An Explanation for the Tena Bar Money
Once he landed, this theory assumes he survived, Cooper faced some problems apart from, most likely, trying to make his way back to Portland or Vancouver.
He can't walk into either city with a large money bag and a parachute, clearly. That would be remembered by eyewitnesses. He must ditch, if only temporarily, both items in a more rural area.
The drop zone is rural, but the ground underneath him was hard and he did not have a full-sized shovel, though he could have placed a smaller one inside the mystery bag. With law enforcement potentially searching for him in the drop zone, he needed to leave it as quickly as possible, never come back and be nowhere nearby given search parties, not knowing exactly where he landed, might look for him in adjoining areas. He needs to put a lot of distance between him and the drop zone and the sooner the better. Anyhow, he has a long hike without sleep and with little food ahead of him and he is a smoker in his 40s. He might also feel like he can not waste energy, not just time, on digging into hard ground with poor or no equipment.
So, Cooper leaves the drop zone with both the money bag and the parachute (given that was never found in the drop zone and would be hard to miss), with him having hastily packed the parachute again. Cooper knows that sand near the Columbia river will be soft and perfect for digging quickly and easily. It is also rural and isolated. He also knows that he can dump the parachute into the river, faking his own death for investigators, and that he can simply follow the river to Vancouver and then Portland, with no need to navigate or wander in the dark.
Cooper has no fixed, preplanned place to head to on the riverbank and ends up at Tena Bar by simply walking westwards, using a compass inside the mystery bag, the stars or finding some signs. Cooper can't go eastwards or northwards, because he has to go to Portland or Vancouver and, plausibly, grab his car.
Not knowing exactly where he headed, being in a hurry and without sleep and it being the middle of the night, he buries the money at a beach that seems deserted during the night but, as we know, was near a farm. Like he mistakenly left his tie inside the airplane. He throws the parachute in the river, later to apparently be spotted by the river by some eyewitnesses before it was lost.
Cooper heads to Portland-Vancouver by simply following the river, gets his car and the sun rises. He realizes he could be spotted digging again or carrying the bag, but he has no choice but getting it, banking on it being early Thanksgiving morning and the place still seeming mostly deserted. If he comes back at some later point, the money could be destroyed, being near water, or have been found or whatever rock he used to mark the spot could have been moved.
So, he hastily grabs the money and heads to his car. Nervous of being spotted and in a hurry, he misses $6,000 falling out, or perhaps it had fallen out in the darkness earlier and he fills the hole again. The money was then buried there for years until being found.