J. L. Greenleaf—The Hydrology of the Mississippi. 31 



over tlie frozen ground to the water channels, and a summer 

 shower may be entirely absorbed by the thirsty land and grow- 

 ing vegetation. In fact, the ratio of '' run off " to rainfall is 

 so uncertain, that engineers consider volumes of flow deduced 

 merely from records of rainfall of small importance, as com- 

 pared with a long series of actual gaugings of the stream. But 

 gaugings are seldom available, and hence the percentage that 

 the flow bears to the rainfall is of more than mere scientific 

 interest. 



From the foregoing it may be concluded that, in studying 

 the hydrology of a watershed, the engineer considers the area 

 of drainage, the rainfall and its distribution through the sea- 

 sons, the temperature, the character of the region as influenc- 

 ing the percentage and regularity of flow, and, most valuable 

 of all, actual gaugings of the streams — if he can obtain them. 



The basis of this discussion of the Mississippi is a report by 

 the writer upon certain water-powers for the Tenth Census of 

 the United States. The data there given have been amplified 

 to cover the flood and low water as well as the average dis- 

 charge of the Mississippi and its tributaries, and have been 

 brought up to date by study of the subsequent gaugings con- 

 ducted by the Corps of Engineers of the United States Army. 

 Acknowledgment is due to this source of information, without 

 which any study of the river would be impossible, and also to 

 the many individual members of the Corps from whom cour- 

 teous replies have been received to letters of inquiry concern- 

 ing details. It is believed that the conclusions here offered 

 are an essentially correct statement of the conditions of flow 

 pertaining to the Mississippi watershed. The yearly reports of 

 the United States Corps of Engineers, which are awaited with 

 interest, will either substantiate still further the diagrams here 

 offered, or furnish data for their alteration. 



Figure 2 illustrates some of the principal features of the 

 branches selected as especially influencing the Mississippi. 

 The shaded rectangles represent the watersheds in the order 

 of their occurrence from the source to the mouth, and the 

 unshaded rectangles the intermediate areas tributary directly 

 to the main river. The horizontal widths of the rectangles are 

 proportional to the respective drainage areas, and their heights 

 show the yearly average flow in terms of cubic feet per square 

 mile per second. The areas of the rectangles may therefore be 

 said to give a measure of the relative influence of the tribu- 

 taries upon the average discharge of the main stream. 



The diagram exhibits the peculiarities of the tributaries, but 

 fails to furnish an explanation, and that information we must 

 seek for. It will be noticed on the diagram, that the rainfalls, 

 shown by the horizontal lines, increase from the north toward 



