180 



PHYSICAL GEOLOGY 



the process of cut-and-fill continues, the curves change in 

 outline as suggested by Figure 188. Finally the stream cuts 

 through the narrowing neck of land between the two limbs 



of a meander (Fig. 189). The cur- 

 rent now abandons the old round- 

 about course because the new 

 route is steeper. The old channel 

 is isolated presently by the shift- 

 ing of the stream to another posi- 

 tion on its flood plain, or by 

 at the ends of the 



FIG. 188. Diagram showing 

 development of a meander. 

 The current directed against deposition 

 the downstream side of the . , 

 meander is on the average abandoned meander, whose Stand- 

 stronger than that directed j n g waters check the edge of the 

 against the upstream side, , . . , , 



and therefore the growing current. Ihe resulting lake is an 



meander migrates down the ox-bow lake. The flood plain of a 



great river, such as the Mississippi 



or Missouri (Plate IX), may contain numerous lakes, which 

 record recent changes in the position of the river (Fig. 141). 

 The extent to which certain great rivers are shifting their 

 channels is shown by surveys of their 

 courses. Figure 190 shows the changes 

 that occurred in the position of a por- 

 tion of the Missouri River between 

 1852 and 1879, and between the latter 

 date and 1894. It shows also the 

 tendency of the meanders to work 

 down the valley. _ 



Ox-bow lakes, like lakes of other 



, ,, FIG. 189. A recently de- 



origin, are temporary features. They vc loped cut-off, 



are filled gradually (1) by the en- What shows that it is of re- 

 croachment of marsh vegetation upon 



their shallow borders, (2) by silts deposited in them during ex- 

 ceptional floods, (3) by wind-blown material, and (4) by wash 

 from the surrounding land. Doubtless many generations of 

 lakes are made and destroyed during the formation of groat 

 flood plains. 



