Yes Kev, as well as the gravel, the river also carries a lot of silt. How this ( and the gravel) is subsequently deposited doesn't just depend upon the maximum height of the floods. It depends upon how long the river remained at its various heights as the flood subsides, and how the river was recontoured as the flood reached that maximum height.
I am not at all sure that I agree with Jerry, that it would be an easy job to re-establish the weed. Planting will not be an easy task in which to succeed. You can form a group Jerry, but getting anything other than notional support on paper from the council would be a miracle. I speak from experience. Worth a go though I guess.
The effluent is probably not having any great effect in that it affects water quality all the way downstream of the outflow. It may be though, that the river bottom just downstream of the discharge point is badly contaminated by sewage based silt and inhibiting growth.
Kev, on the Mersey, there is healthy streamer growth just a couple of hundred yards or so below a major sewage outflow, no streamer here, but the depth is probably the main cause....as soon as the river shallows up again there is lots of weed. But it also has ample sunlight at that point.
Jerry on crayfish: maybe there is a link, but I suspect it might be the other way round. Maybe the ample streamer weed allowed the young crays/eggs to thrive and hide from the chub, and once the streamer disappeared, so did the crays?
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