Contrary to popular belief heavy floods don't wash fish away, there are always slack areas where fish can find refuge, these can vary from huge lakes formed by flooded fields to tiny areas the size of a teacup behind a grassy tussock.
The prolonged summer floods of 2007 (I think) provided a bumper recruitment season for the Severn catchment and the shallow lakes which lasted for several weeks were a haven for fry of all species. .....
Somewhat miss leading this for several reasons - washout does occur when over topping happens, as you acknowledge by the formation of temporary lakes and fish seeking refuge in them. The impact being greatest on fry and year 1-3 fish, which haven’t developed the innate survival strategies to return to the river channel as the flood recedes. The larger fish in the main having developed that innate survival strategy.
The smaller fish then get marooned in those temporary lakes/pools and fall prey to all manner of birds, not only the fish eating species, including gulls, turns (summer) and all members of the crow family, ever the intelligent opportunists they are, as the water dries up. The mortality rate of these year classes under these conditions is very high indeed and causes a discontinuity of stock profile in future years.
Added to this in heavy floods of this type is the violent nature and velocity of the floodwater flow that brings down all nature of debris, large and small pebbles, even moving whole gravel runs 10s of metres from their original position before the flood. Twigs, logs and full trees. Along with all the man-made rubbish that get thrown in the river. Again, these now missiles, will take there toll on all fish causing injury and death to them by being hit by them. There is no doubt, if you are a fish in these conditions you are living in a very hostile environment indeed!
Ray whilst the flow in rivers is lamina (slower at the sides and bed) under flooding conditions it becomes distorted from the idealised model and the speed, pace picks up vastly, creating helicoilous flow that extends to the bed of the river causing Saltation (the picking up of stones, pebbles and crashing them into the next in a chain reaction) which intern allows gravel runs to move as I stated above.
I’ve mentioned earlier I’ve been getting over the last two years significant numbers of year 0-1 tiny barbel in my invertebrate net samples and the reasons why is, they hide in the stones/gravel at that size in their hundreds if not thousands. They frankly stand very little chance of survival once saltation starts because of where they hide.