Ontario Power Generation (OPG) is completing the rehabilitation of Frederick House Lake Dam, a concrete gravity control structure constructed in 1938 on the Abitibi River in northeastern Ontario, Canada. The dam exhibited deterioration associated with Alkali-Aggregate Reaction (AAR), seepage, concrete cracking, and increasing uncertainty regarding the condition of the concrete-bedrock interface. These concerns, combined with evolving dam safety requirements, led to the development of a comprehensive rehabilitation program to extend the service life of the facility. The rehabilitation included structural stabilization, foundation improvements, seepage mitigation, concrete rehabilitation, and operational upgrades while maintaining dam operations. Construction was completed under challenging conditions that included significant seepage, unforeseen deterioration, environmental constraints, and winter construction. This paper presents an overview of the rehabilitation project, key design and construction challenges, engineering solutions, and lessons learned. The project highlights the importance of conservative design assumptions, adaptive construction management, and risk-informed decision making when rehabilitating aging concrete dams.