Here's a useless piece of information because it is probably illegal but still is interesting. Years ago, on a 200 acre pond in western Maine, not too far from North Conway, I helped replace the washed out dam on the pond's small outlet as the water level had dropped by five feet and the camp season was about to start.
It was much easier than one would think. After loading up an aluminum rowboat with bags of dry concrete, we simply set the bags neatly into the water and replaced the old leaky earth & rock dam in one morning. Water soaks right through the paper bags and the concrete sets up, underwater. It turned out to be a very effective dam and lasted for at least five years, and is possibly still there in Maine.
..and a couple weeks later here in New Hampshire, a likely conversation...
"Hello DES....well ya-know....it must have been some type of cement beaver that built that there dam....I dunno"
