Riven vs Volibear Top Matchup
Riven Top wins 44.8% of 864 Low Elo games against Volibear Top, -1.6 points vs expected, 2-patch window 26.19-26.20.
Researched on LoL patch 26.20. Tips are written to hold at any rank; stats come from Low Elo ranked games up to patch 26.20.
How to play as Riven
Keep trades short after level 3 and avoid letting Volibear land a second Frenzied Maul(W); the repeat cast heals him, so extended trades favor his sustain.
Dodge Sky Splitter(E) or shield its damage with Valor(E), then wait for Volibear's shield to end before re-engaging; do not take a long trade while it is active.
After Volibear reaches level 6, avoid forcing a solo all-in; farm safely and look for a gank or a team fight instead.
How to play as Volibear
Sky Splitter(E) grants Volibear a shield if he is inside its blast radius, which can blunt Riven's damage when she trades into the strike.
Sources
These notes come from r/Rivenmains, checked against the champions' abilities.