On the rivers with trout and grayling I find that once the water temp rises in Spring and slowly heats up as we head towards summer, the fish move in general to anywhere fast flowing and well oxygenated..it doesn't really matter how shallowit is so long as they have cover either from trees above or a torrent of white water above their heads, often staying just off the flow in the crease getting food from the 'conveyor belt' that comes past them. At first I used to fish deep slow pools for them thinking it was where the big ones would be but that was totally wrong in hindsight and I have caught good fish in 12-18 inches of water regularly .
If it is a sunny warm day and the river is low I'll concentrate on any white water I can find and fish it as thoroughly as I can. When the trout season ends in Sept we can still fish for grayling but once the temp drops they still feed well but seem to move to the deepest holes which are often slower and shoal up. It's just finding them that takes time but once you find them you've got them. On one of the stretches of the Lancashire Calder that I fish, I regularly watch barbel spawning on the gravel in around May time and I often wonder just how far up the river they have travelled. This winter I'm going to try and find the spots where the chub hold up now the salmon lads have left the river.