The original concrete gravity Cotter Dam was started in 1912 and finished by 1915 when the city of Canberra was being established. The height of the dam wall was raised to a height of in 1951 in order to increase capacity of the reservoir. The dam wall was long and created a reservoir with a surface area of. The uncontrolled spillway was capable of discharging. At that time, the top water level of the dam was above sea level and the reservoir had a capacity of. A subsequent review in October 2006, using more accurate mapping methods, resulted in capacity being re-estimated downwards from the previous estimate of. Additional galleries and drains were constructed between 1984 and 1986. In order to supply the city with potable water, water from the reservoir was pumped to Mount Stromlo, and from there the water flowed by gravity to fill the city's reservoirs. From the 1960s onwards, better quality water could be supplied without pumping using the newly completed Bendora and Corin dams, and Cotter Dam was only used when water was in short supply. However, in December 2004, ACTEW Corporation brought the dam back on line in response to the ongoing drought.
Enlarged dam completed in 2013
Completed in 2013, the enlarged Cotter Dam comprises a new high roller compacted concrete dam wall built downstream from the old high dam wall, along with two auxiliary embankment dam walls along low-lying adjoining valleys. Constructed on rock foundations by AbigroupJohn Holland joint venture, with engineering design by GHD, the main dam wall is long, with the two rockfill embankments and long and and high respectively, both with internal earthen cores. The enlarged dam walls increased the storage capacity of the Cotter Reservoir from the previous to ; while the catchment area remains. The old dam wall remains, inundated by the water held behind the new dam wall and acting as a sediment trap for the new dam's intake tower. The old dam may only be visible in exceptional circumstances of drought. Completion was originally scheduled for the end of June 2011, however construction was delayed until August 2013 due to heavy rainfalls in the summer of 2010/2011, the discovery of an unexpectedly large rock seam at the site of the foundations in 2011, and severe flooding in March 2012. The uncontrolled spillway is capable of discharging from the Cotter Reservoir, with a high water level approximately above sea level.
