be determined and through such physical 

 studies, the dissolved oxygen supply to intra- 

 gravel strata may be quantitatively predicted. 

 Qualitatively, intragravel dissolved oxygen 

 levels will be increased by interchange and 

 will be lowered by ground-water dilution and 

 biochemical oxygen demand (Hobbs, 1937). 



FIELD VERIFICATION OF 



THE SLOPE -INTERCHANGE 



MECHANISM 



The occurrence of interchange has been 

 previously reported (Cooper, 1959). In the 

 field of intragravel flow, investigators 

 have described the conditions under which 

 interchange occurs; however, the mechanism 

 of water flow from stream to gravel has not 

 been defined. 



From ground-water and intragravel flow 

 studies in Indian Creek in 1957 and 1958, 

 techniques for qualitatively detecting inter- 

 change flow were developed. After the theory 

 of interchange was proposed, work was started 

 to verify the proposed slope-interchange 

 mechanism. 



Experimental Apparatus and 

 Procedure 



The technique of tracing interchange was to 

 trace the intragravel flow of dyed water 

 through a study area. Water was tagged with 

 dye, and its flow mapped by appearance in 

 standpipes placed in the stream at various 

 locations and depths. The standpipes used 

 are described by McNeil (1962). 



Downdrafts were detected by (1) placing a 

 capsule filled with fluorescein dye on the 

 stream bottom and observing the movement 

 of dye into the streambed, and (2) Introducing 

 dye through a standpipe 6 inches below the 

 gravel surface and observing its movement 

 to greater depths in adjacent standpipes. Up- 

 welling was traced by introducing dye 18 

 inches below t'le gravel surface and observ- 

 ing its movement to adjacent pipes nearer the 

 gravel surface and to the gravel surface. For 

 each location where interchange was observed, 

 shape of the stream surface was determined 

 with a transit and stadia rod. 



Results 



Observations were made in several convex 

 and concave riffles of Indian Creek, In most 

 cases, to provide a point of zero intragravel 

 velocity, a pool bounded one end of each study 

 section. Observed direction of interchange in 

 concave riffles was invariably an upwelling 

 of intragravel water. In convex sections, that 

 is, where the stream gradient increased in 

 the direction of flow, interchange was from 

 stream to gravel (downdraft). 



In tracing intragravel flow, it was observed 

 that upwelling occurred in certain sections 

 having constant gradient. Upon examining con- 

 ditions surrounding these points of upwelling, 

 it was noted that small irregularities in the 

 stream bottom created waves on the water 

 surface. Points of upwelling were directly 

 below troughs of waves. 



Influence of waves upon interchange was 

 investigated in more detail by tracing the 

 direction of interchange beneath large waves 

 created by placing large rocks on the stream- 

 bed. Results showed that upwelling occurred 

 beneath the troughs of waves and downdraft- 

 ing occurred beneath wave crests. 



While change in slope over a riffle provides 

 uniformity in the direction of interchange over 

 a large area, waves may control the direction 

 of interchange at a point. As part of the Indian 

 Creek study, wave configuration was changed 

 by moving rocks on the stream bottom. By 

 changing positions of large rocks, a point 

 initially below a wave crest would lie under 

 a wave trough. Changing stream surface pro- 

 file in this manner caused a reversal in the 

 direction of interchange. 



DISCUSSION 



Observed dependence of interchange on 

 stream profile was in accordance with the 

 proposed profile-interchange relationship; 

 where the stream gradient was convex, inter- 

 change was downward; where the stream pro- 

 file was concave, upwelling occurred. 



Change in stream gradient over a long 

 distance, for instance 10 feet, provides a 

 unidirectional interchange over a large area. 

 However, the point interchange driving force, 

 inherent in waves, induces a comparable total 



