I used to regularly catch Barbel from different places on the Kennet, it was a prolific water not only for its barbel but also for its other fish including roach, Dace, Chub and trout; it used to have pure gravel on the bottom and an abundance of streamer weed and cabbages, but I'm told that the barbel have diminished quite a lot with only certain areas producing a few large barbel and the streamer weed has all but disappeared in some places and the flow has decreased there is lots of silt and the depths are lower and abstraction is at a peak; no wonder the barbel have started to die out in various places on the river.
I now fish the upper reaches of the Lea which is prolific for Barbel, Chub, Dace and roach etc. The bed is very gravelly and there are lots of good fish loving features thanks to the clubs efforts over the years but a few miles further downstream nearer to Welwyn Garden City the bed Is very silty and the barbel are very scarce around there,
Keith Spears told me once that by the time the water in the Lea reaches the Thames every drop of water has been through at least five human beings through water abstraction.
I don't doubt this as the levels; even in the winter; sometimes gets extremely low.
One thing that doesn't help is that most of the rivers that used to have prolific catches of barbel in them but now have a reduced number of barbel (Ivel, Gt Ouse, Kennet, Avon, Stour, and lower Lea) have otters, which although this may not be the main reason could be a contributory factor along with the reduced flow and silting up of the bottoms caused by abstraction. NB: We haven't seen any otters on the prolific upper Lea yet so once they arrive we can see if they make any difference to our barbel stocks. I suspect it will be detrimental to the quality of our catches; I just hope I am wrong.
I'm no expert but that is how I see it anyway.
Keith