 In the early stages of Pick – Sloan development , tentative plans were made to build a low dam on the Missouri at <unk> , North Dakota and 27 smaller dams on the Yellowstone River and its tributaries . This was met with controversy from inhabitants of the Yellowstone basin , and eventually the <unk> proposed a solution : to greatly increase the size of the proposed dam at <unk> – today 's Garrison Dam , thus replacing the storage that would have been provided by the Yellowstone dams . Because of this decision , the Yellowstone is now the longest free @-@ flowing river in the contiguous United States . In the 1950s , construction commenced on the five mainstem dams – Garrison , Oahe , Big Bend , Fort Randall and Gavins Point – proposed under the Pick @-@ Sloan Plan . Along with Fort Peck , which was integrated as a unit of the Pick @-@ Sloan Plan in the 1940s , these dams now form what is known as the Missouri River Mainstem System . 
