Hatfield Swamp is a freshwater wetland in northern New Jersey of about 2,500 acres at the meeting of the Whippany, Rockaway, and Passaic rivers between Morris and Essex counties. Its origins reach back to the Newark Basin and later to the draining of Glacial Lake Passaic after the Wisconsin Glacier blocked the ancient outlet of the Passaic, leaving a broad belt of swamps. Frequent flooding from rain, snowmelt, and tropical storm remnants shapes its forest and wildlife and limits some species. Named for pre-Revolution mill owner Cornelius Hetfield, it is now public open space valued both for recreation and for absorbing floodwater.