THE PRAIRIES OF THE PACIFIC NORTHWEST. 39 



The prairies of southeastern Washington and their eastward exten- 

 sion into adjacent Idaho occupy a position between the foothills of the 

 Bitterroot Mountains on the east and the sagebrush region of western 

 Adams, eastern Franklin, and western Walla Walla counties of Wash- 

 ington on the west. On the south they are bounded by that high 

 upfold of the lava-rock known as the Blue Mountains. Northward the 

 Spokane gravels, extending somewhat southward of Spokane, with 

 their open growth of yellow pine, mark at the same time the general 

 northern boundary of the exposed part of the great lava sheet and its 

 accompanying prairie formation. Since the supply of water in this 

 region is the chief hmiting factor to plant growth, we shall first con- 

 sider the total amount of precipitation, with its seasonal distribution, 

 after which the water-content of the soil will be considered. 



Hemmed in on all sides by mountains, and especially cut off from 

 the moist winds of the Pacific by the Cascades, the Columbian Plateau 

 has a low annual precipitation. Even on its high eastern border, 

 where these studies were carried on, it is only 21 inches. Table 7 

 shows in inches the mean monthly and annual precipitation at Pull- 

 man, Washington. 



Table 7. 



Month. 



Precipi- 

 tation. 



Month. 



Precipi- 

 tation. 



Feb 



May 



July 



inches. 

 2.55 

 2.18 

 2.02 

 1.50 

 1.84 

 1.20 

 0.57 



Aug 



Sept. 



Oct 



Dec 



Total . . 



inches. 

 0.68 

 1.29 

 1.70 

 3.41 

 2.66 



21.60 



It may be seen at a glance that about three-fourths of the precipita- 

 tion occurs during the resting period. The light showers of July and 

 August seldom have much influence on the water-content of the soil. 

 The soils of this region may be compared to a gigantic reservoir 

 replenished mostly during the resting season and rather thoroughly 

 emptied of its water during the summer. It is not the absolute rainfall 

 figures alone which furnish a criterion of climate, for the maximum 

 duration of the drought period constitutes a limiting factor of the 

 greatest importance. The great problem is the extent to which soil 

 water derived from the winter precipitation is conserved through the 

 weeks of drought. The rains in southeastern Washington are so gentle 

 that there is practically no run-off, and the silt-loam soils have a 

 wonderfully retentive power. 



The prairie soil has originated from the decomposed underlying 

 basalt. It consists of a friable, dark-brown silt loam which has a water- 



