The paper discusses the use of the thermo-hydraulic numerical model to verify the hypothesis of subsoil seepage and erosion development in a section of the Kozłówa Góra dam. This dam is provided with an innovative and advanced system for quasi-3D thermal monitoring of seepage and erosion, which was used for temperature measurements. The herein reported analysis of the measurements showed high applicability of the thermal monitoring method in determining the severity of erosion and seepage processes using numerical modelling.