Upon further inspection, the player can find a Ginseng Elixir between a log and a rock, just west of the dam itself.
South of the dam, about halfway to the border with Tall Trees, a group of scattered crates can be found near the road. Among them is a lockbox containing Chewing Tobacco, money, and the Special Bitters Pamphlet.
This dam is a likely explanation for the lower water level of the Upper Montana River during the storyline of Red Dead Redemption 2. By the time of the first game's story, the water level is much higher than as seen in the second game. It is possible the dam was either damaged, opened up, or left to disrepair.