I've linked before to Christina Fredengren's ground-breaking paper in Fornvännen 2015:3 about human and animal remains found in wet contexts in Uppland province (the area around Uppsala). The study's empirical base is solid and eye-opening. I don't find find the theoretical superstructure that the author briefly sketches onto it (the titular ”water politics”) convincing. But that's not my main complaint about this otherwise excellent piece of research.
Look at the map above, covering a small part of the study area. Bear in mind that due to the relieved pressure of the inland ice, land rises and the shoreline recedes quickly here. The finds from the site and environs of the town of Uppsala map the current channel of River Fyris. Out in open water, during periods when the river mouth was hundreds or thousands of meters upstream of these find locations. Did people really go out in boats and sink remains of humans and animals exactly along a stretch of river that didn't exist yet? No, they couldn't do that. I believe there are two other main possible explanations for this distribution.
1. The reconstructed shorelines on the map are not coeval with the depositions' radiocarbon values. Quaternary geologists have grossly misled Fredengren about the shoreline displacement's timing. (Not likely in my opinion.)
2. The shorelines on the map are coeval with the deposited bones, but these have been flushed way, way downriver in recent centuries before getting picked up, and their find spots are thus not where they were originally deposited.
In either of these cases, Fredengren's discussion of the Uppsala area becomes moot. The map does not show the true situation at the times she discusses. And the concentration of finds in urban Uppsala is an artefact of the town's location.
Anyway, I highly recommend reading the paper.
Fredengren, C. 2015. Water politics. Wetland deposition of human and animal remains in Uppland, Sweden. Fornvännen 110. Stockholm.
I presume that, like most rivers, the Fyris occasionally has flooding episodes. Of course that would cause objects to be swept downstream, if the floods are severe enough. But I have no idea how common it is for the Fyris to have floods of sufficient magnitude. Such floods are common in other places, but most such places have drier climates or flatter topography than the Uppsala region.
Most of the finds seem to be in an area with relatively little change in the shoreline over the centuries. Is there a steep slope there? Are there signs of erosion along the slope?
It's a glacial esker. Some surface erosion but not much. They are extremely stable landscape features until the mechanical excavator is invented. There are loads of intact cremation cemeteries on them from the same era as the bone finds in the river. The eskers were used as roads and the cemeteries were sited for visibility to travellers.
That concentration of finds is right where I would expect material suspended in flowing river water to drop out of suspension due to slowing of the flow as it reaches a much wider section.
John, that's my thinking too.