What would happen if you took the desert sand and used it to replace the river sand? Because you’re now mining and transporting twice, you’ll definitely have higher costs.
They might be. You might find that smoother sand flows more easily, and may be washed away. Certainly the size of grain is important, as smaller grains are carried more easily by the water.
If the intention is simply to displace water, then a better option may be larger rocks. These would eventually be worn down to make more angular sand.
Industry wants the sand because it locks together instead of flowing. Why wouldn’t a river “want” the same thing?
Won’t all the round sand end up at the bottom of a lake or the ocean after a couple of years of seasonal rainstorms?