J. L. Greenleaf — The Hydrology of the Mississippi. 35 



sippi. It flows from the west, bringing the drainage of 

 97,000 square miles. It maintains not onJy a slightly better 

 ratio of flow than the Arkansas, but is subject to a decidedly 

 higher rainfall. The former is eighteen per cent, and the latter 

 averages over thirty-eight inches annually. As a result, the 

 Red River yields an average discharge of 0*515 cubic feet per 

 square mile per second. The mouth of the Mississippi is, in 

 a hydrological sense, situated where the Red River enters 

 it, for at this point the Atchafalaya Bayou taps the river and 

 draws off a considerable share of its waters. 



It will be noticed that two rivers of marked peculiarities are 

 represented in the diagram. The Saint Francis and the Yazoo 

 are the largest of several streams entering in the vicinity of the 

 Arkansas and the Red, which are comparatively small and do 

 not extend far to the west or east. They are consequently 

 directly in the track of the heavy rainclouds from the Gulf. 

 The rainfall upon their watersheds either sinks rapidly into the 

 sandy soil and thus escapes evaporation, as is the case on a 

 large part of the Yazoo watershed, or else flows quickly into the 

 swamp reservoirs that characterize both the Yazoo and the 

 Saint Francis. Hence the ratio of flow is as high as seventy 

 per cent of the rainfall, and the yearly average flow amounts to 

 2-130 cubic feet per square mile per second, for the Saint 

 Francis, and to 2*74:9 cubic feet for the Yazoo. 



The second diagram, giving the average flow and rainfall 

 lines of the Mississippi River, follows directly from the dia- 

 gram of the tributaries. Its special interest lies in the infor- 

 mation it affords as to the resultant effect which the branches 

 have upon the main river. The line of flow, " CCC," gives 

 in cubic feet per second the yearly average of total flow at 

 each point from source to mouth. This line will be seen to 

 corroborate a previous statement concerning the large discharge 

 of the Ohio River. 



The line " BBB " gives in inches, at any point selected, the 

 yearly precipitation in rain and melted snow, averaged for the 

 entire area tributary to the Mississippi above the point in 

 question. If the point selected is at the mouth of some inflow- 

 ing river, the line shows the average rainfall above its tribu- 

 tary watershed, and also the effect of including this watershed 

 in the general average. 



The line " AAA" is by far the most significant of the three 

 in the study of the hydrology of the Mississippi. It represents 

 for each point from source to mouth the yearly mean flow in 

 cubic feet per square mile per second, averaged for the entire 

 area tributary to the Mississippi above the point in question. 

 If the point selected is at the mouth of some inflowing river. 



