19U] Grrinnell: Mammals and Birds of th( Colorado Valley 83 



result of the very infrequent local cloud-bursts or severe thunder- 

 storms. Eveu the two "rivers" named often go completely dry in 

 their lower courses following protracted drouth. At frequent inter- 

 vals along the valley of the Colorado, well-defined but ordinarily dry 

 water-courses leading down from the adjacent uplands record the 

 existence at some time or other of considerable run-off. Among the 

 hills, deep, steep-sided ravines show the profound effects of erosion, 

 even though, in this era, rains come but rarely. Where the water-course 

 drains a large extent of higher country, enough to furnish a volume 

 of water sufficient to find its way through the mesa to the river, it 

 may occupy a broad valley with low confining banks or bluffs. Such 

 a broad, dry flood plain is called locally a "wash"; and as the biota 

 of such "washes" is quite distinctive the term "wash association" has 

 been suggested (see pi. 7, fig. 9). Since a prevalent plant in its flora 

 is the tenaciously thorny catclaw (Acacia greggii), the term catclaw 

 association may be used as an alternative denomination. Both are 

 appropriate terms. 



The vegetation of the catclaw association is the most conspicuous 

 of all of the desert associations, for it includes several species which 

 reach the stature of trees. The largest of these is the desert ironwood 

 (Olncya tesota) which grows abundantly in all the larger washes on 

 both sides of the river, from the lower Chemehuevis Valley at least 

 to the vicinity of Picacho (pi. 8, fig. 10). The branches are leafy but 

 thorny, forming ideal refuges for certain small birds and locations 

 for their nests. The apparently leafless palo verde (Parkinsonia 

 torreyana) is a close companion of the ironwood, and occurs also along 

 the smaller ravines into the hills (see pi. 8. fig. 11 ; pi. 9, fig. 12). Both 

 the catclaw and the palo verde were found in practically every wash 

 on both sides of the river, from the vicinity of Needles to Pilot Knob. 

 The smoke-bush ( Dalea spinosa) is a conspicuous element in many of 

 the washes from near Riverside Mountain to Pilot Knob. 



As an indication of the size reached by individuals of these truly 

 xerophilous trees the following measurements taken by the writer, 

 are here presented : An ironwood growing in a wash about one-half 

 mile back from the river bottom in lower Chemehuevis Valley, Cali- 

 fornia side of the river, was 90 inches in circumference of trunk two 

 feet above the ground, and had a height of 31 feet. A palo verde 

 nearby was 48 inches in circumference of trunk two feet above the 

 ground, and was 28 feet in height. Another palo verde (pi. 9, fig. 12) 

 growing in a wash on California!! territory four miles north of Pot- 



