Changes of river courses in the Dutch delta unintentionally caused by humans

Even long before medieval inhabitants reclaimed land and raised dykes at large scale, humans have had a strong impact on river behaviour in the Dutch delta plain. Physical geographers at Utrecht University have demonstrated that two present Rhine branches – the rivers Lek and Hollandse IJssel – have developed stepwise in the first centuries AD, as a result of human impact on the landscape.

These river branches formed due to land subsidence in the Dutch peatlands and by increased sediment supply from the German hinterland, both of which were human-induced. The study was published in the authoritative journal Geology. ‘What began as minor changes in the delta landscape can have unexpected large-scale outcomes. Insights into Dutch geological history help us to convey messages on why unintentional natural effects of human interventions are to be taken into account in the management of modern deltas.’

Rivers through peatland

In a delta, rivers can switch courses through natural causes. Researcher Harm Jan Pierik explains: ‘Our research shows that human interventions can also trigger the formation of new rivers along courses that natural branches would not have taken.’ This occurred in the downstream peatland areas of the Rhine delta, which had been swamp forest prior to the Common Era. It had been an inaccessible area for thousands of years forming an obstacle for the formation of new river courses. ‘This situation changed from the beginning of the Common Era when growing numbers of people settled along the edges of the swamp, and expanded their land use from the banks of rivers and creeks into the peat land.’ As an unintentional consequence, this caused the formation of the Hollandse IJssel (from AD 100) and Lek (from AD 300).

Subsidence and increased sedimentation

Archaeological data show that on the southwestern side of the peatland, along the estuary that later became the Rotterdam harbour, settlements were present on the banks of tidal creeks. Here, agricultural land use expansion into peatland started in the last centuries BC, testified by ditches and culverts that drained the topsoil. This caused land surface lowering, which in turn enabled flood water and sediments to penetrate deeper inland. Pierik: ‘This made the tidal creeks bigger and longer, reaching ever further into the peat swamp’. This situation was mirrored along the north-eastern side of the swamp - nowadays near Utrecht - where creeks from the Rhine river ran into swamp as well. ‘On this side, land use also intensified when the region became a Roman military border in the first centuries AD.’ In addition to that, the Rhine hinterland in Germany had become widely deforested. This increased hillslope erosion, which led to more sediment transport by the Rhine to its delta. This sediment was deposited on top of the peatland that further subsided under the additional weight, further helping the formation of creeks in the peat.

To be continued below map

Complete transformation

The tidal creeks from the estuary side in the southwest and the creeks from the Rhine river side ultimately connected in the middle of the peatland and grew into the new rivers Lek and Hollandse IJssel. These rivers now partly took over the old main course of the Rhine, which flowed through Utrecht westward to Leiden and Katwijk, and now debouched in the southwestern estuary area near present-day Rotterdam. ‘As these new routes became the most favourable courses for the water flow, the Old Rhine carried less water and silted up. In this way, the lower delta river network was completely transformed unintentionally, within a few centuries following the reclamation activities.’

Significance for present-day deltas

Research into landscape impacts of past human activity in deltas is important, because it enables us to learn about the effects this may have on the long term. ‘With the notion that reclamation activity can start a chain reaction, such as the network reorganisation in the Rhine delta, one can develop different perspectives towards recent increased human pressures in deltas worldwide, for example those of the Mekong and Ganges rivers,’ explains Pierik. ‘In addition to anticipating to sea level rise, these areas also have to deal with land subsidence, for example caused by increased groundwater use. And just as in the Dutch situation, this may not only cause major changes locally, it will affect the entire delta.’

Would you like to know more?

The study into the formation of the Lek and Hollandse IJssel forms part of the larger project: ‘The Dark Age of the Lowlands in an Interdisciplinary Light.’ The project is a collaboration between physical geographers, bio-geologists and archaeologists, who are working on a reconstruction of the Low Countries in the late Roman and early medieval period, which witnessed great social, cultural and economic upheaval. See also the website of the Department of Physical Geography.

Read the full article: Pierik, H.J., Stouthamer, E., Schuring, T., and Cohen, K.M., 2018, Human-caused avulsion in the Rhine-Meuse delta before historic embankment (The Netherlands): Geology, v. 46, p. 1–4, https://doi.org/10.1130/G45188.1