the processes that operate to transport 

 free gaseous oxygen from the atmosphere 

 to intragravel water of salmon spawning 

 riffles. 



Transport Processes 



Stream-intragravel interchange. — The steps 

 involved in physical transport of free oxygen 

 to intragravel water are: 



1. Dissolution of oxygen through air- water 

 interface into stream water. 



2. Transport of oxygenated water to the 

 stream bottom. 



3. Interchange of oxygenated water from 

 the stream into the porous gravel interior. 



These steps are diagrammed in figure 2. 



Since this Is a series process, the rate at 

 any point will control the entire process. 



Rate of oxygen dissolution in standing water 

 is dependent upon temperature, surface area, 

 and difference in partial pressure of oxygen 

 dissolved in water and oxygen in the atmos- 

 phere. It is normally a slow process and may 

 be controlling. The dissolution of oxygen in 

 turbulent stream water, however, is a rapid 

 process compared with subsequent steps and 

 is normally not controlling. This is shown by 

 the near- saturation oxygen level in surface 

 water of unpolluted streams. 



Dissolved oxygen, present at the stream 

 surface, may be transported to the stream 

 bottom through diffusion or turbulent water 

 current. In the case of standing water, for in- 



stance a pool or pond, the water is motionless 

 or in laminar flow. Here the transport of dis- 

 solved oxygen is mostly by diffusion, and a 

 downward movement of oxygen is due to dif- 

 ferences in oxygen concentration between 

 highly oxygenated surface and poorly oxy- 

 genated bottom water. 



On the other hand, a stream or river of the 

 kind used by salmon for spawning will usually 

 be in turbulent flow (Russel, 1942), which is 

 characterized by continuous swirling, eddy 

 crosscurrents, and complete mixing. Conse- 

 quently, oxygenated surface water (saturated 

 with dissolved oxygen) is mechanically car- 

 ried to all depths of the stream (O'Connor and 

 Dobbins, 1956). Turbulent transport is a rapid 

 process and is not controlling. 



For oxygenated water to enter the stream- 

 bed a force must exist to induce flow across 

 the gravel boundary. Consider a stream flow- 

 ing over a smooth- surfaced gravel bed of 

 constant permeability and gradient. Turbulent 

 conditions do not exist at the thin water layer 

 adjacent to the stream bottom (McCabe and 

 Smith, 1956), and there is no reason to expect 

 interchange. For interchange to occur there 

 must be inherent factors in the surface water, 

 streambed surface, or streambed interior 

 affecting interchange. The factors that pos- 

 sibly control interchange include (1) stream 

 surface profile, (2) gravel permeability, (3) 

 gravel bed depth, and (4) irregularity of the 

 streambed surface. 



If the stream surface profile is not curved, 

 if the gravel bed is of constant permeability 



Air 



Stream 



\ o\ . \ Gravel a \ • 



Figure 2.- -Oxygen transport through stream to gravel bed. 



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