I remeber the same thing happen to me on a small pond dam once. Just like you described. I dug out 2 trenchs set traps like you are describing AND stake a conniber between the 2 trenches. I could the beaver the next day. He had pluged the first dam/ trench i made, and was heading to the second one 6 ft away and that conniber caught him. Hope this helps. Good luck!