Tom,
I will try to explain to you what your hydrologist freind was talking about when he refered to the weight of water being considered in the sizing of a pond. Now keep in mind I am a civil engineer with the heart of a golf course architect, therefore I will do my best not to lose you, so here goes.
To size a pond you need to know the surface area of the watershed or area of land that is flowing into it. Next you need to determine what type of soil is under the surface. Clays will allow more water to runoff or over the surface of the land vs. sand that will soak it up, right. Are you still with me. We determine a surface area and the underlying soil types. Next, you determine what the surface material in a post development state will be, basically you need to determine if it will be corn fields, hay fields, fine cut grass, etc. Basically, the denser and thicker the material, the less water will run off or over the surface. A cornfield will let more water run vs. a fairway which will soak it up more. So then you usually assign a value for each of the above items. Surface area (acres), soil type, and finished surface material. Then you run these numbers through a hydrology program called a "Walker" program which while assuming a rain storm event that drops 2.5" of rainfall in a 24 hour period, which then spits out a pond volume in cubic feet or acre-ft. Hope you got the jist of that Tom. Now I know that is too technical for golf courses, but the reality is that those man made ponds that you guys create on golf courses need to eventually dump into a natural water course, and the amount of water and the water quality need to be kept under control, in accordance with local, state, and federal govt. agencies.
So there you go Tom, if you have any other questions feel free to ask.
Oh, I almost forgot to something. When you put together your grading plan on paper or in the field, it is important that you don't "break" watersheds and dump more water into a pond, wetland, etc. than what originally was going into it.
Any other wetland, pond questions, feel free to ask.