The literature clearly highlights the major influence of shear strength variability along concrete joints and dam-rock interfaces on the seismic stability of dams. Assessing these properties is particularly challenging for century-old structures that have undergone several major refurbishments, such as the Isle-Maligne dams. In 2022, a probabilistic seismic hazard assessment for the Isle-Maligne dam site resulted in higher accelerations than those used in previous rehabilitation designs. Consequently, updated seismic analyses were initiated and optimized using a bilinear shearstrength criterion based on on-site investigations carried out in 1989. However, the condition of the dam joints may have changed since 1989 due to concrete aging and subsequent rehabilitation works. A new on-site investigation campaign was therefore carried out in 2024. Core drilling, permeability tests, and geocamera surveys showed fewer cracks and open joints than expected, likely due to the beneficial effects of past injection works. Nevertheless, some unbonded joints remain visible along large aggregates. While these contacts lack tensile strength, they exhibit high roughness, good interlock, and limited weathering. Therefore, the bilinear criterion was updated based on the results of shear-tested unbonded contact specimens and on the engineering interpretation of field observations made in 2024. This avoids the assumption of a risky and hypothetical bonding mechanism associated with chemical cohesion and tensile strength. Pseudo-static, pseudo-dynamic, and rigid-body analyses were performed and confirm that better seismic performance is predicted when using the updated shear-strength criterion. This highlights the importance of carefully selecting shear-strength parameters in dam safety assessments.