Complex sentence: Rome was located at the confluence of the Etowah and the Oostanaula rivers, whose merging forms the Coosa River, in order to have access to the waterways, the major transportation routes of the era.
Split into simpler sentences: Rome was located at the confluence of the Etowah and the Oostanaula rivers, whose merging forms the Coosa River. This gave it access to the waterways, the major transportation routes of the era.