Actually, the 4 foot section takes up a good half of the pond. The 2.5' section is relatively small. According to the pond calculators that I've been using (which, I know are not terribly accurate), I am somewhere around 3000 gallons.
The measurements I'm using are: 15' length, 8' width (both ends are 8', with a 7.5' section in the middle where it comes in slightly - this is INSIDE the plant shelf, FYI). The 4' deep section is about 8' x 8', while the 2.5' section is much smaller since it includes the stairs. When I use this calculator:
http://www.gardenponds.com/kidponcalc.htm...
If I use these dimensions: 8', 8', 7.5', 15', 4', 3' - I get 3300 gallons
If I use these dimensions: 8', 8', 7.5', 15', 4', 2.5' - I get 2900 gallons
I'm not sure if either of these are superbly accurate though, so I'd really like to try using a water meter if I can get my hands on one when we're ready to fill. If not, I'll do the 5 gal bucket fill method and time it to get the gallons. What I may do this weekend if we get the last of the digging done is measure individual sections to use in a pond calculator - so I'll measure the sections on either side of the stairs at the 2.5' depth to get the gallons in those sections, then the 4' section as a separate entity, etc. Might be more accurate that way.
As for the growing mediums, I will check those out. I haven't yet found a local place to get the hydroton. Not sure if I'll find a local place for either of those either, but we'll see. I probably won't do the river rock thing because I want the roots to grow down into the filter if the plants feel the need, but we'll see how it goes. We will at least get the section for the filter dug this weekend, but it's rained an awful lot the last few days and there's still quite a bit of water IN the pond (clay drains VERY slowly), so I don't know if it will be dry enough for us to dig in the pond this weekend.