The Wachusett Aqueduct extends from the Wachusett Reservoir in Clinton, through Berlin and Northborough, to the John J. Carroll Water Treatment Plant at Walnut Hill in Marlborough, Massachusetts. This 9-mile (14 km) long water system consists of 2 miles (3 km) of hard rock tunnel and 7 miles (11 km) of 11-foot (3.4 m) high horseshoe-shaped underground aqueduct constructed of non-reinforced concrete with a brick-lined invert. It is not constructed entirely underground and there are several elevated sections. The final portion of the aqueduct, taken out of service in 2005 with the opening of the treatment plant, is an open channel running from near the treatment facility to Sudbury Reservoir.
Until the 1960s, the Wachusett Aqueduct was the main conduit used to deliver water from the Wachusett Reservoir. At that time, it carried 300 million US gallons (1,100,000 m3) of water each day (13 m3/s). The Cosgrove Tunnel replaced it in 1965 as the primary water transmission aqueduct. Because of the construction of the Walnut Hill Water Treatment Plant in Marlborough, the Cosgrove Tunnel was shut down in 2003 in order to make the large piping connection between the new treatment plant and the MWRA's new MetroWest Water Supply Tunnel. The MetroWest Tunnel extends east 18.6 miles (29.9 km) from Walnut Hill to Weston. During that shutdown, the Wachusett Aqueduct was the primary method of transmission of about 240 million US gallons (910,000 m3) of water per day.