 The Missouri River officially starts at the confluence of the Jefferson and Madison in Missouri <unk> 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 . <unk> 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 <unk> River from the west then widening into the Fort Peck Lake reservoir a few miles above the confluence with the <unk> River . Farther on , the river passes through the Fort Peck Dam , and immediately downstream , the Milk River joins from the north . 
