 At that time, it carried 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.