In many Western European mountains, ancient irrigation practices have been the basis for sustainable subsistence-based mountain agriculture, especially up to the beginning of the 20th century. These mountain zones have proved popular sites for the development of hydroelectric power (HEP). Few attempts have been made to measure the impact of HEP on traditional indigenous irrigation systems dependent on the same resource base. This article examines the physical and socioeconomic impact of HEP development on the bisse irrigation system in the Valais, Switzerland, and discusses new water resource issues, conflicts, adaptations, and innovative responses. The study shows that a two- to three-tier consultation process has led to communes and autonomous collective irrigation institutions (consortages) signing and ratifying long-term conventions that ceded water to HEP companies at the scale of watersheds. Water supplies for irrigation are protected by these conventions at the same time as changed hydrological regimes improve water security in economically viable bisses. Conventions also improved the economic security of some consortages, while at the level of individual households, HEP development allowed diversification of income generation strategies.