 <unk> Dam , about 45 miles ( 72 km ) downstream of Helena , was the third hydroelectric dam built on this stretch of the Missouri River . When completed in 1918 by the Montana Power Company and the United Missouri River Power Company , its reservoir flooded the Gates of the Mountains , a limestone canyon which Meriwether Lewis described as " the most remarkable <unk> that we have yet seen … the tow [ er ] ing and projecting rocks in many places seem ready to tumble on us . " In 1949 , the U.S. Bureau of Reclamation ( <unk> ) began construction on the modern Canyon Ferry Dam to provide flood control to the Great Falls area . By 1954 , the rising waters of Canyon Ferry Lake submerged the old 1898 dam , whose powerhouse still stands underwater about 1 @.@ 5 miles ( 2 @.@ 4 km ) upstream of the present @-@ day dam . 
