over a considerable portion of each study 

 area during 1957, whereas in 1958 oxygen 

 levels were high by connparison. Figure 5 

 presents a comparison of mean dissolved 

 oxygen values obtained for each study area 

 in 1957 and in 1958. The mean values shown 

 were obtained from points that were sannpled 

 both years (see appendix). 



In 1957 sampling was carried out during a 

 period of warm weather, light precipitation, 

 and cloudless days. In 1958 weather condi- 

 tions were quite different; freshets occurred 

 periodically and most days were overcast. 

 Mean daily discharge of Indian Creek was 

 2.0 c.f.s. in August 1957 and 60 c.f.s. in 

 August 1958. 



Wickett (1958) has proposed that certain 

 periods of low stream discharge were asso- 

 ciated with low oxygen levels of the intra- 

 gravel water. The data obtained in 1957 

 supported this contention. The relatively high 

 dissolved oxygen levels observed in 1958 

 were probably the result of more favorable 

 hydrological conditions. 



Spatial differences in dissolved oxygen 

 levels . - - Spatial differences in dissolved oxy- 

 gen content of intragravel water were gen- 

 erally more extreme in 1957 than in 1958. 

 Many points were deficient in dissolved oxygen 

 during the 1957 sampling period, and it was 

 possible to define extensive areas of low (less 

 than 2.5 mg./l.)' oxygen levels. These are 

 shown in figures 6 through 11 for study areas 

 A, B, C, D, F, and G. Points sampled both 

 years are indicated in these figures by dots. 



In 1958 dissolved oxygen levels exceeded 

 5.0 mg./l. at most of the points sampled. 



C5 R 6 



§1 "^ 





2- 



I I 1957 



11958 



li 



II 



IJ 



J 



C D 



Study 



I 



E 

 area 



1 



1. 



M 



I 



I 



1 



Figure 5. --Mean dissolved oxygen values obtained at study areas 

 in late August and early September of 1957 and 1958. 



Relatively few points exhibited low levels of 

 dissolved oxygen. The areas of relatively 

 low and high oxygen values occurring in 1958 

 are also shown in figures 6 through 11. 



A table of dissolved oxygen values observed 

 within each study area appears in the appendix. 



Random Sampling 



By sampling randomly it was possible to 

 obtain estimates of mean dissolved oxygen 

 level of intragravel water within large spawn- 

 ing areas (sampling areas). Spatial differences 

 in dissolved oxygen levels were detected by 

 sampling two or more areas simultaneously. 

 Temporal changes in dissolved oxygen levels 

 w^ere detected by sampling each sampling area 

 two or more times. Standard statistical tech- 

 niques were employed to test for significant 

 differences between estimated mean dissolved 

 oxygen values. 



Two sampling areas on Twelvemile Creek 

 were sampled concurrently in a random man- 

 ner during early September and late November, 

 1958. The lower sampling area, extending from 

 the 12- to 16-foot tide level, incorporated 

 60,000 square feet of streambed and included 

 most of the intertidal spawning area. The 

 upper sampling area extended upstream from 

 the intertidal zone and incorporated 68,000 

 square feet' of streambed. The heaviest ob- 

 served spawning intensity above the intertidal 

 zone occurred in this area. 



The general sampling procedure employed 

 was to place standpipes at randomly selected 

 points 1 day prior to sampling. One dissolved 

 oxygen reading was obtained from each point, 

 and an attempt was made to obtain all readings 

 for both sampling areas on the same day. 



In September 1958, dissolved oxygen read- 

 ings were obtained from approximately 100 

 points within each sampling area. In November 

 1958, it was possible to reduce the sampling 

 effort to 50 points per area, since the varia- 

 bility among readings was considerably less 

 in autumn than in late summer. 



Ninety-five percent confidence interval es- 

 timates of mean dissolved oxygen content of 

 intragravel water within the two Twelvemile 

 Creek sampling areas are given in table 5. 

 These estimates indicated that: 



1. Oxygen levels were significantly higher 

 within both sampling areas during midautumn 

 than during late summer, i.e., there was a 

 change in oxygen levels with time. 



' No physiological significance is attached to a dissolved oxygen 

 content of 2.5 mg./l. This value was selected purely for purposes of 

 illustration. 



2. Dissolved oxygen levels were sig- 

 nificantly lower in the upstream sampling 

 area than in the intertidal sampling area 



