124 



PHYSICAL GEOLOGY 



China have been increased artificially so that, in places, the surface 

 of the river is 30 feet above the surrounding plain, but in spite of 

 man's efforts it has often changed its course and is called " China's 

 Sorrow " because of its great destructiveness (Fig. 112). In 1904 the 

 mouth was 250 miles north of its position 40 years before (p. 133). 

 Natural levees are sometimes high enough to turn the courses of 

 the tributary streams for long distances; thus the Yazoo travels 

 for 200 miles parallel to the Mississippi before entering it, and the 



Fig. 113. — Shallow basins formed as a result of the building of natural levees along 

 the stream are filled at flood time with water from the stream and from the sides of the 

 valley. (Minneapolis topographic sheet, U. S. Geol. Surv.) 



St. Francis for 100 miles. Lakes are sometimes formed when a 

 stream in winding through a valley builds up its levees and thus 

 incloses basins between them and the banks of the valley (Fig. 113). 

 Alluvial Cones and Fans. (1) In Arid Regions. — In desert re- 

 gions streams are fed chiefly by tributaries whose sources are in the 

 mountains where the rainfall is greater than on the arid plains. At 

 rare intervals heavy downpours (cloud-bursts) may occur on the 

 lower courses which, though often of only a few minutes' duration, 

 may fill the valleys, producing torrents of great erosive power. But 

 ordinarily such streams rapidly lose volume as they flow out on the 

 thirsty land, as their lower courses are seldom fed by springs. Dur- 

 ing certain seasons, when the rainfall in the mountains is heavy, 

 some desert rivers are a hundred miles longer than at other times. 

 Srn ;mis flowing from high lands into deserts quickly drop their sedi- 

 ment at the mouths of their gorges, both because their gradients are 

 diminished and because their velocity is decreased as water is lost 



