development were included in the projections. These projected depletions, 

 and the types of development projected, provide a basis for determining the 

 level of impact that would occur if these levels of development were carried 

 through. 



IMPACTS OF REDUCED STREAMFLOW 



Agricultural Projection 



Mean flow values for each projected level of development are compared with 

 recommended instream flows and mean monthly natural flows in figure 28. 

 Possible impacts associated with each development level are presented in 

 table 20. Frequencies of low flow are relative to the 30-year period of flow 

 records. 



Fall . August flows for the low level of agricultural development would 

 fall critically short. Flows necessary to ensure adequate channel catfish 

 spawning during August are 6.4 m3/sec (225 cfs); only one year in 30 showed 

 enough water to maintain this flow. Low flows to 1 m^/sec (36 cfs), which 

 would drastically reduce wetted perimeter, lower velocity, and reduce depth, 

 would result nearly 90 percent of the time (26 of 30 years). 



Flows resulting from the intermediate level of development would fall 

 short of channel catfish requirements all years. One year shows almost enough 

 water but is still short. Flows associated with the high development level 

 fall short all years and would decimate channel catfish spawning in this 

 reach of the Tongue River. 



September flows would be similar to those for August. With the low 

 development level, flows would be adequate to meet rearing requirements only 

 one year out of six. Intermediate and high development level flows would be 

 inadequate in all years. 



Low-development-level flows for October and November would be adequate 

 for rearing one year out of four and one year out of tv;o, respectively. 

 Intermediate and high levels of development, however, would result in depleted 

 flows at least nine years out of ten. 



In summary, agricultural development would have its greatest impact on 

 the fishery of the Tongue River during the fall months, since August and 

 September are months of normal low flows and high irrigation use. 



Winter . The low level of development would result in low flows one 

 year out of two for December and January and one year out of 1.4 for February. 

 The impacts would probably be minimal, however. 



As with the fall period, both the intermediate and high levels of 

 development would be expected to result in severe impacts for December and 

 January. At least 85 percent of the time, rearing flows would not be main- 

 tained for these months. The intermediate projection would provide adequate 

 flows one year out of 4.3 in February, but the high projection would have 

 a severe impact in that month. 



67 



