Raging River
Enchantment
Whenever one or more creatures you control attack, each defending player divides all creatures without [[ keyword: "flying" ]] they control into a "left" pile and a "right" pile. Then, for each attacking creature you control, choose "left" or "right." That creature can't be blocked this combat except by creatures with flying and creatures in a pile with the chosen label.
Sandra Everingham
Rulings
If a creature is put onto the battlefield attacking after the ability resolves, it can be blocked by any creature that could normally block it (including creatures that entered after the ability resolved).
(wotc • 2008-05-01)
If a creature enters (or something becomes a creature) after the ability resolves, that creature will not be able to block any creature that was attacking at the time the ability resolved.
(wotc • 2008-05-01)
Rulings
If a creature is put onto the battlefield attacking after the ability resolves, it can be blocked by any creature that could normally block it (including creatures that entered after the ability resolved).
(wotc • 2008-05-01)
If a creature enters (or something becomes a creature) after the ability resolves, that creature will not be able to block any creature that was attacking at the time the ability resolved.
(wotc • 2008-05-01)