Prior to its opening water had flowed through the Canning Contour Channel to Gosnells. The Canning Dam and reservoir still supplies approximately 20 percent of Perth's drinking water requirements and plays an important role in the context of the development of Perth. The Canning reservoir is also used to store water from the newly completed Kwinana Desalination Plant. Treated water can be pumped from the plant to the reservoir through the new Forrestdale Pumping Station. 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 . The reservoir is at AHD and the highest point of the catchment, Mount Cooke is at AHD. Climatically, the area receives about 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 . 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 which had reduced to between 1975 and 2004. The construction of Canning Dam ended a long period during which Perth's water supply was generally unsatisfactory in quality (either due to salinity or bacterial pollution or both) and in quantity. The project, the biggest public works program of the decade, stimulated significant growth in the local economy and provided desperately needed work for around five hundred men. Several innovative design concepts and construction methods which were new to Australia were introduced on the project, while others which were used on the nearby Wellington Dam, were improved upon at the Canning dam site. At the Canning Dam and indeed all dams, care had to be taken to prevent water seepage between the foundation rock and the structure of the dam. At the Canning in addition to cutting back the foundation to solid unfractured rock, a cut-off trench was sited near the upstream face of the dam, down stream which a rock filled drain containing open jointed pipes was provided to intercept any seepage between the rock face and the concrete of the dam.