Complex sentence: The region's lower western border follows the Arkansas River just outside Little Rock down through Pine Bluff where the border shifts to Bayou Bartholomew stretching south to the Arkansas-Louisiana state line.
Split into simpler sentences: The region's lower western border follows the Arkansas River just outside Little Rock down through Pine Bluff. There the the border shifts to Bayou Bartholomew, stretching south to the Arkansas-Louisiana state line.