The FBI, and the Fazio's claim the sand was only spread 50 yards north and south of the spoil (fact)
It's a fact they stated this claim, but the claim is their opinion, if you get my meaning. Could they be wrong? I think they could, but that's splitting hairs, maybe.
To me, where the plane crossed the river isn't that important. The river is a vehicle that can transport money bags, bodies, and even much larger objects many, many miles. Whether the plane crossed 100 yards, or 7 miles upstream doesn't affect the end result: part of the ransom money was found in the bank of the river the plane crossed some distance upstream.
Timing of the jump is troubling for the money ending up in the river directly from the plane. But, again, that timing is mostly based on the opinions of human beings, and should be weighted as such.
I think talking these things through is extremely helpful in understanding this case. There are so many variables, and so few facts to go on.