I suggest that the reason for a decrease in 

 dissolved oxygen with an increase in tem- 

 perature (above and beyond decrease due to 

 temperature alone) is that the absence of a 

 layer of water over the bar prevents inter- 

 change. Ground-water influence can be ruled 

 out, for the water temperatures are too high 

 to indicate the presence of ground water. 

 Therefore, dissolved oxygen was maintained 

 on a high level as long as surface stream 

 water flowing over the bar permitted inter- 

 change to take place. But as soon as the stream 

 level dropped, no interchange occurred, and 

 water flowing through the bar was subject to a 

 continuous oxygen depletion from biochemical 

 oxygen demand. 



That such depletion can occur may be indi- 

 cated indirectly. The range of 27 flow velocity 

 measurements made in the bar in 1956 (by 

 timing the appearance of dye-marked water) 

 was from 2 to 170 feet per day with an 

 average of 37 feet per day. Based on this 

 average velocity and the length of bar, it is 

 possible that high oxygen content water enter- 

 ing the upper end of the bar would be subject 

 to a biochemical oxygen demand for 2 days 

 at a temperature around 55° F. In preliminary 

 studies on the biochemical oxygen demand 

 of Indian Creek gravels, dissolved oxygen 

 content fell from 9.5 to 2.2 mg./l. in 48 hours 

 at an average water temperature of 52.5° F. 



DISCUSSION 



streambed (such as that observed in Indian 

 Creek in September 1957) is another factor 

 that can interfere with interchange. 



It is also possible that varying amounts of 

 fine materials in spawning riffle streambeds 

 are responsible for some streams pro- 

 ducing more salmon than others. Wickett 



(1958) found a relationship between perme- 

 ability of streambed gravels and pink and chum 

 (Oncorhynchus keta) salmon fry production in 

 British Columbia streams. If high perme- 

 abilities are desirable and a large amount 

 of fines are detrimental to survival of salmon 

 eggs, fines can be removed. This action would 

 increase dissolved oxygen levels and flow 

 rates and enhance survival of salmon embryos. 



Low dissolved oxygen levels of Indian Creek 

 ground water during summer and fall months 

 indicate that areas of ground-water effluence 

 may be harmful to salmon eggs. But on the 

 other hand we found that in both Cabin and 

 Indian Creeks ground water was colder than 

 stream water, in summer and warmer in 

 winter. Therefore, as Needham and Jones 



(1959) point out, ground water may have a 

 tempering effect on stream water and help 

 prevent freezing of streambed gravels. This 

 possibility can easily be investigated, since 

 ground water in salmon spawning riffles can 

 be detected and traced through its distinctive 

 qualities of dissolved oxygen and temperature. 



Ground water in the Indian Creek study 

 riffle was generally low in dissolved oxygen, 

 and dissolved oxygen levels decreased with 

 depth in streambed gravels. Thus, by a process 

 of elimination we can corroborate reports of 

 Royce (1959) and Vaux and Sheridan (1960) that 

 the primary source of high oxygen content 

 intragravel water in salmon streams is the 

 stream itself. 



Therefore, if anything interferes with inter- 

 change of stream and intragravel water, the 

 amount of dissolved oxygen available to salmon 

 eggs will be decreased, and the rate of flow 

 past embryos will be lowered. Silting of the 

 streambed, by lowering permeability 

 of streambed gravels, can definitely interfere 

 with interchange. An algae cover over the 



Although ground water has either a harm- 

 ful or beneficial effect (depending on circum- 

 stances) in upstream spawning areas, it is 

 doubtful if it has any direct effect at all in 

 intertidal areas where great numbers of pink 

 salmon spawn in Southeastern Alaska, Prince 

 William Sound, and other regions. Intertidal 

 areas are often underlain by impervious bed- 

 rock or a clay layer at relatively shallow 

 depths, and streams meander through extensive 

 tide flats composed mostly of mud. Only main 

 stream channels are kept clean. Since there is 

 no place for ground water to come from, 

 intragravel water in intertidal areas must 

 depend exclusively on interchange for re- 

 plenishment of dissolved oxygen and on ebb 

 and flow of warmer salt water for protection 

 against freezing. 



17 



