Your post jogged my memory and got me to thinking.............
After Stacy disappeared, Drew's scuba-diving weights were missing from the garage.
Logic tells me that if you're going to dispose of a body, leaving it in a container would keep the body intact. The body would still decompose, but everything would be contained in the container - all the bones, which could lead to identification if found.
The blue barrel may only have been a means of transport. If Stacy was dumped in a river, Drew would be more likely to remove her body from the barrel, tie the weights around her feet, hands, and neck, so the body would sink to the bottom. Drew would leave it to the forces of nature and fish life to dispose of the remains and the bones would remain on the bottom of the river, weighed down by the weights.
But, along with the forces of nature comes heavy rain and flooding. Rivers become raging torrents when there's floods, rising over their banks. A swift current would tug on the remains, now reduced to mostly bones. In the process, the current would pull bones apart, leaving those weighed down by the scuba weights on the river bottom, and carrying the rest of the bones away with the current.
If the remains are Stacys..........this would explain why the remains have no arms, head, or the lower portion of the legs.
Sorry for the visual................