638 DYNAMICAL GEOLOGY. 



son of the northern hemisphere, and still larger tributaries from the 

 country south of the equator, bringing in floods from the rainy season 

 of the southern hemisphere. Thus it is with many rivers, their wide- 

 spread arms taking advantage of the difference in the seasons or cli- 

 mate of the distant countries whence they get supplies. 



Rivers that rise in snowy heights, like the Rhine, Rhone, and Dan- 

 ube, have their channels kept well filled through the summer, the time 

 of drought, because that is the melting-time of the snows. 



Lakes are the relatively still-water portions of rivers. But they 

 are also reservoirs that store away water in the time of floods, and let 

 them out, gradually, after the floods have passed; and these floods 

 often make temporary lakes along a river course that prolong much 

 the period of high water. .The quiet at the "Whirlpool," in the 

 rapids below the Falls of Niagara, is accounted for by the great in- 

 crease in the depth of the passage and the abrupt expansion in 

 breadth. 



Forest regions, by keeping the soil beneath charged with moisture, 

 tend, like lakes, to make gradual and constant the supply of water to 

 rivers, and give uniformity to the flow ; and hence, when forests are 

 cut away, the rains reach speedily the streams, making them liable to 

 alternate periods of wasteful violence and worthless feebleness. The 

 cutting away of the forests has led, in the French Alps (Dauphiny), 

 to uncontrollable erosion, despoiled fields, and an impoverishment of 

 the people ; and, in America, to annual seasons of dry mill-ponds, an 

 immense sacrifice of available water-power, and the desertion of many 

 a mill-site. 



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 one third this amount. 



The pitch of the Mississippi, from Memphis down (855 m.) is only 

 4.82 inches at low water; from Cairo, at mouth of Ohio River (1,088 

 m.), 6.94 inches; and above the Missouri, from its source, only 11| 

 inches. The Missouri, from its highest source (2,908 m.) descends 

 about 6,800 feet, or 28 inches a mile ; but from Fort Benton to St. 

 Joseph (2,160 m.), about H-J inches; and below St. Joseph to the 

 mouth (484 m.) 9£. (From Humphreys and Abbot.) The average 

 pitch of the Amazon is little more than 6 inches a mile ; of the Lower 

 Nile, not 7 ; of the Lower Ganges, about 4. The Rhone is remark- 

 able for its great pitch, it being 80 inches per mile from Geneva to 

 Lyons, and 32 inches below Lyons. 



During high floods the course of a river is shortened, because the 

 minor bends are obliterated by the overflow, and where the channel is 

 broad and open, the pitch is commonly increased in amount and uni- 



