I'm doing it a bit differently, right now, as my second pond is not nearly full, yet. I've about 8 feet to go before full pool, so I've been dragging mulberry and locust limbs down to the water's edge on the dam. I dig a post hole, say 8-10" in diameter about 2 feet deep. Stuff 2-3 large limbs or trees in and then tamp the clay back in with a sledge handle...I can lean them any direction I want and I don't need Sakrete.
Can't exactly move 'em around much, but it seems to work very well. I'll keep moving up the bank as water rises in spring. Just hope the water comes up and covers most of the dang things eventually. Wife's pretty tired of staring at my "tire reefs" and other "structure" in the older pond. Structure's getting to be a dirty word in our house...