I wouldn't know how to set up an artificial waterfall or drainage system. Where does the water drain to? An evaporation tank of off the map? And what's the significance of the diagonality?
Fluid can be pressurised by pumps, water can also be pressurised by having more water sit on top of it. Fluid under pressure tries to find an empty space to go to, teleporting through other fluid-filled tiles. So if you pour water into a U-bend, both sides will have the same water level, magma would just fill the bottom and refuse to rise up the other side unless you use a pump.
Pressure-based fluid teleportation does not use diagnoal connections, so you can control it.
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View from above. The east/west pipe is your main water line, carrying pressurised water (Connected to a river or reservoir on a higher z-level). Two doors hooked up to levers regulate the water flow to the south. In normal operation, the top door is closed and the bottom door open - you draw water to feed your pretty waterfall, but there is no orthogonal connection. So pressure keeps the main line filled, and the depth gradient will keep the waterfall in action with a nice and steady flow.
The bottom door toggles water flow on and off, the top door turns pressure on and off. If both doors are open, you have a direct connection and fluid will teleport through your pipe to the south at an alarming rate rather than flowing leisurely.
You have a lot of options for getting rid of the water - a pipe leading off the map or into an acquifer, an everburning bin and so on; some people just dump water in the caverns and figure it's the problem of the denizens below.
At the bottom of our multi-level waterfall chamber, you could use a similar system as for the inflow. Again, with the default setting being 'flow on, pressure off'. diagnoal connection means your sewage system won't flow back up into your rooms if you have several similar installations in parallel and get overexcited with some.
Does this make sense? Not detailed enough an explanation, or am I repeating the obvious ad nauseam?