All through history mankind has displayed the need to harness and control free flowing water for his own purposes. In the past water was mainly used for drinking (the Romans developed a highly sophisticated supply system with aqueducts, syphons etc. for this purpose) and for irrigation. Even at this stage then, it was necessary that the discharge in conduits and canals be regulated so as to prevent flooding and also to distribute the available water in a more or less optimal fashion. This was effected by using side-weirs and relief outlets. Nowadays the need for flow regulation in natural and artificial channels and reservoirs is greater due to the growth of world population and the consequent increase in water demand, particularly an increase in per capita demand in the western hemisphere. Consequently, a number of flow regulating principles and applications have been developed. Extreme precision in metering flow rate is not usually a critical criterion when choosing between different regulating devices for irrigation systems and storm-water retention basins. Features such as automatic operation, simple construction and low maintenance are more important.