Towns like Portland, Yakima and Pendleton grew up in the lower, alluvial reaches of major tributaries. Irrigated farming and grazing are in alluvial plains adjacent to tributary rivers. Big dams were built in the river's main stem, where they inundated or blocked a great deal of alluvial habitat. Salmon were pushed out of alluvial areas by human settlement. Now, the salmon that don't come from hatcheries are likely to spawn in headwater areas.
Some alluvial populations remain.
The brights are now the biggest naturally spawning segment of the Columbia runs, one of the few populations that might still serve as a core. One of the ironies my mother would appreciate is that they spawn in the Hanford Nuclear Reservation, part of the Northwest's contribution to the Manhattan Project and the site of stored nuclear waste that is expected to seep into the Columbia some day. Nonetheless, if we are looking for a core area on which to build, the Hanford Reach area is probably it. One of those dams, John Day Dam, anchors the big electric transmission line to the Southwest. These are not small questions, and they won't be answered solely in deference to ecological theory.