A United States–Spanish Cooperative Program is currently being performed between the U.S. Bureau of Reclamation and the Spanish Centro de Estudios y Experimentacion de Obras Publicas applying Landsat satellite digital imagery to eutrophication-related water quality monitoring in Spanish reservoirs. Landsat digital images provided by the thematic mapper sensors were processed with concurrent surface sampling data, to develop regression equations and water surface maps. Limnological parameters selected for the study were related to trophic conditions such as chlorophyll concentrations and Secchi transparency. While thematic mapping based on single-date regression equations has already proved feasible and reliable, more sophisticated processing is needed to develop equations for multiple-date assessment, that is, to enable assessment on any date remote sensing data are available whether concurrent water surface measurements are available or not. The long-term goal of this project is developing updated, cost-effective reservoir surveillance and classification on a regional or nationwide basis.