The river, as luck would have it, hides no sheer drops that would send the tortle under. Kellon is able to wade out into the water, though it soon reaches the point where swimming would be faster if he intends to catch up with the elf before she is swept away - or lost in the gossamer shroud.

Spoiler: Rolls
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Kellon may make an Athletics check. The river is not flowing terribly fast, but this will determine whether they are overtaken by the Mist before he can get back to shore.
Along with the Athletics check, he can roll a contingent Constitution save against the Mist's effects, should the Athletics check fail.