I'll have to second the advice Os just gave. It's the only good way to get dialed in on the smaller rivers. Find something similar, or something in the same drainage, and then make an educated guess based on your previous observations. It's not an exact science, but you'll get to the point of knowing whether your smaller rivers will be fishable.

Fish on.......
_________________________
She was standin' alone over by the juke box, like she'd something to sell.
I said "baby, what's the goin' price?" She told me to go to hell.

Bon Scott - Shot Down in Flames