248 The Realm of Nature CHAP. 



may ultimately be cut through by the river, which establishes 

 a short direct passage, leaving an island ; or the ends of 

 the cut-off portion may be silted up, converting it into a 

 crescent-shaped lake. 



324. Embanking of Rivers on Plains. During a flood 

 the swift, muddy stream rises, and, overflowing the banks, 

 immediately widens out on the level land ; the current is 

 checked at once, and most of the sediment is deposited 

 close to the banks in the form of broad bars of alluvial soil. 

 When the amount of mud in the water is very great, as in 

 the Mississippi, the Po in Northern Italy, and still more the 

 Yellow River (Hoang Ho) which traverses the loess deposits of 

 China, the land on both sides of the stream is raised rapidly. 

 The river-bed also gets silted up, and the great muddy 

 river ultimately flows along the top of a gently sloping 

 embankment, many feet above the level of the plain (Fig. 52). 

 The natural mud walls, called levees, on the lower Missis- 

 sippi are strengthened arti- 

 ficially in order to protect the 

 dwellers on the fertile borders 



-c, of the river. Floods frequently 



FIG. 52. Embankment of a river. BB, * 



original slope of valley. The light make a breach in the wall, and 



shading shows successive layers of . r->\\^(\ a havnii in 



deposit; AA, level of river. a stream, called a bayou in 



Louisiana, escapes, winding 



over the low plain, either to rejoin the main river at a lower 

 level or to reach the sea independently. The Yellow River 

 of China has repeatedly changed its course by the high banks 

 bursting. One such disaster occurred in 1852, when the 

 embankments burst about 500 miles from the sea, and the 

 great stream, half a mile wide, formed a new channel, 

 entering the Gulf of Pechili several hundred miles from 

 its former mouth. In 1887 the banks burst again near the 

 same place, leading to the most fatal catastrophe recorded 

 in history, as the river, inundating hundreds of towns and 

 villages, drowned several millions of people. 



325. Bars, Banks, and Deltas. When rivers enter a 

 tidal sea directly, the effect of the salt water is to cause a 

 rapid precipitation of sediment, which may accumulate at 

 the mouth of the river and form a bar. Bars are often purely 



