as others have said fish where clean water is running in or backwater /eddies
I have fished in the National Park this time of year but have fish the Athabasca all year in spots
what to use depends of course on what you are fishing for
if you are looking for Bulls then yes Imitations minnows or leach patterns
if you are looking for Rockys then nymphs or flys will work .. a Chronamid with a indicator will also catch them or your sinking tip with a terrestrial fly
For your Rainbows there are lots of flys of course,,, any of the above... to most of my fly box;;;
have a fun time
David