Complex sentence: This was a significant success, which kept the flooding out of the Faubourg St. John, Marigny, and other downriver portions of the city, and by providing an outlet for the waters prevented even deeper and more widespread flooding above the Canal.
Split into simpler sentences: This was a significant success, which kept the flooding out of the Faubourg St. John, Marigny, and other downriver portions of the city. By providing an outlet for the waters, the action prevented deeper and more widespread flooding above the Canal.