IRivers anfc ffloofcs* 15 ^ 



lakes. The amount of rainfall upon the area 

 drained by the Mississippi Kiver during one 

 year amounts to about 614 cubic miles of 

 water, while the discharge at the mouth of the 

 Mississippi Eiver is only about 154 cubic 

 miles. The difference between the two figures 

 has been carried up by the process of evapora- 

 tion or stored in vegetation. These figures 

 vary considerably, however, with different 

 years. 



The proportion of rainfall to discharge will 

 vary greatly in different rivers from other 

 causes than having a large evaporating sur- 

 face. This variation is due to the difference 

 in the ability of the soil to retain water after 

 a rainfall. In some drainage areas the ground 

 is more or less impermeable to water, and in 

 this case the water runs readily off, causing a 

 sudden rise in the river; and as suddenly it 

 reaches the low-water mark. In other drain- 

 age areas the ground is very permeable to 

 water, so that the rain penetrates to a greater 

 depth into the earth, where it is held, and by 

 a slow process drains into the rivers, while 

 much more of it is carried off by evaporation 

 and into vegetation than is the case in the 

 drainage district before mentioned. 



The courses of rivers are determined by the 

 topography of the country through which they 

 flow. The sinuous windings, that are found 

 to be a characteristic of nearly all rivers, are 



