An analysis of the characteristics of run-off hydrographs is outlined in this paper, which proposes a revision of current methods for the determination of stream flow from rainfall. It is shown that a continuous rain of constant intensity produces a hydrograph of characteristic shape for any drainage area, and that this shape and the length of time until the run-off becomes constant (time of concentration) are functions of the shape and size of the drainage area. A method of constructing a hydrograph resulting from rain of duration less than the time of concentration is proposed. Based on this analysis, methods have been devised for determining hydrographs for areas lacking in run-off records, from the records of other areas similar in characteristics but different in size, and for the construction from rainfall data of hydrographs for storms or series of storms of varying intensity.