Okay, these are ranked in order from 
warmest to coldest (in terms of air 
temp, since water can only get so 
cold).  1) Lower Salmon, near 
Riggins.  Great play, easy III.
    2)Spokane, play and short urban 
class III run.
    3) Kootenai.  Good play, super fun 
short class IV run.  Near Libby, MT
    4) Clark Fork.  Good play, fun little 
II+ run.  In between Missoula and 
CDA.
  Check out the e-guide for details.  
Enjoy.