I worked on Skagit hydro relicensing on behalf of various tribes, agencies, and private consultants variously from 1976 until I retired in 2016. There are a lot of things, both positive and negative, that can be said of Seattle City Light's stewardship in the Skagit basin. No one can say with absolute certainty whether anadromous fish ascended the Skagit to points upstream of Ross Dam in modern times (1850) until SCL began building its dams on the river. However, the preponderance of evidence indicates that they did not migrate upstream of Diablo Dam.
Why SCL tried to maintain that fish did not migrate through the Gorge bypass reach (the part of the river from Gorge Dam downstream 2.5 miles to Gorge powerhouse is beyond me because that is stupid and provably false. Chinook salmon - and most likely steelhead too - spawned upstream of the Gorge dam site at two places: Cedar Bar and Reflector Bar. These are located between Gorge Dam and Diablo Dam.
The 1921 Smith report noted the presence of major cataracts and cascades in the river between the mouth of Ruby Creek and the site of Ross Dam. These were thought to be, but not proven, fish migration barriers. There were no Indian villages or camps upstream of Newhalem, even though white settlers found places to farm and ranch, which suggests that salmon may not have been present upstream of the cataracts. The last I read about Skagit fish genetics is that the rainbow trout and bull trout in Ross Lake and the upper Skagit appear to be more closely related to their Fraser River counterparts than to those downstream of Gorge Dam. And tribal oral tradition at the time of Skagit relicensing in 1990 made no mention of historical salmon upstream of Ross Dam.
So, is it possible that salmon inhabited the upper Skagit? Yes, it's possible. Is it likely? No, based the cumulative weight of all available evidence.