I would agree with kenc. Without getting into to much minutia, water is relentless and always tries to go in a straight line unless it hits a diversion. If the creek banks are gradually V shaped and meander, then I would be less worried than if they're steep and fairly straight. Historical water flow can be read, but it doesn't sound good.

My two thoughts are, yes get a second opinion from a reputable pond builder, and whatever water comes in a pond has to go out. Building an overflow/drain system adequate enough to handle that much water could be a nightmare, and very expensive.

Silt and nutrient load from that much water could be an issue also.

Good luck, and keep swinging at it. I bet there's a solution somewhere.

AL