December 4, 1783 | Letter Thomas Jefferson to George Rogers Clark, December 4, 1783 Thomas Jefferson expresses concern over supposed British plans to explore North America from the Mississippi River to California and asks Clark if he would be willing to undertake such an exploration on behalf of the United States. Jefferson also addresses the ongoing Congressional debate over the location of the new capitol, believing a site on the Potomac River would be amenable to western states as they form.