190 



K. Bryan — Rock Tanks and Charcos. 



travelers, however, know well how to take advantage of 

 these tanks for the few honrs or days after a rain when 

 they hold water. 



Bock tanks in stream channels. 



The most important rock tanks are irregularities in 

 the rocky beds of streams which hold pools of water 

 after floods. These irregularities are due to the eddies 

 of the current which tend to erode the stream bed un- 



DirecKor? of Flow 



DirecHor? of Flow 



a. Joinh -block IrregulariHes 

 * (5echoD ) 



Direct-ion of Flow 



,d^ 



d Norroal Pohboles 

 (Sechon ) 



Direchoi? of Flow QO dA c ^- 



c. Riffle Hollows 

 (Section ) 



Plunge Pools 

 ( SecHon) 



b. 



Scour 



J 



area of 

 rnc\x\rr)orr) 

 _s^ scour 



Depression 





^y ^^- . 



(Plan) 



area of ^-^r 



rOaxin?urr) 

 5COUT 





Fig. 1. Diagram showing five types of depressions in channels: a, joint- 

 block irregularities; b, scour depressions; c, riffle hollows; d, normal pot- 

 holes; e, plunge pools. 



equally. The inequalities are probably no more common 

 in ephemeral streams than in the permanent streams of 

 humid countries. The beds of ephemeral streams are, 

 however, exposed throughout their length during the 

 greater part of the year, and undrained hollows or de- 

 pressions are on this account easily found. 



The common depressions are of five types: (1) joint- 

 block irregularities; (2) scour depressions; (3) rime 

 hollows; (4) potholes; (5) plunge pools, fig. 1. 



