Salmo g. -- I agree with you the Courter study could have better described what was meant by the "lower mainstem." The reason I believe the hatchery summer runs were planted above the dams is that PGE's website indicates "in decades past, summer steelhead smolts were released upstream of North Fork Dam. The adults entered the North Fork fish ladder adult fish trap and were passed upstream.
In recent years, summer steelhead have been acclimated or stocked in the lower river which has significantly reduced the number of adults returning to the North Fork fish trap. In addition, summer steelhead have not been passed upstream of North Fork dam since 1999."

For anyone who wants to take a further look at the prior studies, you can find them at the links below. If I'm reading the studies correctly, many of the returning hatchery summer runs spawned naturally, and, while producing fewer smolts per spawner, the sheer number of naturally spawning hatchery-origin summers resulted in a significant percent of the out migrating smolts, although few returning adults and minimal introgression with the natural origin winter steelhead.

https://www.researchgate.net/publication...ductive_Success

https://www.fws.gov/Pacific/fisheries/ha...hou2006TAFS.pdf