on the map. This new point was transterred 

 to the corrected map. 



The length of the stream in miles then 

 was measured on the corrected map by using 

 dividers set at map scale to measure 14-mile 

 distances. However, such direct map measure- 

 ments underestimate the true stream length 

 because of the extra distance involved in the 

 meandering course followed by most streams. 

 It is possible, of course, to measure the true 

 stream length in the field, but this would 

 require a great deal of time. Because over- 

 hanging trees obscure the stream channel in 

 many places, complete measurement on aerial 

 photos is not possible either. 



As an alternative, the amount of meander 

 was estimated for the stream by measuring 

 the meander distance of selected segments 

 of stream channel that were visible on the 

 aerial photos. Four or five segments approxi- 

 mately one-fourth of a mile long were meas- 

 ured for each main channel and four or five 

 for the tributaries of that channel. 



A "meander factor" was calculated for 

 each sample segment using the procedures 

 described by Herrington and Tocher. The 

 meander factors for the sample segments were 

 then averaged together. An average meander 

 factor for each stream and its tributaries then 

 was calculated. This factor expresses the 

 number of units of meander distance that are 

 associated with one unit of straight line dis- 

 tance. In this study, factors ranged from 

 1.1 to more than 2.1 for individual stream 

 segments sampled. 



These average meander factors for the 

 stream were used to adjust the stream lengths 

 determined from the corrected map. For ex- 

 ample, one stream that had measured 8.6 

 miles on the corrected map was found to have 

 an average meander factor of 1.47. Thus, the 

 adjusted length of this stream was calculated 

 as being 12.64 miles (8.6 miles X 1.47). 



'Herrington, Roscoe B.. and S. Ross Tocher. 

 Aerial photo techniques for a recreation inventory 

 of mountain lakes and streams. U.S. Forest Serr. 

 Res. Pap. INT-37. 21 pp.. illus.. 1967. 



PRECISION OF RESULTS 



The stream characteristics recorded at 

 each sample transect can be used either 

 singly as a description of the .stream at that 

 particular point or in combination with other 

 samples to obtain an estimate of the stream 

 as an entity. Composite estimates require 

 that numerical values for specific characteris- 

 tics of all samples be totaled to derive a mean 

 or average value per transect or be expressed 

 as a percent of all transects having any 

 particular characteristic. 



Table 1 summarizes the general character- 

 istics estimated for the three drainages in- 

 cluded in this study. Width, depth, and grad- 

 ient of each stream were calculated as the 

 average value per transect. The data for 

 bank stability and vegetation type were ex- 

 pressed as proportions of all banks rated. 

 Data on pool area, riffle area, pool class qual- 

 ity, and bottom material were expressed as 

 proportions of the stream's total surface area. 



The reduction of a large number of indi- 

 vidual measurements to a single figure facili- 

 tates comparison between streams in terms 

 of a specific characteristic. For example, the 

 Main Fork of the Bear River was found to 

 have a much lower proportion of its surface 

 area in pool situations than Henry's Fork 

 and Hayden Fork. However, such composite 

 estimates always are associated with a cer- 

 tain amount of error because they are ob- 

 tained from samples rather than a complete 

 measurement of an entire stream. For this 

 reason, any estimate is expected to approxi- 

 mate the actual situation rather than indicate 

 precisely its magnitude. Therefore, the utility 

 of the estimates depends on the amount of 

 error associated with them. 



The standard error of the estimate was 

 calculated for selected stream characteristics 

 to indicate how large an error was associated 

 with the estimates for the three streams. One 



8 



