Thanks for your words!
Ok, I needed much more time as I had planned to select, as I hope, the right stuff for pouring the river. Finally I decided to try "Heki aqua Nr. 3550" (Thank you for the hint, Marcel!) which I poured in several layers. I'm quite happy overall and the stuff is very easy to handle indeed. There were only two or three air bubbles I had overlooked. But they were no problem, as they could be opened up by drilling and easy filled by the next layer.
As I had described bevor, the whole arrangement is dividided in an underwater and a surface zone. So the unbeloved creeping up the bank and walls can be prevented. As a consequence I had to poure in all parts up to the waterline, i.E. the walls, the piling and the water gauge.
Notice also the shadowing sprayed directly on the riverbed.
So far so good, BUT ...
...now I have to add some wave structure. There are some good tutorials around, but all are showing the same problem: The waves look always too prominent and uniform.
Stop! One artist, I don't know his name, demonstrates the way it should look! That looks wonderful!
So back to the workbench!