I've had trouble tracking down any media reports from the time of Franklin's disappearance, so I can't find any precise location of where the tarp was found.
After some confusion with google maps, which kept taking me to SoCal and Bob's home ground strangely enough, I found the Walnut River which is a tributary of the Arkansas River. From this, it seems the tarp was found about an hour and a half from Franklin's home.
Call me cynical if you like, but I'm not totally convinced the perps were truthful about where his remains were buried, and I can't work out why the tarp would have been laying on the ground but none of his remains visible at all. Why would the perps have removed his body from the tarp and left it lying there? Would wildlife really have been able to remove every bit of Franklin's body?
I did manage to find a previous murder victim discarded at the Walnut River. From 1893. However, it is useful in that it mentions the remains being found entangled in the roots of a huge tree on the riverbank. Depending on where he may have been buried, I'm thinking floods may have disinterred Franklin's remains and washed them downriver, and that possibly there is a coroner's office with just an unidentified bone or two somewhere?
I'm not ruling out the perps discarded the tarp and Franklin in different locations however, and lied about where he is. Especially if his remains would have revealed a different tale from the one defence presented, about his death.