For those not familiar with GRRM's novels but are familiar with the show, Riverrun is the House Seat of the Tully family. So far we've met Catelyn Stark (Née Tully) and her breast feeding sister Lysa Arryn (Née Tully).
Bran notes their sigil as a fish during his infodump genealogy lesson in Season One and the Tully House words: Family, Duty, Honor.
So far HBO has announced casting for several long awaited members of the family, including Clive Russell as Cat's wayward uncle Brynden and Tobias Menzies as the youngest Tully, Edmure. No announcement yet as to who will play patriarch Hoster.