Depends on your location. If the water is coming from higher up in the mountain, a single pulse from Autonomous Activators to deploy/scoop up the sources, followed by a repeater into a not-gate to turn off some drawbridges. Come through the door and it shuts off and redeploys the water. Or the vanilla method with pistons to interrupt the water flow.
Sl1pgatr did it by just cutting a hole into the wall behind a water fall. It's all but impossible to see it unless you're right on top of it.