Since Tom encouraged all of us to put forth theories that could account for his findings, I’ve decided to share mine. Unlike some others here, I welcome correction and criticism. Please fact check me to death.
Cooper ends up in the water of the Columbia. The flight path - similar to the placard find - is slightly west (Perhaps 2 or 2.5 miles west of the FBI flight path all the way down), so he lands somewhere in the vincinity of the I-5 bridge/Tomahawk Island area) The money is still tightly packed into the bag and secured to Cooper’s body. Overnight, in the cover of darkness it floats upstream toward Caterpillar Island and Tena Bar.
Due to the tightly packed bag and lack of full submersion in the river, the number of large diatoms is few and only on the bills nearest the inside of the bag. At some point in its journey, it washes up on the riverside, obscured by snow and other debris where it remains until the the winter thaw raises the water level in May.
After six months the body, the bag, and the strings that hold it all together have deteriorated to a point where packets of money are now falling out. As May turns into June, and the flood waters rise to historic levels., the rest of the money disappears downstream, but a few packets ($5800 worth) wash up on the banks of Tena Bar. While mostly protected from the winter diatoms by the bag, the turbidity of the water and the interaction with the sand remove what few diatoms remain from the colder months. Now, as they float fanned out in the early summer flood water of 1972, they acquire new summer diatoms. The flood waters recede in July and a shallow layer of silt covers the money, and even more sand will cover the money in August of 1974 when dredge material will be pushed over it.
TL;DR: Cooper splashes into the Columbia at or just slightly to the west of where the FBI flight path shows. The tightly packed bag, floating partly out of the water on the body of DBC, protect the cash from acquiring winter diatoms. It spends the winter months washed up on a remote riverbank somewhere between Tena Bar and Tomahawk Island, until the flood waters deliver it to Tena Bar, opening the decayed bag and exposing the found bundles to summer diatoms while simultaneously ridding it of the few winter diatoms that remained.
I’m sure there are flaws with this theory which I am more than happy to have pointed out.