rcahow,
I am contemplating a similar wall structure and looked into concrete pipe, I have it here from 4" to 48" in diameter with assorted lengths. I was going to stand them on end side by side and fill some with concrete and the others with gravel. Then top the wall off with a concrete cap. Here concrete is much cheaper than woood.

1/4 & 3/4 acre ponds. A thousand miles from no where and there is no place I want to be...
Dwight Yoakam