i tried to do something like this. but instead of a huge river system. i made a pipping system that went under my place and a road to a dug out lake area. Once i opened the flood gate from the water source blocks, the water only flowed a few blocks within the pipe system. I then made the bottem of the pipe system out of brick in the V shape form and the water sadly only flowed one or two blocks more then it did with dirt. I would love to watch your guide on how to move water around.
my pipe system was only 2 blocks high and 1 wide. that was i could walk in it. the opening was on a river side wall that was dirtly underneath watersource blocks