Complex sentence: There, at and beyond the western edge of the Appalachian plateau, and terrain became less hilly and navigable waterways like the Kanawha River and the Ohio River led to the Mississippi River, and thence to the Gulf of Mexico.
Split into simpler sentences: There, at and beyond the western edge of the Appalachian plateau, the terrain became less hilly. In addition, navigable waterways like the Kanawha River and the Ohio River led to the Mississippi River, and thence to the Gulf of Mexico.