The Missouri in name officially begins at Missouri Headwaters State Park at 4,045 feet in Montana at the confluence of the Jefferson River and Madison River. The Gallatin River joins the river about 0.6 of a mile downstream as it flows northeast. The Jefferson River originates in southwest Montana near the Continental Divide. The Madison and Gallatin Rivers flow out of northwest Wyoming to meet the Jefferson River.

Meriwether Lewis in his journal entry on July 28, 1805 wrote:

Both Capt. C. and myself corresponded in opinon with rispect(sic) to the impropriety of calling either of these [three] streams the Missouri and accordingly agreed to name them after the President of the United States and the Secretaries of the Treasury and state.

The Lewis and Clark decision not to call the Jefferson the Missouri has spurred debate over what is the longest river in North America since the Missouri and Mississippi are nearly identical in length. With the Jefferson the Missouri would be the longest river.

Lewis (who had followed the Jefferson River to the Beaverhead River) said that on August 12, 1805, he visited Beaverhead tributary of Trail Creek just above Lemhi Pass on the Continental Divide in the Beaverhead Mountains on the Montana and Idaho border at around 8,600 feet which he described:

the most distant fountain of the waters of the mighty Missouri in surch(sic) of which we have spent so many toilsome days and wristless(sic) nights.

However in 1888 Jacob V. Brower, who had championed turning the headwaters of the Mississippi River into a Minnesota state park, visited a site in Montana which today is believed to be the furthest point on the Missouri -- now called Brower's Spring. Brower published his finding in 1896 in "The Missouri: It's Utmost Source."

The site of Brower's Spring at around 8,800 feet in the Centennial Mountains. The site now commemorated by a rock pile at the source of Hellroaring Creek which flows into Red Rock River and then into Clark Canyon Reservoir where it joins the Beaverhead then the Big Hole River before ultimately hooking up with the Jefferson.

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