1914] GrinneU: Mammals and Birds of the Colorado Vail, y 103 



either no trace of riparian tracts or only narrow or interrupted repre- 

 sentations, it would seem that chance is afforded for such mammals as 

 the desert chipmunks and pocket mice to encounter the river itself, 

 with all intervening factors removed. 



At our base camp (no. 4) at The Needles, our measurements 

 showed the actual width of the stream to be 450 feet at the rather low 

 stage of water obtaining at that time (March 4, 1910). At high water 

 the river could not have been much more than 150 feet wider (see 

 sectional profile, fig. A, and pi. 10, fig. 15). In the box canon two to 

 three miles below, the width of the river appeared to be much less, 

 and. because of the precipitous walls, high water would make little 

 change in width. 



The two species of Ammospermophilus were seen at points only 

 about 850 feet apart in a direct line. All the mammals of the Encelia 

 association, as segregated here on the two sides of the river, were 

 trapped at this station within one thousand feet of one another. The 

 same situation obviously held at several other points along the Colo- 

 rado River. The sharp separation of the ranges of nearly related 

 vertebrates by a barrier of such narrow width is, to the best of the 

 writer's knowledge, not known elsewhere in North America. 



As to the opportunities for crossing the river by such individuals 

 as might get to the water's edge, only speculation is now possible. 

 At times of rising water, riparian mammals are undoubtedly often 

 marooned upon islands and finally forced to swim or to take refuge 

 on floating drift. Practically all the riparian species are known to be 

 able to swim readily, and are probably in ways just mentioned fre- 

 quently carried from side to side of the river. 



The ability of typical desert animals to care for themselves if cast 

 into the water is problematical, though a little experimentation would 

 go far to proving the point one way or the other. Their powers in 

 this line may be inferred to be limited because of the facilities normally 

 lacking for putting such powers into practice. For instance, there is 

 fair probability that a Beithrodontomys could safely cross a turbulent 

 stretch of current, where a Perognathus would perish before any 

 chance of reaching the shore. 



Protracted observation along the river brings conviction to the 

 observer that no animal of weak swimming powers is likely to sur- 

 vive many minutes of exposure to the main current. It is a fas- 

 cinating diversion to watch the course of a stick or log adrift in 

 the stream. Such an object pursues an exceedingly devious course. 



