LAND FORMS, THEIR DEFORMATION AND FORMATION 



43 



layers of unlike hardness. These streams have so 

 much energy that they cut young, V-shaped valleys. 

 The stream covers the entire bottom of these valleys. 

 Valley walls are steep because streams are more effec- 

 tive in cutting a slope than weathering is in reducing 

 the slope. Waters are generally clear because stream 

 energy is so great and inefficient that proportionally 

 few materials are carried per unit volume ol water. 

 In addition, young streams are most effective m 

 stream capture, a phenomonon in which a captor 

 stream at a lower elevation erodes into and "robs" 

 water from a captive stream at a higher elevation 

 (Figure 4.6). 



ox bow 



Figure 4.7 Development of stream meanders. Progressive stages 

 (1 to 4) ond retention of port of stage three as on ox-bow v^itfi the 

 current stream {4) ore shown. 



;r .*; 



Figure 4.6 Stream capture of one young stream by another. The 

 orrow indicates the site of capture thot is completed in the illustration 

 to the right. (After a V^ard's Geomorphic model.) 



The filelike action of a young stream cuts the 

 stream bed throughout the stream's length, but cuts 

 the bank only on curves. These bank and stream bed 

 cutting actions are the agents for a moving of curves 

 downstream and widening of the valley floor. How- 

 ever, individual curves feature both cutting and de- 

 positing of materials. Cutting in the upstream part 

 of a single curve is on the inside bank of the arc and 

 in the downstream portion on the outside bank. De- 

 positing in the upstream segment is on the outside; 

 downstream it is on the inside. Therefore, cutting 

 is always on the straight downstream bank of any arc 

 and depositing in the upstream bank. 



Maturity is reached when the stream's slope is 

 uniform and downward cutting of the stream bed no 

 longer occurs. There is no stream bed cutting, be- 

 cause the waters carry a maximum particle load. 

 While no deeper cutting is taking place, weathering 

 reduces the slope of the valley walls. Then weather- 

 ing plus increased lateral meandering (Figure 4.7) 

 of the stream cause progressive development of a 

 broad valley that is outlined by gentle slopes. 



Within this landscape, the meandering, mature 

 stream occupies only a small part of the valley floor; 

 however, during floods, the overflow deposits sedi- 

 ments upon and levels the adjacent land, creating a 

 flood plain (Figure 4.8). Another characteristic of 

 meandering mature streams is the presence of "ox- 

 bows," curving lakes adjacent to the stream beds. 

 These ox-bows are formed when a mature or old 

 stream changes its path of flow. A path is changed 

 by the normal process of cutting on the outer bank of 

 a curve and depositing on the inner bank. The cuts 

 and deposits cause curves to approach circles and, 

 finally, circles to be isolated from the stream bed as 

 ox-bows. 



Further aging of a stream is dependent upon the 

 headwaters, where the character of youth remains 

 for some time. Eventually, the youthful headwaters 

 erode the surrounding land and the land provides 



Figure 4.8 Mature stream. Note the flood ploin (white), ox-bow, and 

 streom cutting, but no natural levees as in an old stream. (After a 

 Word's Geomorphic model.) 



