An additive semi-analytic model of water clarity for the forward problem of calculating apparent optical properties (AOPs) of diffuse attenuation and Secchi depth from the inherent optical properties (IOPs) due to suspended matter in oligotrophic waters is presented. The model is general in form, taking into account algal concentration, suspended inorganic sediment concentration, particle size distribution, and dissolved organic matter to predict Secchi depth and diffuse attenuation. The model's application to ultra-oligotrophic Lake Tahoe, California-Nevada, USA is described. The function of the clarity model is to quantify the relative effect of phytoplankton or phytoplankton-derived organic materials, other particles such as suspended mineral sediment, and dissolved organic matter on the lake's clarity. It is concluded that suspended inorganic sediments and phytoplanktonic algae both contribute significantly to the reduction in clarity, and that suspended particulate matter, rather than dissolved organic matter, are the dominant causes of clarity loss.