Answer: The pond can be constructed with an overflow relief pipe, or it can be allowed to overflow from a slightly lower spot on the outer edge. With almost every pond, there is a spot that is a hair lower than the rest of the edge and that is where it will overflow. For this reason it does not overflow all the sides at once.The overflow is usually just the merest trickle as it would be in a cup that you have barely tilted -- the rain takes quite some time to accumulate depth over that wide an area. You will also find that most ponds are not filled to the lip, leaving several inches to spare between the lip and the water level. This is because the ponds tend to evaporate and need some topping up or refilling periodically. If you know it is going to rain, you would not top it up and thus allow room for some of that extra water. Finally, in my experience the plants in their pots do not move when it rains and the fish seem to hunker down and swim against the current, so it would be unusual to see them get washed out. They might jump out, but I have never seen them washed out.
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