One thing you can do is request a turnaround on here. Sometimes others are floating that same stretch and will work to help you with the turnaround (most intelligent guys know to offer a few bucks for the trouble once they get back to their ride btw).

I hitch all the time on the Yak but I'm camped at the pullout and the temperature isn't a problem. If you come out at about 6:00pm on the Yak there's plenty of work traffic going back through the canyon heading back to Ellensburg. I have however walked from Umtanum back to Big Rock once after dark... sucked!