The river can be a major threshold between two supernatural communities. maybe it exists in the Nevernever as well. Maybe it's the only thing preventing a full-out, large-scale attack because it's only crossable at certain(well-defended) points. You could have sentinels all along the river...maybe they're 'doughnut shops' or maybe they're 'gas stations' or toll bridges but in the Nevernever, they're small fortresses or watch-towers.
Caves could have dragons, ghouls, black court vampires. It could be a hideout for a sorcerer/necromancer.