who were the early explorers there?
Louis Jolliet and Jacques Marquette were the first ones, followed by Étienne de Veniard, Sieur de Bourgmont.
Missouri River
In May 1673, the French-Canadian explorer Louis Jolliet and the French explorer Jacques Marquette left the settlement of St. Ignace on Lake Huron and traveled down the Wisconsin and Mississippi Rivers, aiming to reach the Pacific Ocean. 
where do the kansas and missouri rivers meet Kansas City, Kansas what are its major tributaries? Kansas, Platte, Big Seoux among others
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Missouri River [SEP] Early European explorers
In May 1673, the French-Canadian explorer Louis Jolliet and the French explorer Jacques Marquette left the settlement of St. Ignace on Lake Huron and traveled down the Wisconsin and Mississippi Rivers, aiming to reach the Pacific Ocean. In late June, Jolliet and Marquette became the first documented European discoverers of the Missouri River, which according to their journals was in full flood. "I never saw anything more terrific," Jolliet wrote, "a tangle of entire trees from the mouth of the Pekistanoui [Missouri] with such impetuosity that one could not attempt to cross it without great danger. The commotion was such that the water was made muddy by it and could not clear itself." They recorded "Pekitanoui" or "Pekistanoui" as the local name for the Missouri.