150 
RNAKE RIVER PLAINS OF IDAHO. 
[BULL. 199. 
When the stream bed is 
regions, the water table 
/ 
dry, however, as so frequently happens in arid 
may subside below the bottom of its bed, or 
to dd, and become a plane surface; or, as really 
happens for various reasons, may disappear, there 
being- no saturated subsoil layer. 
Owing- to variations in the conditions on which 
the lives of hillside springs depend, they commonly 
exhibit seasonal variations in volume, and perhaps 
also in temperature. 
CANYON SPRINGS. 
I 
When water disappears beneath the surface of the 
land, it may enter pervious stratified rocks and flow 
through them for long distances, finally emerging 
where the water-carrying beds have been cut across 
by erosion or in other ways furnished with openings 
through which water may escape from them. One 
of the most common methods by which this result 
is attained and one abundantly illustrated in south- 
ern Idaho is by the excavation of canyons in nearly 
horizontal stratified rocks, some of the larger of 
which arc water charged. Although at times hav- 
ing essentially the same characteristics as hillside 
springs, the type here referred to, in numerous con- 
spicuous examples, depends on markedly different 
conditions. The water is conducted to a distance 
through pervious beds having impervious floors, 
and may or may not be covered by impervious lay 
ers. Owing to freedom of escape even when an 
impervious cover is present, the water is not usually 
under pressure except such as results from retarda- 
tion due to friction of flow. There is no storage 
reservoir as in the case of fissure springs, described 
below, and if the source of supply fails the springs 
run dry. 
While the underground courses of the waters sup- 
plying hillside springs are commonly short, being 
measured usually by rods, the journeys of the waters 
feeding canyon springs may be long, and in many 
instances are measurable in miles or tens of miles. 
The conditions favoring the occurrence of canyon 
springs are illustrated by the section forming fig. 3, 
which, although ideal, indicates in a general way 
the mode of origin of the copious springs in Snake River Canyon, 
which are the type of the class under consideration. 
^V X 
Fig. 3.— Section showing 
conditions favoring 
the occurrence of can- 
yon springs. 
