RIVERS. 633 



region it drains ; (2) the amount of rain, mist, or snow of the re- 

 gion ; (3) its climate, — heat and a dry atmosphere increasing the 

 loss by evaporation ; (4) its geological nature, — absorbent and 

 cavernous rocks carrying off much of the water; (5) its physical 

 features, — a flat, open, unwooded country favoring evaporation. 



The annual discharge of the Mississippi River averages nineteen 

 and a half trillions (19,500,000,000,000) of cubic feet, varying from 

 eleven trillions in dry years to twenty-seven trillions in wet years. 

 This amount is about one-quarter of that furnished by the rains. This 

 river is 3500 feet wide at St. Louis, 4000 off the Ohio, and about 

 2500 at New Orleans. 



The mean annual discharge of the Missouri River is about three and three- 

 quarter trillions, or fifteen-hundredths of the amount of the rains over the region. 

 The corresponding amount for the Ohio is five trillions, which is one-quarter the 

 amount of rain. (Humphreys & Abbot.) 



The rivers of some dry countries, as Australia, are great floods in 

 the rainy seasons and a string of pools in the dry. 



2. Amount of pitch or descent in rivers. — The average descent of large 

 rivers, excluding regions of cascades, seldom exceeds twelve inches 

 to a mile, and is sometimes but half this amount. 



The following facts on this point are from Humphreys & Abbot's Report on 

 the Mississippi Basin. The descent per mile is given in inches ; L. stands for 

 the lotc-water slope, and H. for the high-water slope. 



L. H. 



Mississippi R. Mouth to Memphis (855 m.) 4.82 in. 5.23 in. 



" Mouth to Cairo at mouth of Ohio (1088 m.) 6.94 5.96 



" Above the Missouri to source (1330 m.) 11.74 



Missouri R. Mouth to St. Joseph (484 m.) 9.24 



" St. Joseph to Sioux City (358 m.) 10.32 



" Sioux City to Fort Pierre (404 m.) 12.12 



" Fort Pierre to Fort Union (648 m.) 13.20 



" Fort Union to Fort Benton (750 m.) 10.56 



Fort Benton is 2644 miles above the mouth of the Missouri. The whole 

 Missouri from its highest source, a distance of 2908 miles, has a descent of 

 about 6800 feet, — or 28 inches per mile. 



During floods, the pitch of the surface of a stream is increased in 

 amount and uniformity. (1.) The waters are higher in the interior 

 of the country than near the ocean, because of the easy discharge 

 through its mouth. (2.) Owing to the height of the waters, which 

 often cover the banks, the course loses some of its minor bends, 

 and the whole distance is therefore less. (3.) The inequalities of 

 slope between the still water and more rapid portions mostly dis- 

 appear. But when the river runs through a narrow, rocky gorge 



