orgaxiisms during each month at the twelve selected stations indicates 

 that sampling error can accoxint for a large amount of variation (Table 17) • 

 Results at the stations during April, May, and Jxrne i960 are considerably- 

 different from those during the same months in I96I. 



There was a significant change in numbers over the period of 

 study in the upper reach. From a low average number of U7.5 organisms 

 per square foot in July i960, the popxolations built up to a high of 933-9 

 organisms per sqxiare foot in November I96O, emd then dropped to 86.2 in 

 December and 52.6 in January I961. Heavy rains, resulting in high river 

 flows, occurred in late November and may have been largely responsible 

 for the decrease. 



A corresponding decrease was observed in the upper-middle reach 

 between November and December i960. There was no conparable decrease 

 in toteil numbers of organisms in the lower-middle and the lower reaches. 



Discussion 



The BWDst iiBportant contribution of the biological phase of the 

 Sacramento River Water Pollution Survey has been the collection of infor- 

 mation concerning present biological conditions in the Sacramento River. 

 The main value will be realized in future years when the effect of water 

 qimlity degradation due to additional development in the Sacramento Valley 

 can be quantitatively assessed. 



An attempt hats been made to select a limited number of animals 

 that might possibly be used as potential environmental indicators . The 

 selection of these organisms was based on the regularity and frequency 

 with which they were collected. The adaptations and requirements of many 

 of these organisms need to be determined. 



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