DESCRIBING STREAM CHARACTERISTICS 



Stream Length 



Aerial photos are particularly effective for editing maps for stream length. If available, 

 maps with a scale as large as 1 or 2 inches to the mUe are best for use with the photos. End 

 points of each stream should be located first on the photos . The end points are transferred to 

 the map as accurately as possible and the apparent length of the stream and its tributaries can 

 then be measured on the corrected map. The stream's lower end is defined as the point where 

 it flows into another stream. The upper end is defined as the highest point in the drainage that 

 gives evidence of permanent flow of water Dry or intermittent channels can be eliminated 

 so that only the essential portions of any stream under consideration will be included. Dividers, 

 set to the scale of the map, are used to step off the length at quarter -mile intervals to give the 

 unadjusted length of the entire stream or any portion of the stream. This unadjusted length 

 underestimates actual stream length, because small bends and other deviations in stream di- 

 rection cannot be accurately mapped. However, the mapped stream length can be multiplied by 

 a "meander factor" to give an adjusted stream length. 



The meander factor is the ratio between the meander distance as visible in the photo and 

 the straight -line distance for a given stream segment. 



The procedure for determining meander distance on the photo is to rule a thin straight 

 line on a strip of transparent paper and then match this line to short segments of the stream. 

 Two points approximately a quarter of a mile apart in straight -line ground distance are selected 

 on the stream image . The paper is placed over the photo so that one end of the ruled line is 

 over one of the selected points, and a pin is pushed through the paper and into the photo at this 

 point. The paper is then pivoted so that the line lies over a short section of the stream course. 

 A second pin is then placed at the point where the stream image diverges from the ruled line. 

 The first pin is removed and the paper pivoted to coincide with the next section of stream. This 

 process is repeated until the point a quarter of a mile away is reached. The amount of line used 

 is then measured, and the meander factor is calculated: 



Photo meander distance 



Meander factor = 



Photo straight -line distance 



Because the value sought is a ratio, any units of measurement can be used on the photo; conver- 

 sion to ground distances is not required. Several measurements may be made at different points 

 along the stream photo image and an average meander factor established. Thus a stream with an 

 average meander factor of 1.470 and an unadjusted mapped length of 8.6 miles would have an 

 adjusted length of 8.6 X 1.470 = 12.64 miles. 



In some instances, a general average meander factor for a particular drainage basin may 

 be used for all streams in the basin. This saves time without any considerable loss of accuracy. 



^ If work on photos is followed by on -the -ground sampling, the upper end of the stream may 

 be redefined by other criteria. During the fieldwork on the North Slope study the upper end of 

 each stream was arbitrarily defined as the point v^ere stream width dwindled to 4 feet. 



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