I've always found black and olive Wooly Buggers and Kiwi Muddlers to be devastating. If nymphing is your preference then try some big Yuk Bugs or Girdle Bugs as well as some size 18 Hare's Ears and PT's. Be sure to get them into those undercut banks, because that is where the big fish are. Now for some final tips for Flat Creek. First, always approach its banks cautiously.
Many banks are undercut and act as sounding boards.
Blundering along the bank will scare fish for a long distance. Next, slowly walk along and try to spot the fish feeding. This is not the type of place to blindly cast, except when using craneflies and hoppers, because you will scare most of the big fish immediately. You must stalk your fish and plan your cast carefully. Be sure to wear dull colors, because bright ones will alert fish making them move under the banks before you even see them.