According to Eric, the East Multnomah Soil and Water Conservation District representative, we do not have sumac growing on the riverbank – it’s Indigo Bush.
indigo-bush
Indigo bush is a native prairie shrub used for soil stabilization and wildlife plantings that grows abundantly along the Columbia River around Portland. It grows only up to 13 feet tall and forms dense thickets that may deter goose infiltration into our yards.

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The only problem that I can see with Indigo Bush is that it can overwhelm other plants, which may mean we would want to control it in some areas while something else becomes established.