Good Question this. I agree with much that has been said before.
Under the weir sill. Even below the foaming, rushing water its amazing how steady the flow can actually be. And its a good place to start. Especially if you can drop it straight down the edge.
I would tend to use a large bait, like a cajun flavoured lump of meat or a good paste bait on the hook rather than hair rigged. Its often the place because of the back eddies, where debris will collect. If the river holds carp they will often collect there as well. Straight lead on heavier tackle is a good idea.
The main push will be an excellent spot for mixed fish, dace, chub, roach and barbel.
Simple straight forward maggot/caster and hemp feeder will bring up the fish even in apparantly fast water.
My favourite however will be the areas where the water evens out, often narrower and deeper, often with weed beds..................sometimes on the bigger weirs a hundred yards or so downstream.
Here the bigger fish, especially barbel will be. Back to the basic pellet and or boilie approach with pva bag or mini pellet filled feeder scoring well. Tackle for barbel ideally in these conditions, minimum 10lb line size 8 hook.
Good Luck
Graham