Here's my two cents. The south fork does have a nice run of summer run Steelhead but the state has closed all the best water for them because of the poachers that took the Springs, I use to have some amazing days up there, . By August most of the fish are up in the canyon holes which is closed water now. I have been told there are a few Summer fish and Springs in the Middle fork and there is some neat water just above the Mosquito Lake Bridge about ten to fifteen miles from the mouth, it's really neat canyon water and I'm sure you'll be able to find something up there. Most everything below that is braided and pretty skinney water, especially in August. If you try to fish the North Fork good luck unless you get above the falls. There are some glacier creeks right around the falls that dump really ugly dirty water in when the weather is warm so you're going to have to get above that to get any visibility and once above the falls you won't get any slabs since the falls in impassable for migrating fish. Do yourself a favor and either go up to the Skagit and camp or cross the border and fish the Fraser for Sockeye's!