Complex sentence: The river forms the northern border of the Carara Biological Reserve, and is well-known for its American Crocodiles, while at its mouth the marshes are have many waterfowl and wading birds.
Split into simpler sentences: The river forms the northern border of the Carara Biological Reserve. It is a habitat for American Crocodiles, while the marshes located at the river's mouth have many waterfowl and wading birds.