A fractional steps technique for the numerical solution of the shallow water equations is applied to study the water velocity in Lake Mariut, its concentration and the distribution of the temperature along it. Lake Mariut is considering the most productive natural systems in Egypt. The current configuration of this lake is changing rapidly, due to people’s activities and natural processes. Most of its water supply comes from polluted agricultural drains. Several problems affect the conservation of the Lake Mariut, mainly pollution, land reclamation, intensive aquatic vegetation, overfishing and coastal erosion. The shallow water equations for this lake are discretized on a fixed grid and time stepped with the fractional steps method. The method is efficient and simple. The absence of iterative steps in the present technique makes it very suitable for the problems in which small time steps and grid sizes are required and the simplicity of the method makes it very suitable for parallel computer. Therefore, the method provides numerical algorithms which are more efficient than other classical schemes