I also noticed that the further you cast, the more line will be slack or on the water. How do you reconcile getting the float and bait to where the fish are most likely if the result is slack line or an unnatural float down river if one needs to keep reeling in slack (with the resultant pressure on the float). Hope that is understandable...
Keep the line ABOVE the water. This means the rod tip should be high up. And yes, if you cast upstream, you will want to reel in some line to compensate for slack. As the float passes in front of you and heads downstream, click on the release button and let some line out. Just make sure you thumb the line when you set the hook on a take, as you may end with a nasty birdsnest if you don't.
And, as others have said, keep the weight about a foot off the bottom. In an irregular run, with lots of big boulders, it is perfectly normal to feel the weight touch the bottom occasionally.
Once you have established a "corridor" through which the float travels smoothly, repeat your casts exactly the same every time. If you notice anything unusual (i.e. the float stops, twitches, lies on the side or goes under) set the hook like you mean it. Chances are you are having Mr. Salmon checking it out.
Good luck and keep trying. Sooner or later you will be rewarded.
Cheers, Milo