Questions about the end of the film

I just watched this film again for the first time in years. What did Bill Markham the father place on the dam at the end? It looked like dynamite. But then the river appeared to break the dam, while Bill Markham was watching and holding that gadget that one may think was a dynamite ssitch. Then Bill Markham appeared sad to notice the dam break. Why do you think he would have been saddened by that?