I play Riven quite a bit, if you want to counter her you need to know what skills she has and how she plays.
Her e gives her a shield, generally this is when she will go in for a creep. Don't hit her while she's got this up, wait for it to go down then harras. She also has 2 stuns, her q is an aoe knock up and w is an aoe stun. Both are .75s. Be careful.
As far as champions, you want someone with range and good sustain. Any good Riven will eat a half life opponent. Also, if you're going solo top I advise getting yellow armor rune page. Its invaluable against tops. I suggest Nidalee/Ahri/Vladamir.
If you simply must use a melee character top, I'd go with Shen/Lanewick/Tryndamere.