72 PHYSIOGRAPHY 



In time, some of them become valleys, and have permanent streams. 

 Tributary valleys develop tributaries, and the process goes on until 

 a network of watercourses affects the surface. Fig. 39 shows a 

 surface in this condition. 



A valley and its tributaries constitute a valley system. A stream 

 and its tributaries constitute a drainage or river system, and the area 

 drained by a river system is a drainage basin. 



Stages in the history of valleys and streams. Valleys grow in 

 size as they advance in years, as we have seen. When a valley is 

 young, it is narrow, and its slopes are steep. If the land is high, it 

 has a steep gradient, and soon becomes deep. Its cross-section is 

 then somewhat V-shaped (Fig. 46), and its tributaries are short. 

 The mature valley is wider (Figs. 51 and 52), its slopes are often 

 gentler, and its tributaries are longer and older. An old valley is 

 wide, and has a broad flat or flood plain, and a low gradient. 



A stream also, as well as its valley, passes from youth to matu- 

 rity, and from maturity to old age. In its youth, it is likely to be 

 swift, unless it flows through low land. In maturity, it is much 

 steadier in its flow, and when it reaches old age, it winds slowly 

 through its wide plain. Even an old stream, however, may take 

 on the vigor of youth when it is flooded. 



The terms youth, maturity, and old age may be applied to river 

 systems, as well as to single rivers. Every river system, aided by 

 weathering, has entered upon the task of carrying to the sea all the 

 land of its basin which is above base-level. So long as the river sys- 

 tem has the larger part of its task before it, it is young (Fig. 1, PL 

 XVIII, p. 65, and Fig. 46) . When the main valleys have become wide 

 and deep, and the areas of upland have been well cut up (dissected) 

 by valleys, the river system is said to have reached maturity (Fig. 1, 

 PI. XIX, p. 80, and Fig. 51). When the task of base-leveling its 

 drainage basin is nearing completion, the river system has reached 

 old age (Fig. 2, PL XIX, p. 80) . The master stream of a drainage 

 system attains the characteristics of maturity and old age sooner 

 than its tributaries, and in its lower course sooner than in its upper. 



The topography of a drainage basin is youthful when its river 

 system is youthful, mature when its river system is mature, and 

 old when its drainage is old. In an area of youthful topography much 



