PHYSICAL CHARACTERISTICS OF THE ARID REGION. ibs: 
interests demand and favorable opportunities are afforded. In general, the 
opportunities for ponding water in this way are infrequent, as the depres- 
sions where ponds can easily be made are liable to be so low that the 
waters cannot be taken from them to the adjacent lands, but occasionally 
very favorable sites for such ponds may be found. This is especially true 
near the mountains where alluvial cones have been formed at the debou- 
chure of the streams from the mountain canons. Just at the foot of the 
mountains are many places where ancient glaciation has left the general 
surface with many depressions favorable to ponding. 
Ponding in the lower region is somewhat wasteful of water, as the 
evaporation is greater than above, and the pond being more or less shallow 
a greater proportional surface for evaporation is presented. This wastage 
is apparent when it is remembered that the evaporation in an arid climate 
may be from 60 to 80 inches annually, or even greater. 
Much of the waste water comes down in the spring when the streams 
are high and before the growing crops demand a great supply. When this 
water is stored the loss by evaporation will be small. 
The greater storage of water must come from the construction of great 
reservoirs in the highlands where lateral valleys may be dammed and the 
main streams conducted into them by canals. On most streams favorable 
sites for such water works can be found. ‘This subject cannot be discussed 
at any length in a general way, from the fact that each stream presents 
problems peculiar to itself. 
It cannot*be very definitely stated to what extent irrigation can be 
increased by the storage of water. The rainfall is:much greater in the 
mountain than in the valley districts. Much of this precipitation in the 
mountain districts falls as snow. The great snow banks are the reservoirs 
which hold the water for the growing seasons. Then the streams are at 
flood tide; many go dry after the snows have been melted by the midsum- 
mer sun; hence they supply during the irrigating time much more water 
than during the remainder of the year. During the fall and winter the 
streams are small; in late spring and early summer they are very large. 
A day’s flow at flood time is greater than a month’s flow at low water time. 
During the first part of the irrigating season less water is needed, but during 
