The hydropower fleet built in the Upper Mekong River, or Lancang, currently consists of eleven mainstream dams that can control about 55% of the annual flow to Northern Thailand and Laos. The operations of this fleet have become a source of controversy between China and downstream countries, with these dams often considered the culprit for droughts and other externalities. Assessing their actual impact is a challenging task because of the chronic lack of data on reservoir storage and operations. To overcome this challenge, we focus on the ten largest reservoirs and leverage satellite observations to infer 13-year time series of monthly storage variations. Specifically, we use area-storage curves (derived from a Digital Elevation Model) and time series of water surface area, which we estimate from Landsat images through a novel algorithm that removes the effects of clouds and other disturbances. We also use satellite radar altimetry data (Jason) to validate the results obtained from satellite imagery. Our results describe the evolution of the hydropower system and highlight the pivotal role played by Xiaowan and Nuozhadu reservoirs, which make up to ~85% of the total system’s storage in the Lancang River Basin. We show that these two reservoirs were filled in only two years, and that their operations did not change in response to the drought that occurred in the region in 2019-2020. Deciphering these operating strategies could help enrich existing monitoring tools and hydrological models, thereby supporting riparian countries in the design of more cooperative water-energy policies.