Complex sentence: Downtown Pittsburgh, Pennsylvania, officially called the Golden Triangle or Central Business District, is a very dense, urban and walkable downtown located at the confluence of the Allegheny River and the Monongahela River whose joining forms the Ohio River.
Split into simpler sentences: Downtown Pittsburgh, officially called the Golden Triangle or Central Business District, is the urban downtown center of Pittsburgh. It is located at the confluence of the Allegheny River and the Monongahela River whose joining forms the Ohio River.