There is no way to date this. It assumes that Cooper was a no-pull and died in the jump.
The Western Flight Path (or whatever you want to call it) to bypass Portland on the west side would have passed almost directly over Tena Bar. Specifically, the WFP was almost straight down the center of the Columbia River for several miles.
Due to the winds aloft being from the southwest at all altitudes up to 10,000 feet and of about 30 knots velocity, Cooper would have travelled about 1000 feet to the northeast during a free fall which was between about 40 to 60 seconds.
And he would have landed on solid ground but very close to the river's eastern side. He would have also had to land west of the Northwest Lower River Road (which is built on top of a levee) in order to enter the Columbia in the area of Tena Bar.
FWIW, the late Dr. Meyer Louie and I went through the dairy runoff settling pond (which is just over the bank from Tena Bar) about 10 years ago with metal detectors and didn't find a single thing, The detector's manufacturer indicated that they should be able to find anything up to two or three feet beneath the sand under the conditions we experienced.
Additionally, local fishermen who were longtime visitors to Tena Bar told us that they had never seen the river high enough to flood the settling pond or anything from the settling pond passing into the river.
All of this was discussed here in much more detail 10 to 15 years ago.
To respond here to FlyJack's post of a few minutes ago, the jump time of 8:11 PM PST has been the accepted jump time from day one which was 1971. This is not a new revelation.
Further, I reject the FBI's proposed flight path out of hand. No airline crew would fly like that under the hijacking circumstances.
Also, the FBI's FOIA unit has gone out of their way to ensure that the actual transcripts of the communications between the airliner and the Seattle ATC are not released. And those transcripts are in the public domain.