OPTICAL-PROPERTIES OF LAKE COLERIDGE - THE IMPACTS OF TURBID INFLOWS

A 1-year study of the optical properties of Lake Coleridge was initiated following concern that sediment-laden water from the Wilberforce River, diverted into the lake for power generation, was causing substantial discolouration of lake waters and reduction in clarity. The blue colour of Lake Coleridge is largely that of water itself, but it is typically greyer and appreciably brighter than optically pure water because of suspended inorganic sediments. These sediments dominate the attenuation of image-forming light, mainly by the process of scattering, and thus reduce visual clarity as measured by Secchi depth. Phytoplankton biomass is low and has little influence on the visual clarity, but contributes significantly to light absorption. Comparison of our mid-lake Secchi depth measurements with historical data showed a significant reduction from c. 13.4 m to 8.6 m that is attributable to sediment from the Wilberforce River. The visual clarity is still "high", but the reduction represents an impact on aesthetic quality of the lake. Optical modelling suggests that the reduced water clarity has been accompanied by a proportionately smaller reduction in light penetration and an increase in brightness, associated with increased reflectance.