Thanks for the suggestion. I think a toilet valve would be too big to
hide in an inlet in the retaining wall, though. But since my post I
have found some small similar valves on the Internet. I figure I could
jerry-rig one into the retaining wall, without needing to integrate it
to a skimmer or filter. Now I just need to find one that I can hard
plumb to pvc, instead of a garden hose. Any suggestions out there?

Also, does anyone know if the Beckett Medium Waterfall-Stream Pump has
an adjustable flow rate? I'd like to be able to make the flow just
right for the wall spout, especially since later on I plan to move the
outlet up the hill to make a stream-waterfall. I'd lmake the flow
stronger at tht time. Thanks,
Sam