text: Are you using iD to edit the map? If so then I think I understand what is happening. The River Thames is mapped as a large multi-polygon along this stretch of the river. If you try to edit the map with iD at this point, it will usually only download some elements of that multi-polygon. That leads to a peculiar rendering of the river segments that have been downloaded, as you describe. If you force iD to download the remaining elements of the multi-polygon then the display of the river corrects itself and renders accurately. So, I think this phenomenon is a limitation of iD and not something that we can or should try to correct by editing the map.