The Lower Rhine (Germany) in Late Antiquity: a time of dissolving structures

Abstract From the middle of the 1st century AD, the Lower Rhine was part of the frontier of the Roman Empire (Limes). However, this Limes was not an impermeable line, but rather an open corridor that served as march area and as a trade and supply route for the adjacent military and civilian settlements as well as the hinterland. This required access to the river and permanent harbours. When planning military camps and towns along the Rhine, the existing topography and the challenge of a dynamic riverine landscape had to be taken into account. The prefered location for forts and towns were the raised edges of the Lower Terraces close to the undercut bank of a meander. For many years, research assumed that the river bend adjacent to a Roman site had to be an oxbow lake. The main argument being that only an oxbow situation would have protected the Roman sites against strong currents and the risk of demolition of land and settlements by the main stream (abandoned channel hypothesis). A re-evaluation of the extensive archaeological, sedimentological and palynological archive and published data from the Rhine plain, as well as of supplementary data from new boreholes, lead to a considerable increase in the previously incomplete knowledge of the fluvial history, especially regarding the timing of palaeomeander infilling. It showed that most of the meanders and river bends of the Rhine with Roman settlements on their banks not silted up before Late Antiquity (from the end of the 3rd century AD on). Before that those meanders were part of the main stream. The advantage of steep undercut banks was that a quay could be built on a location that ensured mooring all year round, even at low water level. However, it was necessary that stabilising bank protections were present in order to avoid shifting the course of the river with subsequent destruction of the infrastructure. Such an antique bank protection construction (a so called “Packwerk”) could be recognised in front of the Colonia Ulpia Traiana (CUT, Xanten). With this knowledge in mind, other excavated structures on the banks of palaeomeanders, previously mostly interpreted as Roman harbour remains, could be interpreted as bank protections. At Wesel near the strategically important mouth of the Lippe, the archaeological, sedimentological and palynological data showed that a meander that had silted up in Prehistoric times (Bronze Age) had been reactivated in Roman times and silted up again in Late Antiquity. A man-made diversion (perhaps by building a groyne) of the main stream could have been responsible for this. With the beginning of the Late Antiquity crisis of the Roman Empire at the end of the 3rd century, it presumably became increasingly difficult to maintain these water works. This enabled the river to regain its morphodynamics, by cutting of the meanders that were active during Roman times. We hypothesise that this increase in fluvial activities of the Lower Rhine from the end of the 3rd century onwards is due to an anthropogenic trigger: the collapse of the Roman Empire.