L16 LANDS OF THE ARID REGION OF THE UNITED STATES. 
by August or September. In the low valleys the irrigation of wheat and 
other small grains begins about the first of June, and continues until the 
latter part of July. The irrigation of corn and potatoes begins in the 
early part of July, and continues until the middle of August. In the 
middle of July all of the land ealls for water, and if the supply is sufti- 
cient at that time, it is sure to meet all demands at other times. It will be 
convenient to call that time the critical season. In the higher agricultural 
valleys corn and potatoes are not grown, but the irrigation of small grains 
and hay is carried on from the middle of June to the middle or latter part 
of August. Through all this time the volume of the streams is diminishing, 
and if they fail at all it is at the end of the season. The critical season 
for the higher valleys is about the middle of August. 
In order to estimate properly the agricultural capability of a stream, 
it is necessary to ascertain its volume at its critical season. In the investi- 
gations of the past summer, this was accomplished by direct measurement 
in but a limited district. For the remainder of my field of operations I 
was compelled to depend on the estimates of others as to the relation 
between the volumes of streams at the time of measurement and at the 
critical season. 
As will appear in the sequel, the uncertainty attaching to these 
determinations of volumes affects the grand total in but small degree. 
The utility of the large streams is not timited by their volumes so much as 
by the available land suitable for overflow, a quantity susceptible of more 
accurate determination, and the extent of land irrigable by the large 
streams is many times greater than that irrigable by the small. 
No streams are used throughout the year, and few can be fully utilized 
during the spring flood. Wherever it is practicable to store up the surplus 
water until the time of need, the irrigable area is correspondingly increased. 
Enough has been accomplished in a few localities to demonstrate the 
feasibility of reclaiming thousands of acres by the aid of reservoirs, and 
eventually this will be done; but except in a small way it is not a work of 
the immediate future. For many years to come capital will find greater 
remuneration in taking possession of the large rivers. 
In estimating the agricultural resources, it was, of course, necessary to 
