I'm reading Undaunted Courage, and I've also seen documentaries about the Corps of Discovery, and in both, they take about "sending specimens and letters" back to Thomas Jefferson. Does that mean men from the expedition went back? There's obviously no mail system beyond the Mississippi...in the book they are almost to the Rockies and they sent back some magpies and a prarie dog. It doesn't say how.
I am at work and wish I had time for a more lengthy response
I suppose depending on where you are in 'Undaunted Courage' these might be spoilers
As brief as I can make it, following the long winter which the Corps spent camped in a fort near the Mandan villages, the Captains sent their big keelboat back down the Missouri and continued on upriver in a flotilla of canoes and pirogues.
Manning the keelboat were about a dozen picked men from the expedition, including the man who was court-martialled out of the expedition for nearly deserting with his rifle & etc and causing some dissent among the men during that first year of the expedition. The object of the men who paddled the keelboat back towards St Louis was first to survive the trip back through Lakota territory, and then to transport specimens, documents and even a magpie and a prairie dog (plus a confident and now famous letter from Lewis to Jefferson) to either Philadelphia or Washington DC, and in some cases directly to Jefferson's Monticello home
Jefferson was deliriously excited with pretty much all the things which the Corps sent back, sleeping in a buffalo robe, planting various seed and plant specimens in his private garden, and the like
Kee p reading! There's a ton of great info in Undaunted Courage