NyQuiLlama,
Always at ease.
This should be a simple job for an experienced operator to remove the old culvert. I would start on the end away from the water and work my way up taking the last bite out at the water. You will loose some water but the outflow in your small pond shouldn't be overly high assuming you don't run into other problems. For example, the critters may have caused damage that isn't obvious to the naked eye and that damage, remote possibility, could be tied to your digging area and take out more than just your culvert. If you suspect that is the case, you might want to rent a small trash pump and lower your water level before starting digging.
It is critical that you get good material back in the spillway and have a very gradual slope to carry off the excess water in the future.
Good luck and let us know how your adventure turns out.
