PHYSIOGRAPHY 



streams which cross them often give off distributaries (Figs. 100 and 

 101), which are subject to constant changes. These changes some- 

 times affect commerce in a vital way. 



Many deltas are cultivated, and some of them, like that of the 

 Hoang Ho, support dense populations. Delta lands are, however, 

 subject to disastrous floods. It is estimated that the flood of the 

 Hoang Ho River in September, 1887, drowned at least a million 

 people who lived upon its delta, and caused the death of many more 

 by disease and famine afterward. Many villages were completely 

 destroyed, and hundreds more were temporarily submerged. Pre- 

 vious to 1892, this river flowed into the Yellow Sea south of the 

 Shan-tung promontory (Fig. 96) . In that year, it shifted its course 

 in flood time, forming a new channel leading northeast into the 





y : .*, 



Fig. 105. Terrace of the Columbia River, opposite Lake Chelan, Washington. 



Gulf of Pechili, 300 miles north of its former mouth. Comparable 

 changes at earlier times, running as far back as 2293 B. C. are 

 recorded in the annals of Chinese history. 



Alluvial Terraces 



When a river which has an alluvial flat is rejuvenated, the 

 stream sinks its channel below the level of the flat. The remnants 

 of the old flood plain are then alluvial terraces (Fig. 105). Such 

 terraces are also formed in other ways. Thus If a stream is for a 

 time supplied with an excess of load, it aggrades its valley (Fig. 87). 



