 If the Jefferson were included in the Missouri length, it would technically still be considered the longest river by experts of the United States Geological Survey.The utmost headwaters of the Missouri are subject to debate but two commonly claimed locations ultimately drain into the Jefferson.Lewis on August 12, 1805 said he visited the headwaters on Trail Creek just above Lemhi Pass on the Continental Divide in the Beaverhead Mountains at around 8,600 feet which he described