98 RUNNING WATER 



Since the velocity of a stream is so largely dependent upon 

 gravity, it is obvious that the deeper a stream cuts its channel, the 

 less steep does its slope become, and that so long as the region is 

 neither upheaved nor depressed, the river performs its vertical 

 erosion at a constantly decreasing rate. Unless, therefore, the 

 work is done under very exceptional conditions, as in the case of 

 the Niagara, we cannot reason from the present rate of excavation 

 to the length of time involved in cutting out a given gorge. 



Unless the region through which a river flows is upheaved, and 

 thus, by increasing the fall, renewed power is given to the stream, 

 a stage must sooner or later be reached when the vertical cutting 

 of the stream must cease. This stage is called the base-level of 

 erosion, or regimen of the river, and it approximates a parabolic 

 curve, rising toward the head of the stream. Elevation of the 

 country will start the work afresh, until a new base-level is reached, 

 while depression will have a contrary effect and may put a stop to 

 vertical erosion where it was in active progress before. When 

 the base-level is reached, the river cuts laterally, undermining its 

 banks and working like a horizontal tool upon the country-side. 



So long as the slope of the bed is steep, the river runs swiftly 

 and with a comparatively straight course ; when lower slopes are 

 reached, the stream begins to meander, and shift its channel 

 about, undermining its banks, cutting them away in one place and 

 building them up in another, and forming a wide plain. 



Having learned the general character of river erosion, we may 

 illustrate it with a few concrete examples. 



i. A particularly interesting case is that of the little river 

 Simeto in Sicily, since the history of its gorge is so well known. 

 In 1603 a great lava flood from ^Etna was poured out across the 

 course of the stream, and, when cold, solidified into a barrier of 

 the hardest rock. When Sir Charles Lyell visited the spot in 

 1828, he found that in a little more than two centuries the stream 

 had cut a gorge through this barrier of 40 to 50 feet deep, and 

 varying in width from 50 to several hundred feet. The lava which 

 had thus been trenched is not porous or slaggy, but homogeneous 

 and dense. 



