Probably Suiattle run Chinook. I've seen Chinook spawning in the mainstem Sauk into the fall months.
here's a little info: http://www.nwfsc.noaa.gov/pubs/tm/tm35/chapters/03regvar.htm
Summer-run chinook salmon are present in the Upper Skagit and Lower Sauk Rivers in addition to the Stilliguamish and Snohomish Rivers (WDF et al. 1993). Supplementation programs currently exist for spring-run chinook salmon on North Fork Nooksack River and summer-run chinook salmon on the Stillaguamish and Skagit Rivers (Marshall et al. 1995, Fuss and Ashbrook 1995). Hatchery programs also release Suiattle River spring-run chinook salmon and Snohomish River (Wallace River) summer-run chinook salmon (Marshall et al. 1995, Fuss and Ashbrook 1995).