Globally, a significant portion of energy comes from hydropower. However, harnessing hydro energy interrupts the natural state of river flows, thus affecting the ecological processes of the surrounding communities. In this thesis, a water level control model is described to sustain a desired head for a hydropower plant despite the nature of the stream flows. A scientific analysis is carried out on the physical set up of a hydropower reservoir via mathematical modelling. The study depicts that the amount of electric power, Pe(t) generated from a specific hydro reservoir is mainly controlled by the current water level, h(t) and its corresponding outflow volumes, fout(t). However, these two variables are largely constrained by the behaviour of the inflow volumes, fin(t). So, by relating the torque-force balance equations of all the dynamic elements involved we develop a mathematical model that maintains a steady water level at sustainable inflow rates. The Routh-Hurwitz stability criterion and the Final Value theorem are applied to decide the PD-control actions that stabilize the system. The penstock cross-sectional area, Ap is varied to attain the correct fout(t) for the desired h(t). The model behaviour is verified using Simulink simulation software. Eventually, the model accounts for hydroelectric power production patterns that depends on the nature of the stream flow rates.