Runoff 



The low levels of development would have no water quality impact. July 

 would show concentrations greater than 700 mg/1 with the intermediate and 

 high levels of development, which could affect channel catfish migration 

 and spawning. The end of the shovelnose sturgeon and paddlefish runs could 

 be affected, as could egg incubation. 



OTHER LIMITING FACTORS 



Thus far, discussion of impacts of future water withdrawals on the 

 aquatic biota of the Tongue River has been limited to the impacts of low 

 flows. Low flows result in decrease in depth, loss of physical habitat, 

 reduction in velocity, and deterioration of water quality, all of which 

 render the habitat less appealing. There are other factors associated with 

 the lotic coimunity which must be considered. 



Temperature 



As water levels are lowered, the heat budget of the stream is altered, 

 and temperatures increase until an equilibrium (between standing water and 

 the ambient air temperature) is reached. Temperature is a major limiting 

 factor because aquatic organisms often have narrow tolerances. The life 

 cycles of fish and invertebrates, day length, and water temperature are so 

 interdependent that even a small change in temperature can have far- 

 reaching effects. In an artifically warmed stream, insects or fish may 

 hatch too soon to fit into the food chain and successfully compete. If 

 temperatures are near the upper limits of tolerance, the fish will be placed 

 under stress if not killed. Similar competitive disadvantages would result 

 with a cooling of water. 



Turbidity 



Suspended materials limit the penetration of light, restricting 

 photosynthetic activity. Sediment fills the interstices between gravel, 

 thereby eliminating spawning areas and aquatic insect habitat. The ability of 

 a stream to move this material and hence cleanse the stream bottom depends on 

 the discharge, particularly during the spring runoff. Low flows reduce 

 bedload movement and thereby limit fish production. 



Concentration of Respiratory Gases 



Oxygen and carbon dioxide concentrations are often limiting to the 

 aquatic coimunity. As flows are reduced and temperatures increased, concen- 

 trations of oxygen become critical. Eggs are especially vulnerable to 

 oxygen lack because they depend upon oxygen diffusing into them at a rate 

 sufficient to maintain the developing embryos. 



72 



