The effects on algal productivity of changes in light availability because of suspended materials entering a lake from storm runoff or turbidity generated by wind mixing of sediments are often discussed. The relationships between suspended solids, turbidity, Secchi disk depth, light attenuation, and the resulting photosynthetic response of algae are not well documented. Data from several TVA reservoirs indicate that variations in the available light due to turbidity of the water are large and must be described in more detail before accurate predictions of algal dynamics and macrophyte growth can be achieved. Basic relationships between the various measurements of turbidity, suspended solids, Secchi disk depth, and light attenuation are given. This allows frequent measurements of turbidity or Secchi disk depth to be combined with daily solar radiation and less frequent chlorophyll and C14productivity data to yield a detailed estimate of algal productivity within a reservoir or lake throughout the year. For long lakes and many reservoirs, significant longitudinal gradients in suspended solids will result as the inflowing materials settle and the water column clears. The importance of this gradient of available light for modeling algal dynamics is demonstrated with data from several TVA reservoirs.