KvssEi.i.j TEMPERATURE OF SPRINGS. 153 
the air. They may in fact have a temperature even in summer but 
little, if any, above that of freezing water, as, for example, where 
a soil or the surface sheet of rock waste becomes deeply filled with 
ice during winter, which melts slowly during summer, and yields 
nearly ice cold water. Hillside springs supplied with water which 
descends to a depth of less than about 50 feet may be expected to fluc- 
tuate somewhat in temperature from season to season, but are not 
quickly responsive to surface changes. Those which derive their 
water from a depth of approximately -io to 60 feet and are of suffi- 
cient volume not to be sensibly altered in temperature on nearing the 
surface will show no seasonal variation. Canyon and cave springs 
perhaps, as a rule, are supplied with water which has descended more 
than 50 feet into the earth and possibly may have reached a depth of 
several hundred feet, and their temperatures should, therefore, be 
somewhat higher than the mean annual temperature of the region 
where they emerge, but they will not be characterized by a high degree 
of heat unless they come from abnormally warm or hot rocks. In the 
typical canyon springs in southern Idaho the temperature is about 
6 )° F., or 10° above the mean annual temperature, and shows no 
seasonal variation. In these and other similar instances, however, 
in which springs are supplied by subterranean drainage, particularly 
of broad sheets of pervious rocks, there may be some addition of hot 
water rising through fissures in the rocks beneath. That is, fissure 
springs may contribute to the supply of subterranean streams in the 
same way that they do to surface streams. 
Fissure springs are supplied from depths ranging from a few 
score feet, or even less, to hundreds of feet, and come to the sur- 
face with all variations in temperature, from that of the mean annual 
temperature of the region where they occur up to the boiling- point 
for the elevations where they emerge. Those of sufficient volume and 
manifest freedom of flow indicate closely the temperature of the rocks 
from which they are supplied, and the depth of their source can thus 
be approximately determined, if the rate of increase of temperature 
with depth for the locality is known. The rise of temperature with 
increase in depth below the stratum of no seasonal variation, however, 
is not the same in different regions, and may be greatly modified by 
local causes, such, for example, as the presence of still hot volcanic 
rocks below the surface. The hot springs in Clover Creek Valley, 
near Blanche, and again the hot springs at Soda Springs, Bannock 
County, Idaho, are situated near volcanic craters that were recently 
active, and no doubt derive their heat from hot volcanic rocks at no 
great distance below the surface. The temperature gradient beneath 
the Snake River Plains, as indicated by the meager data in hand, is 
above the normal. As will be shown later (p. 178) the wells along the 
Oregon Short Line Railroad give an increase in temperature of 1° F. 
