Australian Capital Territory Implementation Report 2004–2005

Driving Action 1: Rehabilitating Fish Habitat

Highlights

Installation of destratification system for Cotter Reservoir

Destratification unit at Cotter Reservoir

Photo: ActewAGL

View enlargement

Following the 2003 Canberra bushfires, monitoring of water quality in Cotter Reservoir revealed significant problems with low dissolved oxygen levels (less than 1 mg/L) over the majority of the reservoir at certain times of the year. As the reservoir contains a significant population of the threatened Macquarie perch and is used as a source of domestic water, a solar-powered de-stratification system was installed in December 2004. The main purpose of the system was to increase the amount of dissolved oxygen, and consequently to reduce the amount of iron and manganese in the water column of the reservoir. The system used a top-down mixing arrangement, which was designed to reduce the re-suspension of bottom sediments.

As shown by the graphs below, dissolved oxygen values decrease during the summer and late summer months, as microbiological activity in the sediment depletes the oxygen in the overlying waters. Dissolved oxygen values increase in winter, when fresh inflows and wind mixing introduce more oxygen into the water column. The de-stratification system considerably reduced the seasonal pattern of oxygen depletion at the 3 m, 6 m and 9 m depths in the period from January to May 2005.

Riparian restoration program

A significant program of riparian restoration has commenced in the lower Cotter catchment. This area was devastated by the 2003 bushfires, with the loss of most of the pine plantations that were the dominant landuse in this area. As a result of the fires, the future and management of these plantations has been reassessed; significant riparian buffers of native vegetation are to be planted along streams. Work commenced to clear burnt pines, and prepare riparian areas for future restoration.