The aim of this paper is the investigation of the performance of side weirs at the fishways. Side weirs have been used extensively for water level control in canal systems of fishways as a means of diverting excess water into relief channels for protection works of fishways. The general differential equation of the water surface profile along the side weir in rectangular canals is derived by means of the energy equation. The conditional sampling technique allowed to obtain instantaneous profiles of the above hydrodynamical quantity which illuminated the physical picture of the flow over a side weir.