This study investigates the thermal effects of the outer layer materials forming the upstream embankment of the Ajdabiya Holding reservoir on the performance and degradation of the 1.5 mm thick Ethylene Copolymer Bitumen geomembrane lining. The concrete blocks used in the embankment are hypothesized to contribute significantly to the thermal degradation of the geomembrane due to their high heat absorption and retention properties. A comparative analysis is conducted to assess the thermal benefits of replacing the concrete blocks with riprap, as used in the Omar Al-Mukhtar Reservoir. Using heat transfer calculations and field data, results demonstrate that concrete blocks elevate the geomembrane’s surface temperature to 47.2 ℃ due to high solar absorption, accelerating degradation in free zones and fluctuating water-level areas. Replacing concrete with riprap reduces peak temperatures to 36.8 ℃, lowering thermal stress and extending service life. These findings provide actionable strategies for improving geomembrane durability in arid climates.