Original: United States Geological Survey Hydrological Unit Code : 07-14-01-01 - Cahokia-Joachim Watershed The Missouri likely originates at Brower 's Spring at the upper reaches of the Jefferson River , before joining the confluence of the Madison , Jefferson , and Gallatin rivers in Montana .
Simplified: For most of its course , the Missouri flows across the Great Plains , one of the driest parts of North America .