Alrighty, allow me to dip in a bit. Incoming wall of text.
Riven is a combo fighter, as in, she needs both her abilities and her auto attacks to be effective. If the Riven is worth her salt, she will, as Mira said, be weaving her autos in between each ability to absolutely maximize her total damage output, as the damage from her passive is most of the damage you will be taking. As such, whenever possible, robbing her of her passive will greatly reduce her damage output and there are a few ways of doing that.
First off, her passive is a auto attack buff that consumes a charge to deal damage bonus damage. Her passive can store up to three stacks. Since her passive is based on her auto attacks, anything that can block, dodge or parry her autos will cause the charge to consume itself and do no damage. The other option is to blind her. This typically lasts longer than blocking, dodging or parrying, but will NOT consume her passive stacks. Once the blind is over, she will be able to hit you with her passive again like nothing happened. Blinding her, however, will give you a broad window of reduced damage to drop hell on Riven.
Second, her passive triggers only on ability casts. That said, simply stopping her from casting abilities in the first place is a greatly effective way of countering Riven's play-style. This is true for a few reasons actually. It makes her passive nearly nonexistent, it cuts out the damage from her other abilities, and all of the utility from CC to mobility goes by the wayside. The most obvious way of robbing Riven of her abilities is to silence her. This has been the reason Garen is so widely accepted as a counter for Riven.
But one thing that people don't seem to realize is Riven's greatest strength is also her greatest weakness. Her dashes. The very thing that annoys people the most about Riven is also the very thing you can exploit. Two of Riven's abilities are dashes, very short ones, but none the less dashes. Whats more, is one of those two has three casts to it, as it is the core ability to her kit. What makes this important is there are a lot of champs that have ways of punishing her dashes by the way of blocking dashes from occurring in the first place. Champions like Poppy and Cassio have abilities that block dashes, thus cutting a lot of Riven's kit out of the equation. Other characters, like Morg, Lux, Varus, Zyra and the new Xayah to name a few, can snare Riven. To Riven, snares are a pseudo silence. Where Poppy can stop her dashes from going very far, snares stop Riven from being able to cast her Q or E at all. From here, Riven's only two options are to use W and to pop ult, and so unless you are standing right next to her, you find that she is quite harmless. Even as a melee champ, you might take a W and empowered auto to the face, but unless she is oppressively fed, this won't do much damage at all on its own, so feel free to dump all kinds of damage on her, since she really can't fight back, run away or even defend herself at this point.
Also, please don't take slows against Riven, if you can help it. Riven, if given access to her dashes, will simply power through the slow.
Hope this helps!
-The fighting game fanatic Riven main
TL:DR; Blind, Silence or Snare Riven to remove the threat of her passive from the equation.
