if the river is up because of rain rather than snow melt i wait until the river is dropping before fishing,the air temperature is of less importance to me, i caught 2 barbel last winter when the river was up and there was frost on the ground.
The canalised river(Colne,Gade) pounds of the Grand Union locally probably hold more barbel in flood conditions than the river........
however I would wait for the river to drop, but get on it before the colour dropped out.
Dead maggots well soaked in a flavour like The Source. Mix with pellet groundbait to give a dry mix that will just squeeze in a small cage feeder. Maggots on the hook. Fish slacks, steady areas next to cover and other known holding areas. Don't wait for 3ft twitches, strike at any positive indication. Give each swim an hour before moving.