Every spring during snow melt, and a heavy rain I worry. Pretty much a creek at that point. I have needed to dress the outflow with rock piles every few feet down the outflow and inflow to slow water and erosion. Far more water than I or the construction group anticipated. Too bad that all dries up in June or so. I don't have a drainage pipe, and rely on the overflow to handle it.
The guess is there is farm tile directing water into the pond drainage covering a larger area then the 8 or so acres of runoff the map says.
We are anticipating 2-3 inches of rain in the next couple of days, and I already have a lot of water running through. I have some work tonight collecting some more stones to break up water flow into the pong this evening. I am really hoping this delivers more food to my hungry fish! The BCP are croaking (though secretly I am not too sad, there are too many mouths in the pond, and PCB can be a problem)