The Town of Winchester is located fully within the Mystic River watershed, which covers an area of approximately 76 square miles, or roughly 1% of the land area in Massachusetts. The watershed partially or wholly encompasses 22 communities north and west of Boston. Its headwaters begin in Reading, and form the Aberjona River which flows through Winchester and into the upper Mystic Lake. From the lower Mystic Lake, the Mystic River flows through Arlington, Somerville, Medford, Everett, Chelsea, Charlestown, and East Boston before discharging to Boston Harbor. According to the Mystic River Watershed Association (MRWA), the Mystic River watershed is home to approximately 8% of the state’s population (nearly half a million people), making the Mystic one of the most densely populated and urbanized watersheds in Massachusetts.
The Aberjona River watershed has an area of approximately 27.5 square miles, or about 25% of the Mystic River watershed, making it the largest sub-basin. In addition to Winchester, the watershed partially encompasses the communities of Burlington, Wilmington, Lexington, Reading, Stoneham, and Woburn. The primary tributaries to the Aberjona River include Horn Pond Brook, which drains Horn Pond in Woburn, Sweetwater Brook, and Spot Pond in Stoneham.
The Aberjona River flows from north to south through the Town of Winchester for a total of 3.9 miles. From the Winchester / Woburn line, the River flows for approximately 1.8 miles before being joined by Horn Pond Brook. From the confluence of Horn Pond Brook, the Aberjona River flows 2.1 miles to the point where it empties into the Upper Mystic Lake. There are 1.1 miles of the Horn Pond Brook in Winchester, with the Winchester / Woburn town line being less than a tenth of a mile downstream of Scalley Dam (Horn Pond Dam), where the Brook originates.