Many people explored the land west of the Mississippi, but it seems most likely the question is asking about just after the Louisiana Purchase, if so then the answer is Louis and Clark. However if that is not the answer you are looking for you must resubmit the question with more specificity.
Lewis and Clark explored the northerly land west of the Mississippi River by foot.
Thomas Jefferson
The French
land west of the Mississippi river
Texas
lasalle explored the two rivers(Mississippi river and Ohio river) because he wanted to be the first one to find the north west passage and wanted to build a colony at the mouth of Mississippi river.
The Yazoo River is in west central Mississippi and flows into the Mississippi River.
The state of Mississippi is east off the Mississippi River. Mississippi is bordered by the Mississippi River to the west.
Spain originally claimed the land west of the Mississippi. However, Russia claimed Alaska and some of the west coast. Mexico had won independence from Spain in 1810. Then Napoleon of France conquered Spain and all of its lands.
It is west of the Mississippi River.
The land west of the Appalachian Mountains and east if the Mississippi River.
The Mississippi River is west of the state Alabama