The consequent change in land cover from vegetation to water surface after inundation is the most obvious impact attributed to the impoundment of reservoirs and dam construction. However, river regulation also alters the magnitude and variability of water and energy fluxes and local climatic parameters. Studies in Mediterranean, temperate and boreal hydrological basins, and even a global-scale study, have found a simultaneous decrease in the variation of runoff and increase in the mean evaporative ratio after impoundment. The aim here is to study the existence of these effects on a regulated tropical basin in Colombia with long-term data, as such studies in tropical regions are scarce. As expected, we observed a decrease in the long-term coefficient of variation of runoff of 33% that can be attributed to the impoundment of the reservoir. However, we did not find important changes in precipitation or the expected increasing evaporative ratio-effect from the impoundment of the reservoir, founding for the latter rather a decrease. This may be due to the humid conditions of the region where actual evapotranspiration is already close to its potential or to other land cover changes that decrease evapotranspiration during the studied period. Our study shows that the effects from impounded reservoirs in tropical regulated basins may differ from those found in other climatic regions.