Thermal dispersion of heated effluent in a lake near nuclear power plant is analysed. Lake Bathymetry is established by data collection at a pre-planned matrix of sample points in the lake. Threedimensional geometric model of the lake is developed based on the geometric data collected, using a high accuracy GPS and a dead weight based depth meter at respective sample points of the lake matrix. A turbine type digital flow meter is used to measure the velocities at the intake and blow down points of the lake. Numerical analysis of flow and thermal dispersion is carried out using PLIC-VOF two-phase model with the two-equation k-epsilon model for turbulence closure. Numerical results for varying flow and blow down temperature conditions and wind speed are studied. It is observed that the thermal gradients are steeper in the curved area near the blowdown point. Small increase in main inlet (inlet II) velocity suppresses the dispersion of high-temperature contours significantly. Thermal discharge and dispersed temperature is monitored using temperature sensor mounted floating buoy at various locations in the lake. It is established that the thermal dispersion is influenced by wind velocity and the presence of water hyacinth in the lake.