Cobble Mountain Bypass and Diversion Gate Project

The first step in the Commission’s water treatment and distribution process is to convey raw water from Cobble Mountain Reservoir in Blandford and Granville to West Parish Filters Water Treatment Plant in Westfield. As part of the Commission’s efforts to modernize West Parish Filters and ensure system redundancy and reliability, a series of improvement projects are planned for the raw water conveyance systems between Cobble Mountain Reservoir and West Parish Filters.

One of the initial projects is an update to the Diversion Tunnel Gate House, which were originally constructed in 1928. The update will provide anti-icing mechanisms that will allow for the use of the diversion gates in cold weather, which is currently not possible. Another planned project is a by-pass through the Cobble Mountain power plant to allow for water flow in the event the power plant is offline for any reason.

Design and construction kicked-off in FY21 with more construction planned in FY22. The estimated construction cost for both projects is $4.5 million.

Other future projects related to raw water conveyance include penstock repairs and improvements to the 42-inch conveyance pipeline to West Parish Filters.

Diversion Gates at Cobble Mountain Reservoir, originally constructed in 1928.
Diversion Tunnel Gate House

In July 2021, the Commission successfully completed improvements at the Cobble Mountain Dam Diversion Tunnel Gate House.  The project primarily consisted of piping modifications to the existing outlet works to mitigate winter icing conditions that historically caused hazardous conditions and prevented the Commission from using the Diversion Tunnel Gate House during cold weather conditions.

Diversion Gate Valve Chamber, shown here after improvements were completed.


Water flows from the Diversion Tunnel, with the Tunnel Gates open 100%, following the completion of improvements.  

By-pass Facility at the Cobble Mountain Hydro-Power Station

Construction of the by-pass facility at the hydro-power station is anticipated to begin in 2022 and will improve system redundancy.


Cobble Mountain Hydro-Power Station, originally constructed in 1930, will be upgraded as part of planned improvements to the raw water conveyance system.