Once you learn her, Riven wins every lane. I'm not kidding. I've fucked up 1900-GRUNKER Yoricks, 1650-GRUNKER Rumbles, 1700-GRUNKER Udyrs, 1700-GRUNKER Olafs and 1500-GRUNKER Kennens without breaking a sweat with her. The only lane Riven doesn't win is one that gets camped by enemy jungle and mid simultaneously while her own team neither comes up to help nor pushes the rest of the map while Riven stalemates a 3v1.
I'm wondering wether it is incidental or not, that the Kennens are the lowest elo of them all. 'Cause I've got no clue how they lose that.
Also, Olaf.
While certainly trickier than, say, vlad, irelia, warwick or gangplank, kennen is quite managable.
Start Doran's blade. Level EQWWWR.
Play passive from level 1-3. Use shield to go in to last hit creeps. Try to bait him into harrassing you when you do so. (Most kennens will at least try to toss a shuriken at you). Your creeps will attack kennen while his creeps attack your creeps, pushing the lane ever so slowly towards your turret. Because of her passive, Riven excels at turret farming. With a little practice you should be able to farm under turret from level 2 onwards without missing any CS. Farm as close to turret as possible at all times and he'll have to be defensive lest your jungle shows up. Starting at level 3, you can counter-harrass when he makes a mistake.
Learn the energy cost and cooldown of his cocaine ball. If he's at less than 50 energy, he can't cocaine ball away and you will outdamage him in a trade. The cooldown is also fairly long. If you can bait his cocaine ball with Valor, your gapcloser will be up again before his and you will be able either to harrass him or force him out of farm range. Try to get him to 70% health or less by level 6. As soon as he falls to ~40% energy, he won't be able to cocaine ball or ultimate. Hit R, flash to him, Stun +ignite, do your Q/aa combo, using valor to gapclose i he flashes and finish off with wind slash. I have about a 75% kill ratio on kennen with that combo. At the very least you will force him to burn summoners and leave lane.
If you have a halfway decent jungler, try to set up a gank around level 3. Most kennens will go QWQE, especially if you're playing passive, so he won't have cocaine ball. If you can get kennen to burn his flash then, he will be defenseless once you go for him at level 6.
In a straight 1v1 fight, Riven will outdamage Kennen. If you can get even slightly ahead, the lane is winnable.