Dumping someone in a river will decompose and spread the body quickly, but it will surface in a couple of days, before it has time to decompose and be consumed. They're more buoyant than most people think. They'll float cinderblocks, chains and small anchors. Encasing them preserves them in one place, though. Fortunately, I've never had to hide a body, so I haven't thought too much about how I'd do it.
Louisiana swamps, chainsaw, alligators..
Just saying...