Go With The Flow is one of the hidden treasures in Far Cry: New Dawn. The exact location of the mission is presented in the picture below. This location is hard to find. You can learn about this stash in Prosperity, in one of the captured outposts, or while you are exploring the world.
The note starting this mission is in a mountain cave. Watch out for the wolves roaming this area. Slide down the rope to the opposite shore of the river. You will find the key to the stash.
To find the stash, follow the white flags. They are placed along the river. On your way, you will encounter a bear and a small group of bandits. You shouldn't have problems with defeating them.
The stash is in the cavity right behind the waterfall. Avoid receiving fall damage by using the grappling hook on the nearby rock. The stash contains resources, ammo and perk points.