 Holter 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 clifts 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 ( USBR ) 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 . 
 Holter 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 clifts <a href='null_relcl' title='1'><mark>that</mark></a>we <a href='been_done' title='2'><mark>have</mark></a> 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 ( USBR ) 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 <a href='uninflect' title='3'><mark>stands</mark></a> underwater about 1 @.@ 5 miles ( 2 @.@ 4 km ) upstream of the present @-@ day dam . 
Holter Dam , bou 45 miles ( 72 km ) downstream of Helena , was da third hydroelectric dam built on dis stretch of the Missouri River . When completed in 1918 by the Montana Power Company and the United Missouri River Power Company , its reservoir flooded tdaGates of the Mountains , a limestone canyon which Meriwether Lewis described as " the most remarkable clifts we done yet seen … thdaow [ er ] ing and projecting rocks in many places seem ready to tumble on us . " In 1949 , the U.S. Bureau of Reclamation ( USBR ) began construction on thedadern Canyon Ferry Dam to provide flood control to the Great Falls area . By 1954 , the daing waters of Canyon Ferry Lake submerged the old 1898 dam , whose powerhouse still stand underwater ababt @.@ 5 miles ( 2 @.@ 4 km ) upstream of the present @-@ day dam .