50 ANIMAL LIFE IN DESERTS 



in others the floods are so violent, or the soil is so 

 unfriendly, that nothing grows (Fig. 19). 



These apparently dry beds form Hnes along which 

 subterranean water drains, and in many of them 

 it may be found by sinking a weU to a depth of a 

 few feet. This subsoil water determines the pre- 

 sence of a relatively dense vegetation (Fig. 21), 

 consisting often of a fairly large number of species ; 

 these in turn provide food and shelter to very many 

 animals. The bed of the stream gives Hfe lavishly ; 

 as lavishly as it destroys it. A sudden heavy rain- 

 storm, such as is common in deserts (page 13), 

 falls, the water runs rapidly off the bare ground 

 and produces a devastating flood. Shakespear saw 

 such a flood as this in the Batin (lower part of 

 Wadi Rummah) in Central Arabia ; twenty minutes 

 of heavy rain converted the dry bed into a rushing 

 torrent 3 feet deep, 50 feet across. Such floods are 

 not rare in Arabia. Philby has suggested that the 

 desolation of the once populous Wadi Hanifa was 

 due to a great flood which obUterated the settle- 

 ments in the bed of the Wadi, and spared those 

 on the sloping ground around ; he records a flood 

 in Wadi Dawasir which filled weUs, and destroyed 

 a hamlet, and another which drowned 150 men, 

 450 camels, and thousands of sheep, if local report 

 is reliable. Palmer records a flood which destroyed 

 forty bedawin and their flocks in the Wadi Solaf 

 in Sinai, and doubtless examples might be multipUed. 

 Certain desert areas are traversed by great 

 perennial rivers, such as the Tigris and Euphrates, 

 the Nile, and the rivers of Transcaspia. In Lower 



