The Stillwater River is part of the Nashua River watershed. This river is part of the Massachusetts
Water Resources Authority system that supplies drinking water to the greater Boston
area. The Stillwater River rises in Princeton, Massachusetts, the watershed generally known as the Upper Worcester
Plateau, or the Monadnock Upland.
This watershed tops at Wachusett Mountain
, the highest feature in the area. Water flowing east from this high ground feeds the Nashua River Watershed, and water flowing west feeds the Ware River
or the Millers River
, both heading to the Connecticut River.The Stillwater flows 8.1 miles (13.0 km) through Princeton and Sterling
before joining the Quinapoxet River
at the Wachusett Reservoir
in West Boylston
to form the south branch of the Nashua River.