Pink Nighty -
Assuming that you found your South Fork summer steelhead post late 1970s they likely were hatchery fish. Those hatchery fish were planted in both Canyon Creek and the upper South Fork. During the 1980s there were some exceptional summer fish to be found in both the South and North forks, BTW regardless of the fork (or even the mainstem) those fish would come readily to a skated or drag free dry flies.
Historically summer steelhead comprised a relatively small portion of the north Sound steelhead populations. the following was taken from the Feds Puget Sound steelhead recovery plan.
For the Stillaguamish they estimated there were 33,700 winter fish in the basin, 3,311 Deer Creek summer fish and 530 Canyon Creek summer fish.
For the Nooksack the estimate was nearly 31,000 winter fish, and 1,820 SF summer fish
For the Snohomish the estimates were 57,719 winters, 728 NF Sky summers and 1,655 Tolt summers.
For the Skagit the estimates were just over 57,000 winters, while there were no estimates for any discrete summers though we know there were small numbers scattered across the basin.
Curt