I hope you don't mind, I'm not a pro per-say, but have been guiding/chartering on a river for 15 years much like the one you discribed in your question. The Tittabawassee (in Michigan) is about as low and clear most times, than a pristeen trout stream. And I to, have had 25 fish days fallowed by 3 fish days. It's my thinking that walleye, especially during pre-spawn, will both drop back down/out of a system, or shoot upstream when any fresh/higher/colder water presents itself. This leaves a void, especially if the water temps/bait fish, have not tempted more males and the bigger females to begin their river forray. The females will generally hold the males in/around/close to, any likely spawning areas once they start there spawning drive. These usually smaller males in the bigining though, march upstream fallowing the fall bait fish. They over time, deplete this food resource and will start moving around even more. I have at times traveled 12 miles downstream to spots I know are the deepest and darkest holes, and have found walleyes there podded-up big time. And at other times, when getting skunked in spots that produced the day before, heard about guys killing the walleyes 20 miles upstream, where the day before there were none. The biggest mistake you can make while fishing rivers is to only have a few spots. Hope this helped. Capt: Dan Manyen, Walleye Express Charters.