I only need to read a change of about 1m but I will need to do it from a at least 1m above that or the thing will drown when the river is in flood.
The water is flowing, disturbed and far too dirty, floating debris of all sorts and sizes, to offer a flat surface.
That is why I was asking about the tube ...
A float seems like a good plan, polystyrene perhaps?
However if ultrasonics don't work in a tube then I probably wont be using ultrasonics
I will try it anyway I think whilst expecting to be disappointed, I may learn something from the attempt that points to another solution.
Thanks for the input
Al