Chinook eat small fish, including galaxiids, as well as crustaceans, molluscs and insects. A slim baitfish streamer is a sensible first fly, with a Woolly Bugger or small crustacean pattern for a slower approach.
For general lake work, begin with a six or seven weight and a sinking option suited to the depth. Make a repeatable countdown and start with a steady retrieve. Change one element at a time: first depth, then speed, then the length of the pause. When a fish follows, continue fishing the fly rather than immediately lifting into another cast.
From a boat, set a drift that covers the edge instead of repeatedly crossing it under power. From the bank, fan the casts and work the water in front of you before moving. The purpose is to find a feeding level you can cover reliably.