Kalu River, the third longest river in Sri Lanka, discharges the largest amount of water into the ocean while causing floods along its route from the most upstream major town, Ratnapura, to the most downstream town, Kalutara. It has become necessary to either totally control these floods or instruct people to adjust their activities to the rhythm of the river and prepared them to live with floods with minimum damages. This paper presents a model developed to determine water levels along the river from Ratnapura to 79 km downstream Kalutara using the HEC-RAS hydrodynamic model. The model was calibrated and verified for both steady and unsteady flow conditions. It provides water levels and inundation areas along the river for different discharges. A set of tables, which could be used by people with less technical knowledge, were prepared to predict flood levels at downstream locations based on observed water levels at upstream locations.