THE WORK OF RUNNING WATER 73 



of the surface has not yet been much changed by erosion (Fig. 68, 

 Fig. 1, PI. XVIII, p 65), and the surface is often ill-drained. In 

 an area of mature topography, much of the surface has been reduced 

 to slopes by erosion (Fig. 1, PL XIX, p. 80), and is well drained; while 

 an area of old topography is one which has been brought down to 

 general flatness by erosion (Fig. 2, PL XIX). Those parts of a 

 drainage basin near the master stream may take on the charac- 

 teristics of age, while other parts farther from the trunk stream are 

 not advanced beyond maturity, or even youth. 



Rate of land degradation. It has been estimated that the 

 Mississippi River carries enough mud, sand, etc., to the Gulf each 

 year to make a layer about 1-400 of an inch (1-5000 of a foot) thick, 

 if it were spread out over the basin of the river. If to this we add 

 the material carried to the sea in solution, the rate of degradation 

 of the Mississippi basin is about one foot in 3,500 years. This, per- 

 haps, represents about the average rate at which the lands of the 

 earth are being lowered by erosion at the present time. 



Conditions affecting the rate of erosion. Summarizing some 

 things that have been stated in the preceding pages, we may say 

 that the rate at which running water wears down the surface over 

 which it flows depends largely on (1) the volume of water, and this 

 depends chiefly on the amount of precipitation; (2) the velocity of 

 the water, which depends chiefly on its gradient and its volume; 

 (3) the character of the surface, especially the resistence of its mater- 

 ials; and (4) the load which the water carries. To work most effec- 

 tively, the water must have tools (gravel, sand, etc.,) enough to 

 enable it to cut rapidly, but not enough to retard its flow seriously. 



Exceptional Features Developed by Erosion 

 Canyons and gorges. Valleys which are narrow and deep are 

 often called gorges if they are small, and canyons if of larger size. 

 The sides of gorges and young canyons are sometimes nearly vertical 

 (Fig. 1, PL XV, p. 60), but the sides of large canyons are rarely 

 so. The distinction between a canyon, and a valley which is not a 

 canyon, is not sharp. 



The Colorado Canyon (PL XIV, p. 53) is the greatest canyon 

 known. Its depth is about a mile. Though narrow at the bottom, 



