This study presents a cost-effective method to reconstruct lake bathymetry using only digital elevation model (DEM) data, without relying on field measurements or optical imagery. By simulating lake-level decline and using shoreline topography, it accurately infers underwater terrain. Validation across 12 lakes on the Tibetan Plateau and Lake Mead demonstrates good accuracy and generalizability, enabling scalable lake storage estimation for hydrological and ecological applications.