I have caught some of my best fish in flooded rivers, especially when the river just starts to drop after a flood, or "fine off" as some say.
The reason for this is that after using a lot of energy to fight the floods, the fish become starved and have to replace that energy by eating. Find the fish, and you could be in for a bonanza.
I found a shoal of roach, together with a few chub and dace once on the River Leam and caught a tremendous catch. This was after a week of very heavy rain.
But you must be prepared to pick your days.
And you must know your river.