They either move out and then repopulate once the waters have receded or drown. Has ever been thus and wildlife has managed to exist like this for millions of years
The problem with dredging rivers and the type of works that Mr. Price has carried out is that whilst initially it allows for a greater volume of water to move downstream (which in turn can cause problems for those downstream) but reduces flow levels in the summer which leads to silting up - a process which is accelerated by removal of bankside vegetation which makes the banks potentially unstable. After a few years you are faced with a silted-up river channel that needs dredging again.
There has been a lot of research and work carried out on the best method for flood prevention and the current thinking is to try to slow down and disperse the water (like a good old fashioned water meadow, you know those things that all got destroyed in the last century).
If you simply dredge wider deeper rivers then the water flows faster downstream to where your neighbour hasn't dredged and their land floods worse than it used to.
Mr. Price may have relieved flooding in the area temporarily but will have potentially made it worse downstream and the effects will be short lived.
Flood management is far more complicated than simple examples of pint glasses and sand or a bloke with a JCB who thinks he knows best...
Listen bud, river maintenance and dredging works have been carried out for decades if not centuries in some areas. Then your gang get put in charge, fudge all gets done and everything floods. And floods badly, too. If they can't do it or don't want to do it, put it in the hands of organisations who know what they are doing.
Many many parts of these catchments are either wholly man-made, re-routed or improved by man or surrounded by man made structures all of which interfere with the way water moves across a catchment. It does not take a genius to realise that man will have to manage how water reaches the sea also. You seem to be suggesting that wildlife should take precedence over people's homes and livelihoods being flooded and destroyed. Fudge that will bells on top. Rotational maintenance and drainage work is far less disruptive to wildlife than you make out and certainly less disruptive than thousands of acres being under water for days or weeks killing every fudging thing.