 The Lewis and Clark decision not to call the Jefferson the Missouri has spurred debate over what is the longest river in North America, since the Missouri and Mississippi are nearly identical in length.The Missouri traditionally had been called the longest river in North America.However, 72 miles of it have been trimmed off in re-channeling its streambedmainly for the many hydroelectric power plants in the region so that it is now sometimes referred as second to the Mississippi in terms of length.