An investigation of water transparency characteristics and light attenuation by waters of a turbid subtropical impoundment showed that allochthonous inputs of silt during summer floods, impoundment morphometry, and the warm monomictic thermal cycle were the main factors regulating the temporal and spatial variations in water transparency . Statistically significant relationships between the Secchi disc transparency, turbidity of the surface water, mean diffuse attenuation coefficient and beam attenuation coefficient, were established . These relationships allowed for an approximation of the I per cent of surface light intensity depth to be made by using any of the four parameters . The attenuation of blue light was greater than that of red light, owing to the effects of suspended clay particles on the spectral attenuation of light.