38 REPORT OF COMMISSION VII 
Potential sources of water subject to development for livestock, wild life, 
or human consumption, can sometimes be recognized on photos by the staining 
of dry ground caused by seeps, or by the presence of known hydrophyllic vegeta- 
tion. Retention and preservation of the water resource by reducing runoff can 
be accomplished by damming, spreading over underground storage basins, and 
by revegetating denuded watersheds. Planning for all of these and related 
activities can be aided by photo-interpretation. 
Flood control surveys rely heavily on streamflow and precipitation data 
gathered in the field. Valuable supplemental information can be obtained from 
photo classifications of vegetation types, as well as from soil and geological 
classifications. One survey adapted photos to the task of determining long- 
  
F1G. 3. Composite oblique photograph showing usefulness of aerial photography for 
delineation and interpretation of watersheds. 
time flood crest potentials. Old inhabitants of the area were questioned as to 
their recollections of heights of maximum floods, and locations of areas or ob- 
jects known to have been flooded or not flooded. Check points located in this 
manner were spotted on photos, and then, by stereoscopic examination of land 
features and topography, a line of highest known flood stage was established. 
Snow surveys have been relied upon for a number of years to provide infor- 
mation on water content and depths of snow packs. Hydrologists use the survey 
data in predicting available water yields for irrigation and stream flow, and for 
judging flood potentials. Aerial photographs are being used to aid inventories 
of snow cover. One technique followed is that of preparing an aerial mosaic of 
a snowshed, and studying sample points selected by use of a superimposed grid. 
In addition to indicating presence or absence of snow cover at sample loca- 
tions, stereoscopic study of the photos reveals essential information on slope 
and nature of topography, ground cover, etc. 
Oblique photos, such as illustrated by Figure 4, are easier and cheaper to 
procure than overlapping verticals flown to provide corresponding coverage. 
They also yield less precise data, but nevertheless can be used to determine 
indexes for judging extent of snow cover. 
Under present usage techniques, there is relatively little interpretation of 
photos required. Snow is normally very conspicuous in contrast to vegetation