204 K. Bryan — Rock Tanks and Char cos. 



The adobe flats in which, charcos occur are the result of 

 sedimentation from flood water streams which spread as 

 a thin sheet of water over large tracts of country and 

 deposit mud as sediment. This process produces smooth 

 plains having a gentle slope in the direction of drainage, 

 and generally unmarked by channels, or at most having 

 very insignificant channels 2 to 5 feet wide and a few 

 inches deep. However, in places where the current is 

 exceptionally swift, part of the mud laid down by past 

 floods is removed and a relatively large channel is formed. 

 It is characteristic of these channels that they begin with 

 a series of little cliffs, 6 inches to 18 inches high, which 



Fig. 11. Miniature canyons near head of a ehareo, La Guituni Valley, 

 Pima Co., Arizona. 



lead down to numerous small channels and rill marks 

 which collect together into a single channel which pur- 

 sues a somewhat sinuous course in the direction of the 

 drainage, and finally ends more or less abruptly. In 

 many instances the channel ends in a vertical wall 3 to 

 5 feet high. It is evident that concentration of the cur- 

 rent of a flood normally spread in broad sheets over the 

 flat, digs the original channel, and movement of water into 

 this channel toward the end of the flood or during minor 

 floods causes erosion of the fanlike set of miniature 

 canyons at the upper end, 1ig. 11. The lower end of the 

 channel is fixed at a place where the current again be- 

 comes less swift, due to the spreading of the flood to 

 occupy a larger cross-section. The channels thus formed 



