Various methods have been used in the past to estimate the amount of sediment deposited in Lake Argyle. Before the dam was built, sources of sediment and the amount of sediment that would enter the new dam were investigated, since sedimentation in the proposed reservoir was seen as a potential problem. Sediment samples were taken from rivers in the catchment and the sedimentation rate estimated from annual flow and sediment loads at the Coolibah Pocket gauging station. Later, in 1986, Wark used 10 cross section surveys to estimate the sediment deposited in the lake. He calculated the dry density of the soft sediments found on the lake bed and the coarse sediments in the channels from sample cores collected in mid 1987 and 1991. In 1991/1992, echo-sounding of traverses spaced 200 m apart made it possible to create a digital terrain model of the sediment surface and to estimate the coarse sediments deposited in the Ord River channel underneath the reservoir. Volumes were calculated by taking the sediment surface from a pre-dam surface derived from ‘aerial stereo-paired photographs taken in 1955 and 1961’. The sedimentation rate was reduced by 50% to 14 Mm3 per annum or 12 million tonnes per annum after the latest survey in 2006. A Geoswath sounding system captured the total channel bed for 84 km along the Ord River and 8 km along the Bow River. The pre-dam surface was extended in the southern portions of the lake by generating a digital elevation model from ‘1968 aerial stereo-paired photographs’. Currently the sediments occupy 5% of the reservoir or 500 Mm3. Extrapolating the current sediment rate indicates that 15 % of the reservoir may be filled when the dam is 100 years old (2071).
Lake Argyle sedimentation surveys
Renee N M Dixon - WA Dept of Water , 22 October, 2010