J. L. Greenleaf — The Hydrology of the Mississippi. 37 



in the general average. The line " AAA " is not a simple 

 exponent of totals, as is the line " CCC " of total flow. It is a 

 function of the varying average of rainfall, the varying tribu- 

 tary area as the branch streams are passed, each with its large 

 or small watershed, and the varying percentage of flow to rain- 

 fall in force upon these side basins. In the use of this line 

 area enters as a factor of equal importance with volume in the 

 study of the hydrology of tiie river. 



It will be noticed that the effect of any one tributary area 

 upon the line " AAA " is much less marked toward the mouth 

 of the Mississippi than would be the case if its watershed were 

 located at the upper waters. The obvious reason is that it has 

 the weight of all the large areas tributary above it to work 

 against before it can influence the general average. If, for 

 example, the Yazoo and Wisconsin, which have nearly equal 

 drainage areas, were to change places, the latter would cause 

 but slight alteration in the lower end of the line, while the 

 Yazoo, in its new position, would raise the line to a flow of over 

 one cubic foot per second. 



It remains to see what general deductions the diagram 

 affords. In the first place, it shows that there is abundant 

 reason for the popular division of the river into the upper and 

 lower Mississippi. The line " CCC " gives the upper river a 

 gradual increase of volume to where it joins the Missouri. At 

 that point begins the lower Mississippi, and the great accretions 

 from the Missouri, and especially the Ohio River, immediately 

 force the line to a higher level. Then the Arkansas and Red 

 Rivers make the rate of increase much greater than for the 

 upper Mississippi. But the line "AAA" exhibits in a decided 

 manner a difference still more fundamental between the upper 

 and the lower divisions of the river. A glance along this line 

 from source to mouth will make apparent the great depression 

 that it undergoes from the Missouri south. The upper river 

 is a high flow stream throughout its length. The lower river 

 belongs just as decidedly with the low flow class. 



The depressing effect that the prairie streams have on the 

 Mississippi is marked. Wherever a drop occurs in line "AAA" 

 it is caused by a river upon which prairie influences prevail. 

 The Crow Wing, Crow, Minnesota, Iowa, Des Moines and 

 Illinois exert such effect. The Arkansas causes a decided drop 

 in spite of the lar^e area above that its watershed is averaged 

 with. The Red River alone, of all its class, does not lower the 

 line. This is due in part to its large rainfall, and partly to the 

 line being already well pulled down by the Arkansas to meet 

 it. As for the effect of the Missouri, it is deserving of special 

 notice. A ]3rofound drop in line "AAA" occurs at its mouth. 

 If it were not for this, the main river would continue a high 



