Longitudinal and temporal distributions of optical properties, and the relative role of selected light attenuating constituents in regulating light penetration, in Cannonsville Reservoir, NY, in 1995 are documented. The analysis is supported by field measurements of light penetration; die angular distribution of irradiance (reflectance); and the spectral quality of penetrating light; laboratory measurements of turbidity, chlorophyll, gelbstoff, and microscopy-based individual particle size and elemental chemistry, and calculations of absorption and scattering coefficients and their components. Longitudinal gradients in attenuating components, and therefore optical properties, prevailed along the main axis of the reservoir; light attenuation and the relative role of scattering decreased with the approach to the dam. Large temporal variations in optical characteristics occurred. These dynamics were mostly controlled by variations in phytoplankton biomass in spring and early summer, when the reservoir was full, but resuspended inorganic tripton (non-phytoplankton particles; suspensoids) became the regulating component as the reservoir was drawn down. Quantitative optical frameworks (models) are applied to simulate light penetration conditions that would have prevailed for scenarios of reductions in phytoplankton biomass and/or inorganic tripton.