Now all "we" need is measurements of current, but I can't help but think that if the barrel was buried in the bank, on the area marked as "Dow Chemical," the barrel (even if exposed again) might have gone unremarked as belonging to Dow. If the barrel broke and the remains fell out, the barrel may have travelled downstream. Not knowing for certain what the next part discovered is, and being uncertain where it was found (especially if it were upstream), I can't speculate more. I'm not familiar with that stretch of the Des Plaines at all. I really think that a scientist familiar with the river could work out where/when things happened using weather and river measurements.