Complex sentence: By now, Newtown Creek had become a major industrial waterway, with the city starting to dump raw sewage into it in 1866; it was bounded along most of its length by retaining walls, the shipping channel maintained by dredging.
Split into simpler sentences: By now, Newtown Creek had become a major industrial waterway, with the city starting to dump raw sewage into it in 1866. It was bounded along most of its length by retaining walls, and the shipping channel was maintained by dredging.