 The Stillwater flows  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.The Stillwater and Quinapoxet rivers are the two major tributaries to the Wachusett Reservoir, which serves as the primary source of water for 2.5 million consumers in 43 communities of central and eastern Massachusetts.The U.S. Geological Survey (USGS), in cooperation with the Massachusetts Department of Conservation and Recreation (DCR), operates stream-flow monitoring gages near the mouths of both rivers.