River Monsters

River Monsters is a main quest in the Fallout: New Vegas add-on, Honest Hearts.

Detailed walkthrough
Head to Pine Creek. From here you can either enter the nearby cave to set explosives and close off the entrance, or kill the yao guai outside the cave. The cave option does not require combat. Completion of either option ends the quest successfully, and the particular method chosen has no effect on the rest of the game.

If opting to collapse the cave entrance, enter the cave. From here, explore the cave, looking for each of three marked positions on the floor where you must place the explosives. Note that the "mining explosives" are not "items" carried by the player; they are a part of the cave and only need to be enabled. They are visually identical to C-4 plastic explosive.

There are three yao guai in a central area of the cave, but killing them is not a requirement for this quest as they can be avoided while "placing the explosives." When all three explosives are set, exit the cave and trigger the mining detonator, sealing the cave and completing the quest.

If you opt to kill the yao guai outside the cave, simply follow the map markers to the targets and dispatch them. Once all of the target yao guai are killed, the quest is complete.