Hi Rockytopper, took your advice and started another thread on habitat - good call.
Regarding the falls, the latest thinking is that perhaps we'll pump water up the hill a bit and create a natural looking stream/waterfall that flows back into the pond to help with aeration, turn-over and aesthetics.
Combining primary and aux spillways per your suggestion could be enabled by several PVC siphons in parallel, enough to carry 4 cf/sec for the 25y storm event. I'd like to hear more about your stair steps on the side of the pond and how it may look.
Rainman suggested that I look at what AaronM did, since he has great production from an even smaller pond. Lots of good stuff there. You're probably already familiar with it, but he went with an Aquascape design with stepped down sides, liner, and a gravel covering for spawning beds - it appears that a primary benefit of the stepped down sides is that it will help to keep the cover material (gravel) in place.
Below the main pond we're looking at installing a smaller pond (puddle) for aesthetic purposes. It is kind of a silt basin now, so we thought we'd just excavate down a little bit and create a marsh/bog type thing. This almost sounds like the marsh/bog type filter that AaronM created for his pond, only his is placed before the pond, not after. RT, Do you think it feasible to pump the outlet of this puddle/marsh/bog to supply the waterfall so that we could take advantage of the filtering the marsh would provide before returning it to the pond? Of course this would involve a much higher "normal" flow rate in the stream leading from the pond to the marsh, as compared to pumping directly from the pond, but we could design the stream accordingly (typically the marsh would only get the overflow from the pond, probably less than 10 gpm or so).
