Riven Guide

Tips for Playing Against Riven

-I put his matchup as "hard", and it is, but I do think it is on Tahm Kench to win.
-Play very safe early, going as far as to try not let her get a single Q on you. Throughout the game, don't be in too gankable a spot, as she can interrupt your W easily.
-If she's on your side of the minions with close to no cooldowns, you can Q + Grasp, but be careful not to take an extended trade before first back.
-If you survived early, and didn't get killed in two combos, (She stacks Conq really fast!) then you should start to turn the tides at Giant's Belt, as she doesn't do %HP damage, aside from her ult (which is just a % missing health execute).
-Try to eat her during her Q3, as you'll avoid getting knocked back.
-Careful when she gets her Eclipse, as she will chunk your max health, turning the tides back in her favor if you're not super ahead of her.

Heartsteel

Heartsteel is amazing against Riven. She lacks %HP damage, and it provides tongue range and speed to chase, escape, and poke her easier. It also pops her really easily, as the flat damage from Tahm's passive and Heartsteel's passive is very nice against squishies.
Iceborn Gauntlet

Iceborn Gauntlet and Dead Man's Plate are nice choices against Riven. I prefer Heartsteel when I'm ahead, especially if their team has nice squishy targets for me, but these provide armor, escapability, and chase.
Doran's Ring

Haven't tried this myself, but I imagine D-Ring would allow you to just sit back and max range minions early without running out of mana. Might be worth a try! Just be extra careful not to take any damage from her in this case. Also, as a last note, Swifties are great as always, but I find them extra nice against Riven!