Flies need to be small and any un-weighted or lightly-weighted Hare and Copper, Halfback or Pheasant Tail can be effective.
During the day try any mayfly pattern while caddis patterns are effective especially in the evening and during the late summer try cicada patterns, or a Daddy-Long Legs or Hoppers in the evening. Beetle patterns are worth trying in the early summer months and try a small lace fly from February to March.
Small Woolly Buggers fished across and down through some deep pools and under vegetation can induce a strike in the lower reaches. Otherwise try a wee-wet such as Greenwell's Glory, March Brown or Invicta especially if trout are rising in the early evening during the warmer months.
Spinners: Small bladed spinners such as a Mepps up stream and retrieved just slightly faster than the current so they are just above the river bed. As most fish are rainbows try brighter colours such as red and gold.