by whom?
Montana Power Company built the second dam.
Black Eagle Dam
Montana Power Company built the second dam, PPL Corporation purchased it in 1997 and sold it to NorthWestern Corporation in 2014. 
what river forms the border between iowa and nebraska UNANSWERABLE which are the main tributaries of the missouri river? James, Big Sioux, Grand River systems etc. where does it originate from? It officially starts at the confluence of the Jefferson and Madison in Missouri Headwaters State Park near Three Forks, Montana. does it have any dam over it? Yes, Black Eagle Dam, Ryan Dam, Canyon Ferry Dam, etc. when was the first one built? 1890
wiki:5672276
Black Eagle Dam [SEP] Introduction
The maximum power output of the dam is 18 MW. Montana Power Company built the second dam, PPL Corporation purchased it in 1997 and sold it to NorthWestern Corporation in 2014. The reservoir behind the dam has no official name, but was called the Long Pool for many years. The reservoir is about long, and has a storage capacity of to of water. The dam is a "run-of-the-river" dam because it can generate electricity without needing to store additional water supplies behind the dam.