Numerous brooks and streams flowing out of the Ramapo Mountains to the west feed the river as it runs south.
Just south of its source at the foot of Lime Kiln Mountain, the Mahwah passes through the Ladentown Swamp, an extensive wetland area that is one of the last unspoiled aquifers in Rockland County, New York.
Approximately 4.5 mi (7.2 km) south of its source the river passes under Sky Meadow Lane and shortly thereafter enters a stretch where it is surrounded by Harriman State Park on both sides.
It also hosts waterfowl species, including wood duck in its northern reaches, mallards and Canada geese throughout its length.
Pileated woodpeckers, wild turkey, whitetailed deer, black bear and coyotes are common in the surrounding woodlands and mountains.