The determination of the magnitude of the maximum flood that may be expected to occur on a river is of importance in many problems connected with engineering works. The problem is a complex one because floods are produced by combinations of a large number of conditions, many of which are themselves variable quantities. Floods which have occurred on some rivers have been greatly in excess of others on the same river. These great floods come rarely on any one river, but an extraordinary flood occurs on some river every year. A study of their past frequency gives the best indication of what may be expected in the future.