Ask Hashinshin, he'll tell you all about how a 44% win rate champ is ruining league.
But srsly riven is weak af, champions like her are good at one thing, punishing you for wasting a CD or overextending. They have no real raw power, only the potential to outplay your mistakes. You would need to say what champion you're playing or at least what type for better specifics on matchups, but typically you dont want to give her an opening that she can get on you with her Q's while still having her W and E up to get away, without having a CD up to chase so she cant take the trade for free. Riven is only going to wittle you down then all in you when you're low enough that she knows her combo can kill you. Before she has 40% cdr her CD's are to long for extended fights.
Examples of how you fk up vs riven are if you were something like darius, and you try to poke with your Q while her CD's are up. You just wasted your biggest damage CD, and she can use her dashes to get in, win trade, and get out before your Q comes up again. So just be careful with your CD's and positioning around her, and you win.