 In the early stages of Pick – Sloan development , tentative plans were made to build a low dam on the Missouri at Riverdale , 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 USBR proposed a solution : to greatly increase the size of the proposed dam at Riverdale – 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 . 
 In the early stages of Pick – Sloan development , tentative plans <a href='uninflect' title='1'><mark>were</mark></a> made to build a low dam on the Missouri at Riverdale , 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 USBR proposed a solution : to greatly increase the size of the proposed dam at Riverdale – today <a href='null_genetive' title='2'><mark>'s</mark></a>Garrison Dam , thus replacing the storage <a href='null_relcl' title='3'><mark>that</mark></a>would <a href='been_done' title='4'><mark>have</mark></a> been provided by the Yellowstone dams . Because of this decision , the Yellowstone <a href='drop_aux' title='5'><mark>is</mark></a>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 . 
In the early stages of Pick – Sloan development , tentative plans was made to build a low dam on the Missouri at Riverdale , 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 da USBR proposed a solution : to greatly increase the size of tdaproposed dam at Riverdale – today  Garrison Dam , thus replacing thdatorage would done been provided by thedallowstone dams . b/c of this decision , the Yellowstone now the dagest free @-@ flowing river in the cdaiguous United States . In the 19da , construction commenced on the five mainstem dams – Garrison , Oahe , bigg 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 194da, these dams now form wat is known as the Missdai River Mainstem System .