Complex sentence: The Neponset River forms the southern boundary of the city of Boston, through the neighborhoods of Readville, Hyde Park, Mattapan and Dorchester, with the northern border of the city of Quincy.
Split into simpler sentences: The Neponset River forms the southern boundary of the city of Boston through the neighborhoods of Readville, Hyde Park, Mattapan and Dorchester. It also forms the northern border of the city of Quincy.