Half a Century of Civil Engineering in the Bahlui River Hydrographic System: The Unexpected Journey from Gray Structures to Hybrid Resilience

Water reservoirs are critical components of hydrological systems that mitigate floods and droughts, but their long-term performance under climate change and variable socioeconomic conditions remain insufficiently documented. This study examines the Bahlui River basin (northeastern Romania), where 17 reservoirs constructed mainly between the 1960s and 1980s have been operational for more than five decades. Using the most recent technical reservoir reports, land-use evolution, and present operational functions, the contribution of man-made reservoirs to flood attenuation and drought buffering over time was appraised. Flood mitigation is the most consistent and reliable function, with peak-flow reductions commonly exceeding 60–90% of design discharges at the basin scale. Engineered drought mitigation functions (irrigation and industrial water supply) have decreased significantly as a result of socioeconomic changes started in 1989. However, the gradual expansion of green infrastructure, such as wetlands and riparian vegetation, has improved passive water retention and low-flow buffering capacity. These unanticipated developments have resulted in variable levels of hybrid hydrological resilience. The findings show that, while artificial reservoirs have strong flood-control capacity over long periods of time, their contribution to drought mitigation is increasingly dependent on the integration of ecological components, emphasizing the importance of green-gray interactions in long-term reservoir management.