Both Stephen E. Ambrose in "Undaunted Courage" and Elliot Coues (ed.), "The History of the Lewis and Clark Expedition" make it clear some soldiers volunteered from the existing US Army, other men were enlisted for the expedition and others served as civilians.
In Ambrose, cross reference the names of volunteers with the names of their commanding officers to find out what regiments the volunteers for the expedition came from. Coues has a biography of Sergeant Patrick Gass, a lifer God bless him, who served into the War of 1812 and lived to be 99.
Bernard De Voto in "The Journals of Lewis and Clark" has a breakdown of personnle by name in Appendix II.
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