Currents come into play when creatures are moving or fighting in swift-flowing water (see “Aquatic Combat") or battling strong winds while flying.
A current drags creatures along its path. When a creature enters a current or starts its turn in one, it moves a distance and direction according to the current’s strength (as specified in an adventure or as determined by the DM) and in the direction it flows. This movement is a slide effect, with the distance and direction determined by the current.
A creature that wants to fight against a current can spend squares of movement to reduce the distance the current slides it. The creature can reduce the distance partially, or decrease it to 0, provided that the creature has enough movement to do so.
If a current slides a creature through another square that has a current, the creature ignores that other current. If a creature starts its turn in a square that contains more than one current, the strongest current applies. If the currents have the same strength, the creature chooses which one applies.
Published in Rules Compendium, page(s) 207.