it really depends on the gameplay. The reason why many people think riven is countered by garen is his strong lanesustain, his execute and his Silence.
His sustain allows him to swallow the riven damage and just rest for a Little moment to heal up, and such is even worse if you run grasp of the undying like a good garen should.
Rivens mobility and skills are heavily, and i mean HEAVILY dependant on her skills. screw what i just said, her skills are the entire mobility she has. and a silence pretty much destroys that. including a stun a knock up and a shield. if riven all Outs garen, she Needs to go melee, so he just E shreds her armor first while having his W up to lower her damage, even if she fully uses the maximum of timelimits for her Q ticks she loses a lot of damage, and THEN silences her when shes already low, lands one AA and then ults her.
The Problem in all of that is that garen denies her the earlygame snowball she Needs. sure, a riven can devastate a garen, ive seen it myself pretty damn often, but when it Comes to laning phase garen simply has so many Advantages that he might aswell count as a Counter.