Hydrology
The Canning Dam Catchment lies within the Darling Scarp which forms part of an Archaean Shield composed largely of granite with some invaded linear belts of metamorphosed sedimentary and volcanic rocks.
The dam wall is situated in a narrow gorge running east and west, with rock sides sloping upward from the river bed. Behind the dam wall, the south branch of the Canning River joins the main stream, with the impounded water forming a lake which stretches back in three major arms to the east south-east and south.
The catchment has an area of 804 square kilometres (310 sq mi). The reservoir is at 200 metres (660 ft) AHD and the highest point of the catchment, Mount Cooke is at 582 metres (1,909 ft) AHD.
Climatically, the area receives about 900 millimetres (35 in) of rainfall per annum with most of this falling between May and September. There is widespread variability of rainfall across the catchment however, from between 700 and 1,300 millimetres (28 and 51 in).
Since 1975 long-term average rainfalls at the dam wall have decreased by 20 percent and streamflow into the catchment by approximately 60 percent—the average annual inflow between 1948 and 1974 was 52 gigalitres (1.8×109 cu ft) which had reduced to 22 gigalitres (780,000,000 cu ft) between 1975 and 2004.
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