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



flow stream to the Grulf. The extremely small rainfall and 

 percentage of flow of the Missouri, coupled with its half million 

 square miles of drainage area, determine the main river to its 

 mouth. The pronounced high flow character of the Ohio 

 results in the lifting of the line "AAA" at once through forty 

 per cent of the drop occasioned by the Missouri, but even the 

 Ohio, magnificent as it is in drainage area, and with a high 

 rank among the tributaries for its volume of flow, cannot 

 redeem the character of the Mississippi after its debasing union 

 with the Missouri River. 



It is interesting to notice that the dual action of the Missouri 

 and Ohio has its imitation toward the north. The low flow 

 Minnesota causes, on a small scale, a drop in the line "AAA" 

 similar to that produced by the Missouri, and below it the 

 Saint Croix and Chippewa are the first of a series of branches 

 culminating in the Rook. River, which more than make good 

 the depression. 



The influence of the heavy i^ainfall and large percentage of 

 flow in the south is shown by the gradual rising of the line 

 "AAA." The Saint Francis and Yazoo and lesser streams, 

 draining all together only 43,000 square miles, make a percept- 

 ible headway in the averages, even against the inertia of all 

 the vast drainage area lying to the north, east and west of 

 them. If their condition of rainfall and flow prevailed over 

 the entire 1,259,000 square miles forming the Mississippi water- 

 shed, the average discharge would be fully three million cubic 

 feet per second ; more than forty times its actual volume. These 

 figures indicate what the Mississippi, grand river that it is, 

 might be if the trade winds and moist air currents from the 

 Gulf of Mexico did but sweep the semi-arid regions to the 

 northwest, and pour a plenteous rainfall over their vast 

 extent. 



Thus far the discussion has been confined to a study of aver- 

 age flow, and is therefore open to the criticism that all averages 

 are liable to, for it states nothing concerning the variations of 

 flow characteristic of the rivers. Indeed, the average flow may 

 even be looked upon as a theoretical quantity, inasmuch as it 

 has no real and tangible existence, except for a few brief periods 

 during the year. JNo apology need be offered for its presenta- 

 tion, however, for a study of averages is an important aid to the 

 understanding of the actual relation of flow which the rivers 

 bear to one another. Attention will now be given to the vari- 

 ations in the volume of flow of the tributaries of the Missis- 

 sippi, and the relations which these variations bear to the 

 climatic and topographical features of the watersheds. Figure 

 4 and the subsequent notes present particulars concerning rain- 



