148 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
reaching them from the atmosphere not evaporated or absorbed by- 
plants disappears below the surface. 
The run-off is contributed directly to surface streams, but these 
would be intermittent, flowing only during and immediately following 
rains or the melting of snow and ceasing to flow during intervening- 
periods, were it not for the return to the surface of previously sub- 
surface waters. This return is accomplished in part by seepage from 
the sides and bottoms of valleys, and in part by springs. The manner 
in which water is retained for a greater or less time by porous and 
fissured rocks and rock waste and returned more or less gradually to 
the surface thus becomes important in the study of both the surface 
and subterranean water supplies. It is to the subsurface or subter- 
ranean waters that attention is here invited. 
The water absorbed by porous material, such as constitutes soils, 
sand dunes, etc., is in part returned to the surface by passing up- 
ward, owing to what is termed capillary attraction, as the surface 
dries, and in part through the action of plants in absorbing water 
through their roots and transpiring it through their leaves. The 
latter process is an important adjunct to direct evaporation, and in 
densely plant-covered regions the quantity of water so passing into 
the air is frequently greater than that escaping by direct evaporation, 
even from freely exposed water surfaces. The water not removed 
by evaporation or by plants is in part retained as soil moisture, and 
if there is an excess above what the soil can permanently hold as a film 
on the surface of the particles composing it, such excess descends or 
moves laterally, under the influence of gravity, to a lower position and 
is available for the supply of springs. Water entering fissures in 
rocks evidently has, in general, greater freedom to flow than that in 
porous material, and in many instances is also an important source for 
the supply of springs. 
The force which causes subterranean water to flow is the same that 
controls the movements of surface streams, namely, gravity, but cer- 
tain modifications due to difference in temperature, capillarity, etc., 
fully discussed by King/* will not be considered at this time. 
SI'KINGS. 
Four varieties of springs may for convenience be recognized, 
namely, hillside, canyon, fissure, and cavern springs, the classification 
being by reference to types and with the understanding that more or 
less intermediate gradations occur between well-characterized exam- 
ples. In the case of each variety there is a wide range in volume, the 
minimum in each case being what is termed seepage. 
«A highly instructive discussion of the way in which water percolates through porous material 
may be found in a paper on the " Principles and conditions of the movements of ground water," by 
F. H. King, in the Nineteenth Ann. Rept. U. S. Geol. Survey, Pt. IV, 1899, pp. 59-294. 
