flow over a smaller area. Beneath the wave 

 are adjacent areas of upward and downward 

 flow which provide a rapid circulation of water 

 over a small area. 



It is to be noted that the assumption of equal 

 hydraulic and energy gradients no longer ap- 

 plies. Although there is no apparent basis for 

 predicting the direction of interchange beneath 

 a wave, there is certainly an energy dissipa- 

 tion through the wave. For the general case 

 of equal energy and hydraulic gradients up- 

 stream and downstream from a wave there 

 must necessarily be a point of inflection in 

 the energy line and, in turn, adjacent areas of 

 downward and upward interchange. 



Another consideration in comparing profile 

 and point interchange is the location of points 

 of interchange in the stream. Assuming con- 

 stant gravel permeability, interchange due to 

 the axial profile can be expected to be in 

 the same direction across the stream. On the 

 other hand, the occurrence and size of waves 

 vary with velocity of flow. Accordingly, point 

 interchange can be expected to be low in calm 

 water near the stream shore and most exten- 

 sive at midstream points where turbulence 

 is greatest. 



Finally, how do conditions causing inter- 

 change change with time and variations in 

 stream discharge? Changes in the direction 

 and extent of interchange will result, essen- 

 tially, from changes in the stream surface 

 configuration, the interchange driving force, 

 and an increase or decrease in gravel per- 

 meability. 



Interchange over a large area will be in- 

 fluenced by changes in stream surface con- 

 figuration through stream discharge fluctua- 

 tions and shifting of the stream bottom. The 

 extent of interchange will be governed through 

 variations in gravel permeability resulting 

 from siltation, gravel compaction, organic 

 content, and gravel shift. 



Point interchange, too, will depend upon 

 stream discharge, in this case, however, 

 through its effect on surface wave configura- 

 tion. During low stream discharge the water 

 surface is comparatively calm and point inter- 



change will be reduced accordingly. Relative 

 dissolved oxygen levels tend to verify this: 

 McNeil (1962) has shown through extensive 

 intragravel dissolved oxygen sampling that 

 the intragravel dissolved oxygen content in- 

 creases with stream discharge, and Wickett 

 (1958) has proposed that low oxygen levels of 

 intragravel water are associated with periods 

 of low stream discharge. 



SUMMARY 



Studies of interchange of stream and intra- 

 gravel water were conducted in 1957, 1958, 

 and 1959 as part of a project that is supported 

 by the Bureau of Commercial Fisheries to 

 study the effects of logging on pink salmon 

 production. Interchange was first qualitatively 

 demonstrated in a salmon spawning riffle in 

 Indian Creek in Southeastern Alaska. Then, 

 experimental research was carried on at the 

 University of Washington Chemical Engineer- 

 ing Laboratory to determine variables that 

 control interchange and, finally, additional 

 field studies in Indian Creek provided a quali- 

 tative verification of dependence of inter- 

 change on stream gradient and other factors. 



The theory of interchange postulates that 

 steps involved in physical transport of free 

 oxygen to intragravel water are (1) dissolu- 

 tion of atmospheric oxygen into stream water, 

 (2) transport of oxygenated water to stream 

 bottom, and (3) interchange of oxygenated 

 water from the stream into the porous gravel 

 interior. Factors controlling interchange are 



(1) gradients in stream surface profile, 



(2) gravel bed permeability, (3) gravel bed 

 depth, and (4) bed surface configuration. 



This theory was partially verified in the 

 field as follows: 



1. Interchange was traced by following in- 

 tragravel movement of dyed water through a 

 study riffle. Water was tagged with dye, and 

 its direction of flow mapped by appearance in 

 standpipes placed in the stream at various 

 locations and depths. 



2. Downward interchange was detected by 

 (1) placing a capsule filled with fluorescein 

 dye on the stream bottom and observing dye 



