Creatures in the log don’t have to attack, but nonattacking creatures don’t affect how the attacking logmates can be blocked. Notably, not attacking with one creature in the log doesn’t make the others automatically unblockable.

The defending player(s) don’t have to block any of the attacking creatures in the log. They just can’t block any of them unless they block all of them.

If the creatures in the log are attacking multiple defending players, those players declare their blocks in turn order. If any of the defending players declines to block a creature in a log attacking them or a permanent they control, other defending players can’t block attacking creatures in that log attacking in their direction.

If a defending player blocks the creatures in a log attacking in their direction, other defending players aren’t obligated to do the same. If they don’t, however, the first defending player’s blocks are illegal, and the game rewinds and blockers are declared again. Defending players are encouraged to come up with a plan for incoming logs before this happens.

Log Flume doesn’t copy any spells or abilities that target a creature in a log. Those spells and abilities just target each creature in the log.

If a spell or ability targets multiple creatures in the log, it targets each creature in the log for each instance of “target” that would otherwise target only a single creature in the log.