12 TIMBEB OF THE EDWAKDS PLATEAU OF TEXAS. 



conditions; especially upon the manner in which the rainfall comes, the 

 topography of the area (the run-oft' being greater in the country of 

 steep slopes and deep canyons than on the level uplands, unless it should 

 happen, that here the rock strata are flat, unbroken, and impervious), 

 and upon the presence of hindrances to rapid run-oft' which detain the 

 water until more of it sinks into the (Mirth. The physical action of 

 this ftood water has been the principal agent in causing the dissection 

 of the ancient plateau into its present aspect of hills and canvons. 

 The drainage channels cut by the run-off waters constitute the present 

 river systems of the area. (See frontispiece.) 



RIVERS. 



Two classes of streams are to be distinguished: (1) Those having 

 their origin within the plateau area and cutting deep canyons in 

 it before emerging upon the Coast Plain; (2) those which rise at 

 the base of the escarpment, and which consequently, although owing 

 their water supply to the plateau region, are structurally features of 

 the Coast Plain. 



Of the former class the Colorado is the foremost; for, although it 

 heads in the Red Beds and Staked Plains, it is to be regarded in the 

 main as a drainage system of the Edwards Plateau, from which it 

 receives not only its major branches, the San Saba, Llano, and Peder- 

 nales, but also several important minor ones, like Beaver, Sandy, 

 Spring, Cypress, Barton, and Onion creeks. Next in sequence west- 

 ward come the Blanco, the San Marcos, and the Guadalupe, of which 

 the last two receive strong reenforcements from the San Marcos and 

 Comal springs, respectively, as they emerge from the plateau. West of 

 San Antonio come, in order, the Medina, Hondo, Sabinal, Frio, Nueces, 

 and Devils rivers. In the case of all of these except the Colorado, 

 Guadalupe, and Devils rivers, while there are constantly flowing 

 springs and water holes within the plateau area, the water, upon 

 emerging from the escarpment, especially during dry seasons, often 

 entirely sinks out of sight in the broken limestone bed, reappearing 

 farther toward the coast. In the three exceptions named there is a 

 flow which, though subject to fluctuations with the season's rainfall, 

 is at all times relatively strong. 



Of the second class of streams are the San Marcos, as reenforced by 

 San Marcos Springs; the Comal, fed by Comal Springs; the San 

 Antonio, fed by a series of springs; Los Ulmos, heading in the springs 

 at Fort Clark; and the San Felipe, heading at Del Rio in the San 

 Felipe Springs. In all these rivers the flow is remarkably constant, 

 though naturally fluctuating somewhat with longer periods of drought 

 or excessive rainfall. The quantity of flow will be found in the table 

 of flow of springs on page 14. 



