When the creek and river flows get skinny the trout get spooky. They know they can be easily seen by predators (herons, raccoons...and fishermen). The upside is that the fish are still hungry and don't have quite as much food available as they did in the summer. This makes them relatively easy to catch so long as you are sneaky!
Wear drab clothing, be aware of your shadow and take advantage of whatever natural or man made concealment is available.