Hahaha we're the opposite. I don't think Julian and River is sudden. In fact, while reading I felt that the parts with Noah (S1) we're fast-paced, whereas the development with Julian (S2) gave more time for feelings to actually spark.
Julian and River def makes more sense, but somehow I'm still with Noah. I'm still not sure, but I don't get the impression that River loves Julian romantically.
I have a feeling she misunderstood noah and beth's relationship again and was all "welp, another sign it's about time i completely move on"
But if ever I'm wrong and her feelings for julian actually became romantic then she ends up with noah in the end (if, somehow), I just hope the author doesn't ruin julian's character. He's been so good so far

I DON'T CARE IF EVERYTHING'S TOO SUDDEN! WHAT'S IMPORTANT HERE IS THAT, THE ENDGAME SHOULD NOT BE THAT TRASHY GUY NOAH