Like almost everything gardening on GardenWeb, we should probably know where you are (zone). You have the advantage of dealing with a natural pond and don't need as much water control or dealing with pots to anchor plants. My only general thought is to remind you that the upper flat 'grass' area should have a slight rise against the pond so that excess fertilizer doesn't run off and end up in the pond.

Zone 4...thinking about a native praire grass mix?

As I have mentioned various times, I am a big fan of Acorus (sweet Flag). I am surprised to see that it supposedly will grow in zone 4 since I believe it is native to the Middle East. Like most here, I have a liner built pond (and stream). Sweet flag is really useful there because it will grow directly in water but it is also happy in damp conditions. Mine seems to grow less than two feet tall (rather than the listed three feet) in clumps.

I specifically suggest it (or Mondo Grass or Lirope if they would grow in zone 4, I didn't check), because you have a huge area to cover. I originally bought a gallon pot full of a clump of Sweet Flag and soon realized that I could split it into several little clumps with a shovel. As it turned out, it can be torn apart into individual plants. I would think you could take a single clump and divide it up into a pile of small clumps and spread them in a small area. In a year or so, you could dig them all up and divide into individual or very small clumps and spread them all over your edge and into the water.

Of course, my spreading theory might not be as easy or fast as it is for me in zone 7, and in fact might be impossibly slow. My problem is to keep it under control.