Archimedes screws have become increasingly popular throughout Europe to drive hydropower plants using residual flow from a stream or river. This heightened usage of such screws during the past few years is attributable to their robustness, simplicity, and fish friendliness. Presently the dimensioning of a hydropower site equipped with an Archimedes screw is dependent mainly on the experience of the design engineer. This is so because there is no appropriate theory for the optimal hydraulic design of screws used at hydropower sites, especially for the water-inflow conditions. In this paper, a new analytical model for the water inflow is derived, and the optimal values of the inflow parameters are determined to achieve a planned inflow to the screw.