pleasing than the higher levees in Pasco and Kennewick. Water is pumped from ponds behind the levees to protect downtown Lewiston.

To accommodate floods, the reservoir is drawn down at the dam below the normal five-foot operating range. At the dam a special bypass for adult fish is provided, but juvenile fish bypass facilities cannot operate. Navigation stops.

Why does the Corps have to dredge? It is required to maintain the authorized navigation channel depth and the flood carrying capacity of the river. Why is there an earth-fill section at the dams? The earth-fill section closes the channel that was maintained around the concrete structures during construction.

Where does the silt come from? It comes from agricultural, mining, logging and other erosional activities on private and government lands upstream in Idaho, Oregon and Washington. Why is dredged material deposited in water?

It is cheaper and used to create islands to restore some of the wildlife habitat lost when the reservoirs were built. The four dams inundated over 50 islands and created only two of any size.

New York Island supports a large goose nesting population, and Chief Timothy Park occupies the other one.

The sediment analyses I read do not support Mr. Johnson’s argument. Detailed studies in 1992 and since show no extreme contamination; the levels in Lower Granite sediments are no higher than state averages. This is with a paper mill and urban discharges from Lewiston, Clarkston and Asotin. Agricultural chemical pollution was not excessive either.

I call myself a practical environmentalist. Radicals drive nails in trees, burn logging trucks, set fire to mountain lodges and exaggerate to get dams breached.

John McKern
Walla Walla


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