What happens when it rains?

I was often asked if a pond would overflow if it rained. I suppose so, but the water would only go into the surrounding landscape. I ran into many ponds with an overflow pipe built into them. These were concrete ponds and to me, that was just an extra expense and unnecessary. If the pond is going to overflow, let it. Now a flood can be a different story. If your pond floods like ours did in New Orleans after Katrina, you fish will probably swim away and you may lose your plants. But your pond will be OK.
If your pond is build at grade, you could have problems because all those chemicals and fertilizers and run off will end up in your pond when the rain stops. If your pond is built the way I recommend, about 4" above grade, then you will be fine.