All streams atop the mountain are headwaters, and feed most of the 19 streams and rivers in the Sourland region, which then flow into two main watersheds, the Delaware and Raritan Rivers. Streams and rivers do not recognize political boundaries: residents of New Jersey and Pennsylvania whose water supply is taken from the Delaware and Raritan are using, in part, water that emanates from Sourland Mountain.

Development on Sourland Mountain has always been constricted by scarcity of water.  Hard bedrock impedes penetration of water into aquifers (underground geological formations containing or conducting ground water), resulting in limited supplies of water that can be withdrawn through domestic wells, which are the primary source of potable water in the region.

The forests of the Sourlands are inextricably connected and of crucial importance to the water resources  of the Mountain. Contiguous forests, wetlands, floodplains and vernal pools protect the Mountain’s water quality and quantity. Forested riparian areas protect water quality by buffering streams from surrounding land uses and by reducing water temperature, stabilizing stream banks, filtering pollutants from runoff and providing habitat for stream life.

There are no public sewers on Sourland Mountain, and the bedrock at shallow depths limits installation of effective septic systems.  Improperly treated sewage can flow along the boundary between soil and bedrock, entering fissures and seeping into the aquifers where it mixes with ground water and creates serious health hazards. Failing septic systems can introduce contamination into surface waters.

Other threats to Sourlands water quality include sedimentation, construction of impervious surfaces, installation of storm sewers, compaction of soils, deforestation and disturbance of riparian areas, and agricultural runoff consisting of animal manure , commercial fertilizers and pesticides.