THREATENED ARIDITY ON THE PACIFIC SLOPE 163 



other localities than in proximity to streams and lakes ; .the arid 

 are regions completely deforested, away from streams and lakes, 

 natural or artificial, but which bear an often rich and varied flora 

 of herbaceous and suffrutescent vegetation ; the desert are tracts 

 without vegetation. The two former are abundantly represented 

 in this region by very large areas, as we shall see. The third, or 

 the desert, does not exist here. It is common to speak of " the 

 desert regions of eastern Oregon," for example, but the fact is 

 that in no place has aridity reached its third and last stage. 

 When herbaceous vegetation is absent, as on certain alkali flats 

 east of Steins mountains, or on drifting sand-dunes along the 

 Columbia and on the plains of eastern Washington and north- 

 eastern Oregon, it is due to local soil conditions, not to absence 

 of sufficient precipitation. 



To facilitate a more detailed examination of the various forest 

 conditions, as modified by increasing aridity, the region under 

 consideration will be divided into certain classes or zones. These 

 zonal distinctions have reference solely to the amount of pre- 

 cipitation which each class receives without regard to altitudinal 

 limitations, and will be designated arid, semi-arid, subhumid. 

 and humid areas. 



THE REGIONS OF ARIDITY 



The regions of greatest aridity north of the 42d parallel of lati- 

 tude between the Rocky mountains and the Cascades are found : 

 in Idaho, on the Snake River plains; in eastern Oregon, on the 

 plateau areas between the Snake and the Owyhee rivers on the 

 one hand and the Steins mountains on the other; in the region 

 bounded by Crooked river and Malheur lakes and river on the 

 north, Steins mountains on the east and the northern boundary 

 of Nevada on the south, and in the Deschutes depression be- 

 tween the Blue mountains and the Cascades ; in eastern Wash- 

 ington, north of the Snake and east and south of the Columbia 

 river. 



The aridity which prevails upon these areas is of various de- 

 grees of intensity, depending more or less upon local conditions 

 and the proximity or distance of humid, snowy mountain ranges. 

 In eastern Washington the driest section is situated at the east- 

 ern base of the Cascades, and extends eastward some 60 or 70 

 miles, gradually merging into uniform semi-arid and subhumid 

 conditions as the moisture-condensing Bitter Root ranges are 

 approached. In eastern Oregon the most arid tracts are found 



