 The Missouri River officially starts at the confluence of the Jefferson and Madison in Missouri Headwaters State Park near Three Forks , Montana , and is joined by the Gallatin a mile ( 1 @.@ 6 km ) downstream . The Missouri then passes through Canyon Ferry Lake , a reservoir west of the Big Belt Mountains . Issuing from the mountains near Cascade , the river flows northeast to the city of Great Falls , where it drops over the Great Falls of the Missouri , a series of five substantial waterfalls . It then winds east through a scenic region of canyons and badlands known as the Missouri Breaks , receiving the Marias River from the west then widening into the Fort Peck Lake reservoir a few miles above the confluence with the Musselshell River . Farther on , the river passes through the Fort Peck Dam , and immediately downstream , the Milk River joins from the north . 
 The Missouri River officially <a href='uninflect' title='1'><mark>starts</mark></a> at the confluence of the Jefferson and Madison in Missouri Headwaters State Park near Three Forks , Montana , and is joined by the Gallatin a mile ( 1 @.@ 6 km ) downstream . The Missouri then <a href='uninflect' title='2'><mark>passes</mark></a> through Canyon Ferry Lake , a reservoir west of the Big Belt Mountains . Issuing from the mountains near Cascade , the river <a href='uninflect' title='3'><mark>flows</mark></a> northeast to the city of Great Falls , where it <a href='uninflect' title='4'><mark>drops</mark></a> over the Great Falls of the Missouri , a series of five substantial waterfalls . It then <a href='uninflect' title='5'><mark>winds</mark></a> east through a scenic region of canyons and badlands known as the Missouri Breaks , receiving the Marias River from the west then widening into the Fort Peck Lake reservoir a few miles above the confluence with the Musselshell River . Farther on , the river <a href='uninflect' title='6'><mark>passes</mark></a> through the Fort Peck Dam , and immediately downstream , the Milk River <a href='uninflect' title='7'><mark>joins</mark></a> from the north . 
The Missouri River officially start at da confluence of the Jefferson and Madison in Missouri Headwaters State Park near Three Forks , Montana , and is joined by tdaGallatin a mile ( 1 @.@ 6 km ) downstream . Thdaissouri then pass through Canyon Ferry Lake , a reservoir west of the bigg Belt Mountains . Issuing from thedauntains near Cascade , the daer flow northeast to the city of Great Falls , where it drop over the Great Falls of the Mdaouri , a series of five substantial waterfalls . It then wind east through a scenic region of canyons and badlands known as the Missouri Breaks , receiving the Marias River from the wedathen widening into the Fort Peck Lake reservoir a few miles above the confluence with the Musselshell River . Farther on , the river pass thru the Fordaeck Dam , and immediately downstream , the Milk River join from the north .