How can I get roof rain runoff into pond with an aboveground wall?

• Run your down spouts into barrels.
• Plumb these barrels together into a central feed.
  • These can run underground if you'd like, so long as the pipes have the proper slope, gravity will be your friend.
• Have a sump pump (or other, float-activated pump) pump the water up and over the retaining wall.

Depending on how full your pond normally is and how much rain you get, the addition of all the runoff from your roof could end up overfilling the pond on a regular basis, meaning that you've spend a lot of time, effort & money moving water around, just to have it go down the overflow tube and into the storm sewers, when you could just let the runoff do that for free via gravity. You may want to do some calculations along these lines on your own before doing all the plumbing to ensure it's worth the effort.

You could call it an "inverted syphon"... I'm searching for a word here, but it's not coming to me.

Build a leak-tight pipe that starts at the house above the top of the pond wall. The pipe goes down, underground, laterally to the pond, up the exterior of the pond wall, then over the wall and dumps into the pond. This pipe will be pressurized so you have to use competent plumbing pipe (sewer pipe will do) and actually glue/caulk it.

It will need a screen at the top to keep out junk. Also you will want to "tee" in a cleanout pipe that goes downgrade, with a valve; open the valve and blast water down it with a hose to clean out crud, and also leave the valve open all winter to winterize the pipe.

The pressure in a pipe is decided by the height of the water above it, and due to how pressure works, the only factor that matters is height. So as long as the pipe intake is above pipe outlet, the water going down the intake side will apply more pressure than the water sitting on the outlet side, and water will move that direction.

This is the same concept that happens under your sink and under every toilet bowl. In that case the objective is to create a "plug" of water to stop sewer gases, but you notice this, um, what's the word, help me out here Admiral Ackbar, does not impede the flow of water at all.

I would build a decorative arbor over the path. Its purpose would be to camouflage a basic gutter and downspout arrangement that dumps into the pond. Simple as can be.

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This answer incorporates some of the info provided in comments. To simplify Harper's answer a bit, I'm pretty confident this system will work just fine:

During a time when the pond is low, you can dig your trench and install pipe underground, entering the pond underneath the wall.

When the pond fills (from roof runoff or other means), you'll have standing water in the pipe, but never higher than the level of the pond. Make sure to slope your underground pipe downward into the pond by about 1/4" per foot, so that if the pond goes completely dry then you don't have standing water in your pipe.

I might try putting some sort of filter in the pipe next to the house, where it is easily accessible and always above the maximum water level, for easy removal and cleaning. I would also put some sort of grill over the end of the pipe in the pond, to prevent leaves or ASCII fish from getting into the pipe.

You'll mostly have flow going from the house out to the pond, so the Bti used in the pond for mosquito control might not find its way into the pipe. But if you find that the pipe is causing mosquito problems then you could always pour a little Bti into the pipe using your filter access point.