How can I keep the birds from pooping all over my pool waterfall??

If there are branches above the waterfall, I would add some strips of that scare tape.... https://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=bird+scare+tape&rh=i%3Aaps%2Ck%3Abird+scare+tape

Maybe even add one of those plastic owls whose eyes light up.... https://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=bird+scare+owls&rh=i%3Aaps%2Ck%3Abird+scare+owls see they even have an eagle now

I think the only way would be for your yard to be less attractive to birds. No fruit trees or tomatoes or berries in your garden, no bird feeders, and no water for them to bathe in. But then you're left with a yard that's less attractive to people too :-) Maybe you could try setting up a net of sorts to keep them away from it, but then you've got a big net in your yard, and they'd probably just sit on the net and poop through it. Some people put up fake owls to scare away other birds. I think the birds get used to those pretty quickly though, like the crows sitting on the scarecrows :-) If you come up with anything, please share - I'd love to know about it!

put beachballs in the pool. AS they are blown around, it scares the birds off.

Would maybe an Umbrella over the water fall provide the needed protection. You could periodically wash top of it off with water hose.

What about an outdoor dining canopy? They are available in a variety of sizes, perhaps large party size might work?

One other thought comes to mind, a pipe frame the size needed & use outdoor shade cloth with it which is much more durable than a plastic tarp as well as attractive, unless you want to spend the money for heavy vinyl awning material. Make it such that it could be removed to clean, put grommets on the edges of cloth & mount to pipes with shower curtain rings.

This may not work either, but I'm trying....

If it’s possible to hang a artificial owl on a pole it will keep them away .

I have an owl planter and a couple of solar owls on the rocks with light up eyes.

Could you post a pic of it so we can see the area you are wanting to protect? We all may not come up with any real solutions but what could it hurt?

Sorry to hear your problem, but the birds probably use it as a perch when drinking.

Train you dog to chase them off.

https://www.youtube.com/watch?v=hfGRnZ3zYYY

There are some ideas here as well: https://www.thespruce.com/how-to-keep-birds-off-porch-6746337