The Great Swamp Watershed is a 55-square-mile region in Morris and Somerset Counties of the state of New Jersey, and includes portions of ten different towns.
A watershed is a geographical area all of whose streams flow into a single body of water. In the Great Swamp Watershed, five streams – the Upper Passaic River, Black Brook, Great Brook, Loantaka Brook, and Primrose Brook all flow into Great Swamp and exit as the Passaic River via Millington Gorge.
The large south-central green/gray shaded area is the Great Swamp National Wildlife Refuge. Note that this protected area is only a small portion of the total watershed!