Eventually six different leaders for the Red River-Arkansas Expedition were considered before, finally, in November of 1805, Jefferson settled on a young man who seems to have been about thirty-five years of age--we don't know exactly how old he was--whose name was Thomas Freeman.
Thomas Freeman, unlike Meriwether Lewis, was not an officer in the American Army, he was a civilian. He was a civil engineer and a surveyor who in fact was an Irish immigrant to the United States. He had a whole range of political contacts across the spectrum in early America. He was a friend of Alexander Hamilton, for example, of the Federalist Party. Played a role in surveying Washington, D.C.
And he seems to have been brought to Jefferson's attention by William Patterson, that mathematician at the University of Pennsylvania who had tutored Meriwether Lewis. Patterson suggested Freeman in the summer of 1805, and in November of 1805 Freeman had a private dinner at the White House with Thomas Jefferson, where Jefferson, after two years of searching, finally turned over that letter of exploring expedition--or, exploring instructions--and inscribed "To Thomas Freeman, Esquire" across the top of it.
So Jefferson finally had a leader for his expedition. It was a civilian leader, however, and that's one of the aspects of this party that makes it different from Lewis and Clark. It's led by civilians rather than by the American military.
Jefferson was concerned, however, that a military detachment was going to have to be added to the Expedition, and so, with the advice of Meriwether Lewis, who had done a careful appraisal of all the officers in the American Army, Jefferson selected a Virginian by the name of Richard Sparks, captain Richard Sparks, to lead the military contingent that would be attached to this southwestern probe.