I don't know how to people, so I'm leaving those questions to people who do.
Shop vacs, on the other hand, I know at least a little about. The one I have works by evacuating the air from a container to create the suction, so you can't get away with not emptying it by cutting a hole in the bottom for the water to fall out of. You could, however, get another pump, like a sump pump for drying basements. Put that in a bucket with lots of small holes drilled in it, to act as a sieve and not suck anything else into the pump, and it could empty the pond. Unfortunately, according to the internet, sump pumps cost ~$200 new.
Can you redirect the waterfall? Put a piece of gutter or something under it to catch the water and redirect it to someplace else?
Also, you might consider fish. I hear koi are voracious plant eaters, so you could put fish in it to keep the plants down, possibly after cleaning it some first.
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Shop vacs, on the other hand, I know at least a little about. The one I have works by evacuating the air from a container to create the suction, so you can't get away with not emptying it by cutting a hole in the bottom for the water to fall out of. You could, however, get another pump, like a sump pump for drying basements. Put that in a bucket with lots of small holes drilled in it, to act as a sieve and not suck anything else into the pump, and it could empty the pond. Unfortunately, according to the internet, sump pumps cost ~$200 new.
Can you redirect the waterfall? Put a piece of gutter or something under it to catch the water and redirect it to someplace else?
Also, you might consider fish. I hear koi are voracious plant eaters, so you could put fish in it to keep the plants down, possibly after cleaning it some first.