192 K. Bryan — Rock Tanks and Char cos. 



Riffle hollows occur when the bed of a stream is com- 

 posed of alternate layers of hard and soft rock, as 

 illustrated in figure 1, c. Erosion of the softer rock is 

 carried below the grade established by the harder 

 rocks which project in the stream bed and constitute 

 obstacles to the stream flow. Such hollows are com- 

 monly from 3 to 12 inches deep, and vary in size accord- 

 ing to the spacing of the harder portions of the rock in 

 the stream bed. Riffle hollows make very shallow pools 

 unless deepened by pothole action, or unless they grade 

 into plunge pools. 



A pothole is formed by the grinding or drilling of 

 an original hollow in a stream bed by sand, pebbles, or 

 bowlders rotated by the current. The top of a pothole is 

 nearly circular, and the diameter generally increases be- 

 low, as shown in figure 1, c. The diameters of potholes 



Level of Damp 5oil Channel sand and gravel 



f flood plain . / shifted by each flood 



5ana\| Clay 



Hardpan 



/ 

 Water / 



/ /"arer^ g. 



-20fh< 



lav/ 



Hardpan 



Fig. 2. Cross-section of the stream channel and waterhole in the Vekol 

 Valley, Maricopa Co. 



range from 3 inches to 10 feet or more, and the depths 

 from 6 inches to 8 feet or more. 4 Potholes develop in 

 all streams actively eroding their channels in consoli- 

 dated rock. They are, however, more likely to be found 

 in gorges and below waterfalls. They are thus asso- 

 ciated with and grade into plunge pools. 



Plunge pools are formed by the impact of water and 

 the sand and gravel which it carries, at the foot of water- 

 falls, fig. 1, d. A fall differs from the changes in grade 

 of stream beds described in connection with riffle hollows 

 in that it is usually sufficient to cause a flexure in the 

 flood surface of the stream. Consequently, a very great 

 velocity, accompanied by eddies and back currents, is 

 present at the foot of the falls. The erosive effect at 

 the foot of the falls increases with the discharge of the 

 stream in flood, and the quantity and character of the 



4 Elston, E. D.: Potholes, their variety, origin and significance, Sci. 

 Monthly, vol. 5, pp. 554-567, 1917, and vol. 6, pp. 37-51, 1918. 



