ranged harass > no ranged harass
ranged CC > no ranged CC
sustain > no sustain
Riven beats Ryze if she can survive long enough that he runs out of mana, which he does early game.
Max shield and she should be able to buy enough MR and sustain that it'll end up being a farmfest.
This is also disregarding Jungle ganks, which Riven has a significantly easier time escaping.
© 2015 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.