In a draft analysis of the potential effects of removing the Lower Snake river dams, the US Army Corps of Engineers (USACE) says breaching the dams could send a wave of sand and silt down the Columbia river that would settle behind the McNary and John Day dams. USACE says the released sand may plug irrigation pumps, and could require dredging to clear navigation channels. USACE also said that habitat used by young salmon could be buried by the flow of silt and sand.
Conservationists who support breaching acknowledge that salmon and other fish might be damaged by the initial release of sediment, but they say the river would quickly recover and the long term benefits would be worth it.
The USACE analysis found that between 75-155M m3 of sediment has accumulated behind the four dams on the Snake river. If the dams were breached, the free-flowing river would create a gully through the layers of accumulated sediment, picking up about 40-60M m3 and carrying it downstream. Most of the sediment is expected to settle when the water is slowed behind McNary, the next dam downstream.
The USACE study also raises concern that resuspension of sediments could expose heavy metals and chemical contaminants like DDT.