Fish start trickling in in late June and early July (hence summer steelhead). Most of these early fish are wild it seems. There are hatchery fish early on in the mix, but they start coming in en masse in September, and by the time harvest opens up on the 15th of October the river is usually full of them.

The water in July isn't as low as you might think it is since Dworshack regulates the river level throughout the year.