Sounds like the warning signs at every boat ramp I've been to are, obviously not working.
Boats are not spreading this stuff around. Water running into the lakes is whats bringing it.
+1. And they have stuff to kill it, it just kills everything else also. Look at toledo right now. Cold winters knock it back pretty good and I've noticed on rayburn that when the water rises and falls that a bunch gets left high and dry on the bank.