The Stillwater River is part of the Nashua River

Nashua River

The Nashua River, long, is a tributary of the Merrimack River in Massachusetts and New Hampshire in the United States. It is formed in eastern Worcester County, Massachusetts, by junction of its north and south branches near Lancaster, and flows generally north-northeast past Groton to join the...

 watershed. This river is part of the Massachusetts Water Resources Authority

Massachusetts Water Resources Authority

The Massachusetts Water Resources Authority is a public authority in the Commonwealth of Massachusetts that provides wholesale drinking water and sewage services to certain municipalities and industrial users in the state, primarily in the Boston area.The authority receives water from the Quabbin...

 system that supplies drinking water to the greater Boston

Boston

Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region. The city proper had...

 area.

The Stillwater River rises in Princeton, Massachusetts

Princeton, Massachusetts

Princeton is a town in Worcester County, Massachusetts, United States.It is bordered on the east by Sterling and Leominster, on the north by Westminster, on the northwest by Hubbardston, on the southwest by Rutland, and on the southeast by Holden....

, the watershed generally known as the Upper Worcester Plateau, or the Monadnock

Mount Monadnock

Mount Monadnock, or Grand Monadnock, is the most prominent New England mountain peak south of the White Mountains and east of the Massachusetts Berkshires, and is the highest point in Cheshire County, New Hampshire...

 Upland. This watershed tops at Wachusett Mountain

Wachusett Mountain

Wachusett Mountain may refer to:* Mount Wachusett, the highest point in Worcester County, Massachusetts* Wachusett Mountain , the name of a ski area on the same mountain...

, the highest feature in the area. Water flowing east from this high ground feeds the Nashua River Watershed

Nashua River Watershed

The Nashua River Watershed is a major Massachusetts watershed.-Description:From its impoundment at the Wachusett Reservoir in Clinton, Massachusetts, the South Branch of the Nashua River flows north and joins the North Branch of the Nashua River in Lancaster, Massachusetts. The North Branch of the...

, and water flowing west feeds the Ware River

Ware River

The Ware River is a river in central Massachusetts. It has two forks, the longest of which begins near Hubbardston, Massachusetts...

 or the Millers River

Millers River

The Millers River is a river in northern Massachusetts, originating in Ashburnham and joining the Connecticut River just downstream from Millers Falls, Massachusetts...

, both heading to the Connecticut River

Connecticut River

The Connecticut River is the largest and longest river in New England, and also an American Heritage River. It flows roughly south, starting from the Fourth Connecticut Lake in New Hampshire. After flowing through the remaining Connecticut Lakes and Lake Francis, it defines the border between the...

. The Stillwater flows 8.1 miles (13 km) through Princeton and Sterling

Sterling, Massachusetts

Sterling is a town in Worcester County, Massachusetts, USA. The population was 7,808 at the 2010 census.- History :Sterling was first settled by Europeans in 1720 and was officially incorporated in 1781....

 before joining the Quinapoxet River

Quinapoxet River

The Quinapoxet River is part of the Nashua River watershed in northern Massachusetts in the United States. It is part of the Massachusetts Water Resources Authority water system supplying drinking water to the greater Boston area....

 at the Wachusett Reservoir

Wachusett Reservoir

The Wachusett Reservoir is the second largest body of water in the state of Massachusetts. It is located in central Massachusetts, northeast of Worcester. It is part of the water supply system for metropolitan Boston maintained by the Massachusetts Water Resources Authority . It has an aggregate...

 in West Boylston

West Boylston, Massachusetts

-Library:The West Boylston public library was established in 1878. In fiscal year 2008, the town of West Boylston spent 1.6% of its budget on its public library—some $37 per person.-External links:******...

 to form the south branch of the Nashua River

Nashua River

The Nashua River, long, is a tributary of the Merrimack River in Massachusetts and New Hampshire in the United States. It is formed in eastern Worcester County, Massachusetts, by junction of its north and south branches near Lancaster, and flows generally north-northeast past Groton to join the...

.

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

Massachusetts

The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...

. The U.S. Geological Survey (USGS), in cooperation with the Massachusetts Department of Conservation and Recreation

Department of Conservation and Recreation (Massachusetts)

The Department of Conservation and Recreation is a state agency of the Commonwealth of Massachusetts, situated in the Executive Office of Energy and Environmental Affairs. It is best known for its parks and parkways. As of May 24, 2011 the Commissioner of the DCR is Edward M. Lambert, Jr...

 (DCR), operates stream-flow monitoring gages near the mouths of both rivers. This and other continuous monitoring serves to maintain the overall quality of water within the reservoir. The water of these tributaries to the Wachusett Reservoir has been of high quality for decades.
About 47% of the Stillwater sub-basin is permanently protected open space. The Massachusetts Water Resources Authority

Massachusetts Water Resources Authority

The Massachusetts Water Resources Authority is a public authority in the Commonwealth of Massachusetts that provides wholesale drinking water and sewage services to certain municipalities and industrial users in the state, primarily in the Boston area.The authority receives water from the Quabbin...

 owns much of the land. The Town of Holden

Holden, Massachusetts

Holden is a town in Worcester County, Massachusetts, United States. The town was founded in 1741, and the Town Square was donated by John Hancock, former Governor of Massachusetts.The population was 17,346 at the 2010 census.-Geography:...

 owns over 600 acres (2.4 km²) as the Trout Brook Conservation Area, and the Massachusetts Audubon Society

Massachusetts Audubon Society

The Massachusetts Audubon Society, founded in 1896 by Harriet Hemenway and headquartered in Lincoln, Massachusetts, is a nonprofit organization dedicated to "Protecting the nature of Massachusetts." Mass Audubon is independent of the National Audubon Society, and in fact was founded...

owns several hundred acres in the Wachusett Meadow Wildlife Sanctuary in addition to other properties within this sub-basin.