Lake images that are light gray or white seem to blend with the image tones of adjacent 

 rock slopes, and may easily be overlooked (fig. 2). Stereoscopic examination reveals them, 

 however, and field investigation shows that such lakes are almost always heavily silted; their 

 water is opaque, and all underwater detail is obscured. 



Occasionally alpine meadows and dry lakebeds have photographic tones similar to those 

 of opaque lakes. Stereoscopic study of the photo images, however, reveals textural differences 

 between the water surface of a lake and the vegetation of a meadow. When meadows surround 

 marshlike lakes, the exact position of the shoreline is difficult to delineate on photos. In such 

 cases, ground inspection often shows that the area is subject to spring flooding followed by a 

 lowering of water that exposes meadows during the late summer. Dry lakebeds, on the other 

 hand, can be recognized by their basinlike conformation. 



RECORDING THE LAKES BY DRAINAGE UNITS 



The photo interpreter can quickly identify and count all the lakes within each drainage unit. 

 Any convenient method may be used to identify them if they are not already named. It was help- 

 ful in this study to number each lake initially by both drainage unit and the number of the aerial 

 photo on which it appears. For example, lake 3-10-EBW-9-198 would be the third lake in drain- 

 age unit 10 on photo EBW-9-198. The center of each lake was pinpricked on the photo and the 

 lake number was written on the back of the photo next to the pinprick. All the lakes in one drain- 

 age unit were later numbered consecutively on the map and cross -referencedto the photo numbers 

 for rapid retrieval of the photo on which 

 a particular lake was shown. These 

 numbers were used on the drainage unit 

 map to show the location of the lake. 

 When large numbers of small lakes less 

 than an acre each are clustered on the 

 photo, it may be unnecessary or difficult 

 to indicate the precise position of each 

 on the map. In such instances, the total 

 number of lakes less than 1 acre and of 

 a single type can be simply recorded for 

 each drainage unit, and no specific 

 locations need be shown. 



Figure 2. - -An opaque lake image, compared 

 with a clear lake image. Lakes that ap- 

 pear cloudy on the ground may produce 

 light images on panchromatic photographs; 

 such images may blend with the tones of 

 rock slopes. (Enlarged X2) 



