WATER SUPPLY OF THE EDWARDS PLATEAU. 13 



All of these streams, but more especially the first group, which cut 

 through the plateau, are channels for the flood waters of the region, 

 which frequently descend with great volume and velocity, causing 

 serious losses. Their permanent flow, however, is sustained by the 

 springs of the region, which are fed by the retained waters. This 

 leads us next to an inquiry as to the water-receiving capacity of the 

 Kdwards Plateau limestones. We have seen above that not only are 

 the edges of the successive strata exposed either by normal outcrop 

 or by erosion, but that the strata are much interrupted by clefts and 

 fissures, minor faultings, and caverns, until the entire area possesses a 

 verv large capacit} T to take up the precipitated water. The magnitude 

 of these fissures may be appreciated when one observes such volumes 

 of water as that which flows from Kickapoo Springs suddenly disap- 

 pear in the limestone bottom of the creek bed a phenomenon also to 

 be observed in the Nueces, and, indeed, in most of the minor tribu- 

 taries of the region indicated above where they emerge upon the Coast 

 Plain. 



SPRINGS. 



Some of the water retained by the limestones and their superficial 

 soils is removed by evaporation, of whose intensity we have already 

 learned. Some of it sinks to subterranean reservoirs and does not 

 reappear unless the reservoir is tapped, when either nonflowing or 

 artesian wells result. The rest reappears in the form of springs, of 

 which Hill and Vaughan (p. 307) distinguish two types gravity 

 springs and fissure springs. 



GRAVITY SPRINGS. 



These springs are characteristic of the region itself, and are simply 

 the flow of water from exposed rock strata in the natural course of 

 seeking its level. Such springs form the heads of the various streams 

 of the region. The famous Kickapoo Springs at the head of the west 

 fork of the Nueces are an example. 



FISSURE SPRINGS. 



On the southern and eastern margin of the plateau is a line of 

 springs which extends from Del Rio to San Antonio, and thence along 

 the escarpment border to Austin and northward. These are fissure or 

 artesian springs, whose flow is due to hydrostatic pressure. The 

 water is forced up through the fractures caused by the gigantic down- 

 throw or faulting which caused the escarpment itself. So great is the 

 flow from some of these springs that full-fledged rivers boil from the 

 earth within the space of a few square rods (PI. I, fig. 2). 



An approximation of the run-off from these springs may be had 

 from the table following, taken from the geological report of Hill 

 and Vaughan (p. 311). 



