So the bass in Ririe don't get large because it is too cold? So why do they get big in the Snake River--it is also cold ?

Answer:

Cold coupled with elevation.  Indeed it is cold in the Snake River, but it’s also nearly 1,000 feet lower in elevation (assuming you are referring to the American Falls area, where bass fishing is good).  As such, it has a longer growing season than Ririe does.  There are also springs that dump in and buffer water temps.