28.09.2017: Not just since the recent past humans form their environment. For thousands of years now humans settle in Central Europe which is why it is often hard to tell how a landscape would look like in its “natural” state. Here, humans turned the natural landscape into a cultural landscape with consequences even for future land-use options. Led by the GFZ a team of scientists published an interdisciplinary study on the human impact on the system of the rivers of Havel and Spree in the area of Berlin and Brandenburg City. They were able to show that already in the Late Middle Ages the human impact on the river system was massive.
The European Union demands of its members the maintenance of river systems, and, if necessary, their renaturation to preserve or re-establish a “good ecological status”, respectively. Since especially metropolitan regions look back to a long history of cultivation this is a rather demanding task: How does a “good ecological status” of a river system look like? A “natural” state may turn out as highly impacted by humans in the past.
To find out how the river system of Havel and Spree changed during the past the team led by first-author Knut Kaiser of the GFZ now reconstructed the past 2,000 years. The results are now published in the scientific journal Geoarchaeology. Kaiser: „Our results on the historical landscape evolution may support current renaturation activities by scientifically defining their backgrounds and aims.“
Already in the Middle Ages dams caused rising water levels
For the first time the team systematically reconstructed the development of water level changes within stream lakes of the river system of the middle Havel and the tributary lower Spree of the past 2,000 years. To do so, the scientists used existing and new archaeological and palaeo-ecological data sets from the investigation area. Remnants of aquatic or land plants, for example, that are found in sediments indicate if a section was submerged or above water table during a certain time.
The findings point to a rise in water level between 1180 and 1250 AD, during High Middle Ages, of about 1.5 meters. The scientists assume that this was the result of a system of dams that spanned roughly 150 kilometers of the river and dammed up the water to operate water mills. This had massive impacts on the river ecosystem. Previously dry areas were flooded, and existing lakes enlarged, other areas turned into wetlands.
Kaiser: „For renaturation activities our results imply that a natural state of the river must be reconstructed with caution focusing on larger temporal depth.“ (ak)
Original study: Kaiser, K., Keller, N., Brande, A., Dalitz, S., Hensel, N., Heußner, K.-U., Kappler, C., Michas, U., Müller, J., Schwalbe, G., Weiße, R., Bens, O., 2017. A large-scale medieval dam-lake cascade in central Europe: Water level dynamics of the Havel River, Berlin-Brandenburg region, Germany. Geoarchaeology , DOI: 10.1002/gea.21649