60 University of California Publications in Zoology [Vol. 12 



two-year-old growth, succeeding which is a stand of still older growth. 

 Because of the progressive trend of the process it is as a rule the 

 oldest growth which becomes subject to the razing action of the river 

 as its loops travel down the valley.- 



Occasionally cut-oifs occur, thus interrupting the symmetry of 

 behavior, and the river quicklj' establishes a new channel, portions of 

 the old being left as lagoons, though these are usually short-lived 

 because of the rapid sedimentation at recurring times of general ovcr- 

 riiiw. The bottom land immediately adjacent to the channel, where 

 the latter has been fixed for some time, is usually higher than the 

 lateral tracts. The depressions on one or the other side of the tlood- 

 bottom are generally drained at the lower end of the valley by a series 

 of sloughs emptying into the main channel just above the constriction 

 of the valley where the hills converge. 



At high water these lateral depressions are submerged to a depth 

 of cis much as twelve feet, as shown by actual measurement of the upper 

 limit of the mud marks on the tree trunks. Whatever the water 

 touches is discolored by a coat of fine sediment, and one thereby gains 

 at any time of the year an accurate estimate of the depth of inundation 

 in any part of the flood bottom at the previous period of high water 

 (see sectional profiles, figs. A and B). 



In some places the overfloM' depressions have no drainage outlets. 

 After times of flood, the water in them disappears by evaporation, 

 which proceeds at a very rapid rate in this excessively arid and hot 

 region. Areas of alkaline deposits are left, often stretches of bare, 

 baked hard-pan surrounded by concentric belts of halophilous vege- 

 tation. 



At rather infrequent intervals along the large valleys the river, 

 in swinging, cuts beyond its usual limit and assaults the desert mesa. 

 The process tends to add to the area of the regular flood plain and 

 conduces to the formation of lateral bluffs. It is noteworthy that 

 this aggressive work of the river is much more conspicuous on the 

 west side of each valley than on the east side. The law of westward 

 cutting of north-and-south flowing streams in the northern hemi.s- 

 phere, as brought about by the earth's rotation, is thus clearly illus- 

 trated in the Needles-to-Yuma section of the Colorado River. 



The local use of the terms "first bottom" and "second bottom"' 

 in the large valleys is worth adopting in general discussions relative 

 to ranges of animals. The first bottom is the portion of a valley 

 subject to regular overflow, and is clearly marked by the presence of 



