Apart from the destruction of the environment by altering the characteristics of the river, holding the water back with a weir in the summer could simply encourage the depositing of silt.

Plus of course the water that would feed the river in the summer, has been lost out to sea, the important thing is to keep that water as much as possible in the higher reaches.  That would be completely negated by deepening and widening.

You cannot just mess about with the natural flow of a river without suffering some possibly unwanted consequences.

I fish the Upper Severn which twists its way across its flood plain, this is the sort of water level changes that can be expected.

https://rloi.naturalresources.wales/ViewDetails?station=2034.

It is already very wide for its normal flow, to the degree that much is too shallow to hold fish.  Trying to deepen it would be a joke, a pool several feet deep can be filled in overnight with stones brought down by a sharp rise in level.