Going back a few pages:
I think we need to think this through the eyes of Cooper.
He completes a daring skydive from 10000 feet, belted by freezing winds and pelted in the face by freezing rain. He plummets through the darkness before pulling the cord and get roughly jerked back into the air before miraculously floating safely onto the beach at Tena Bar.
He’s not standing on the beach at Tena Bar, filled with adrenaline and shivering from the cold. He has no idea where he is. Somewhere along the Columbia he guesses. But before or after the turn north? He has no clue.
Jacked with adrenaline, fear, elation, and panic, he decides to bury the money just beneath the sandy surface right where he landed. When he’s done, he flees into the rural darkness with his business suit soaked through and his wet loafers giving him unbearable blisters.
Then, six months later, he somehow learns - probably from local papers - that the Columbia is flooding. Again, he is filled with fear and panic because he foolishly buried his hard earned loot mere feet from the river’s edge. He left it there all winter praying that no one would discover it.
He hurries to where he think he buried it. It was over six months ago in the dark so backtracking is nearly impossible, but he does it. In the cover of darkness, he sloshes through calf-deep water and by a miracle locates the exact spot he buried. He digs under the water through water-logged sand and brings up bundle after bundle. It’s dark and he has no time to count - despite the fact that he risked life and limb to get the money. What does a few thousand matter after all?
He escapes with the loot, leaving nearly $6000 behind to lay there undisturbed for 8 years.
I can only speak for myself, but he idea of Cooper surviving the jump, landing in an unknown location in the dark, burying money just yards from the water, not returning until 6 months later, finding the exact spot under feet of water, probably under the cover of darkness, retrieving the cash, but not being careful enough to account for all of it, seems outlandish.