Simplify the following text for a Grade 10 native English-speaking person.
Original: The Trempealeau River ( pronounced TREM-puh-lo ) is a tributary of the Mississippi River , 55 mi ( 86 km ) long , in the Driftless Area of western Wisconsin in the United States .
Simplified: The Trempealeau River is a tributary of the Mississippi River in the western part of Wisconsin , United States .