The $2/$5 rock who calls your flop and turn bets and then raises the river is not bluffing. He made his hand on the river or he was slow-playing the whole way. Either way you're beat.

Why they do it

$2/$5 rocks who raise the river after calling two streets have a specific hand in mind. They were either setting you up or they just got there. There is no version of this where they're turning air into a bluff on the river after investing two streets. It doesn't happen.

The adjustment

Fold everything except the stone cold nuts. Don't talk yourself into a call because the pot is big. The pot being big is exactly why they waited until the river to raise.

What it's worth

Calling a $2/$5 rock's river raise with anything less than the nuts is a guaranteed losing play. One correct fold here per session saves you a significant chunk of your buy-in.