I can tell you that an oversized watershed can be worrisome. My 1/4 acre pond has about 25 acres of watershed, maybe 10 times what it should. When I had it renovated last year I intentionally added extra freeboard to the dam so that it can fill up more so than most ponds before going over the emergency spillway (read low spot on the end of the dam). This allows the water to build up more before topping the dam. I also have a 15 inch drain pipe that I have seen 1 foot under water as the pond was trying to overfill. Before the pond was renovated it had a 20 inch drain pipe and overflowed the dam once every couple years. No damage occurred however and the pond was 40 plus years old so it had been doing this off and on for while without compromising the dam. A lot of luck was involved, I'm sure. The pond was originally 10 foot deep and before renovation it was only 2 foot deep...it had 8 foot of silt in it that had to be removed. I would guess that the old pond was too shallow for fish to survive many winters by the time it was 20 years old.

I am not encouraging you to build this pond with an oversized watershed, as my pond already existed when we bought the place and is actually used to slow the water down before it reaches the front yard.

High amounts of incoming water can also cause temperature turnovers, rapid silting, and fast PH/water quality swings which can be hard on fish populations.

All that said, I would not intentionally build a pond with too much watershed. Like John says...look into diverting some of that watershed.

Fish on!,
Noel