Welcome Gigi. Yours is a dilemma I haven't read about here on the forum. It does seem that a berm several feet above the historic flood level could work. Our pond building expert Otto probably has solved a problem like this. Send him a pm if he doesn't jump in here in a day or so.

In any event, let us know how you resolve your issues. Keep a thread going about your experiences. If your heart is set on a fishing pond, then you will have one!