Salmonid habitats in mainstem reaches of the Columbia and Snake rivers have changed dramatically during the past 60 years because of hydroelectric development and operation. Only about 13 and 58% of riverine habitats in the Columbia and Snake rivers, respectively, remain. Most riverine habitat is found in the upper Snake River; however, it is upstream of Hells Canyon Dam and not accessible to anadromous salmonids. We determined that approximately 661 and 805 km of the Columbia and Snake rivers, respectively, were once used by fall chinook salmon Oncorhynchus tshawytscha for spawning. Fall chinook salmon currently use only about 85 km of the mainstem Columbia River and 163 km of the mainstem Snake River for spawning. We used a geomorphic model to identify three river reaches downstream of present migration barriers with high potential for restoration of riverine processes: the Columbia River upstream of John Day Dam, the Columbia-Snake-Yakima River confluence, and the lower Snake River upstream of Little Goose Dam. Our analysis substantiated the assertion that historic spawning areas for fall chinook salmon occurred primarily within wide alluvial floodplains once common in the mainstem Columbia and Snake rivers. These areas possessed more unconsolidated sediment, more bars and islands, and had lower water surface slopes than areas not extensively used. Because flows in the mainstem are now highly regulated, the pre-development alluvial river ecosystem is not expected to be restored simply by operational modification of one or more dams. Establishing more normative flow regimes, specifically sustained peak flows for scouring, is essential to restoring the functional characteristics of existing, altered habitats. Restoring production of fall chinook salmon to any of these reaches also requires that population genetics and viability of potential seed populations (i.e., from tributaries and tailrace spawning areas, and hatcheries) be considered.