Table I. --Relation of peak flow to average annual flow for some Idaho rivers with good 



or excellent stream records 





Average : 













annual : 



Peak : 



Elevation : 



Length : 



Watershed 



River : 



flow : 



flow^ : 



of gage : 



of record : 



area 





^C.f.s.m. 



^C.f.s.m. 



^Feet 



leases 



Sq.mi. 



Cub 



4. 24 



36. 86 



5,320 



21 



19.4 



Boundary 



1.98 



33. 81 



1,770 



34 



97 



Mission 



1.71 



22.96 



2,800 



6 



23 



Clearwater (Kamiah) 



1.68 



21.24 



1,162 



54 



4,850 



Moyie (Eastport) 



1.23 



18.60 



2,620 



35 



570 



Yaak 



1.20 



15.80 



1,850 



8 



766 



Moyie (Eileen) 



1.16 



14.57 



2,124 



39 



755 



Big Lost River (Wild Horse) 



.86 



11.14 



6,820 



20 



114 



Robie 



.50 



10.32 



4,960 



14 



15.8 



Thomas Fork 



.44 



7.69 



6,280 



15 



113 



Bannock 



.36 



5.91 



5,240 



16 



5.75 



^Peak flow for period of record. 

 ^Cubic feet per second per square mile. 

 ^Above mean sea level. 



Peak flows which can be expected in Idaho and Montana, in watersheds west of the 

 Continental Divide, are listed in tables 2 and 3 for four different return periods. The 

 selected return periods are: 2.33 years (usually termed the mean annual return period); 

 5; 10; and 20 years. In these tables, peak flow is a function of average water yield 

 and an expected return period, and is expressed in units of cubic feet per second per 

 square mile (c.f.s.m.). In table 2, average water yield (the independent variable) 

 increases by selected increments in inches, and in table 3 by selected increments in 

 c.f.s.m. These tables were constructed in accordance with the methods described in 

 Appendix 1, and are based on sources of data listed in Appendix 2. 



2 



