A three-dimensional, numerical free-surface-flow simulation model is developed to investigate hydrodynamics of a lake and the predictive capabilities of the numerical model are validated by comparing them with field results obtained from Lake Naini, Uttar Pradesh, using environmental isotope (δ18O, δ D and 3H) techniques. This has been carried out in order to understand the hydrodynamics of a lake (i.e. ci rculations, mixing and transport of water within the lake.