The geomorphological and sedimentological legacy of the historical Lake Lorsch within the Weschnitz floodplain (northeastern Upper Rhine Graben, Germany)

Felix Henselowsky , Peter Fischer, Elena Appel, Barbara Jäger, Nicolai Hillmus, Helen Sandbrink, Thomas Becker, Roland Prien, Gerrit Jasper Schenk, Bertil Mächtle, Udo Recker, Olaf Bubenzer, Andreas Vött

The artificial historical Lake Lorsch (1474/79 to 1718/20 CE) in the northeastern Upper Rhine Graben (Germany) is known from various historical sources, e.g. for fish farming, as a significant anthropogenic imprint of the Weschnitz floodplain. Nevertheless, there have been no geomorphological and sedimentological investigations about the (quasi-)natural context for the creation of the lake, its importance as a potential sediment archive and the subsequent use of the lake area until modern times. No relics of the lake can be observed in today's landscape. We investigated the geomorphological setting of the area using a high-resolution digital elevation model, groundwater level data, geophysical prospection, as well as sedimentological information from four sediment cores. This allows us to determine that the location of the lake is geomorphometrically deeper in relation to its receiving waters of the Old Weschnitz and was strongly fed by groundwater. Sedimentary analysis (core LOR 21A, unit 2) exhibits lake deposit, with characteristics indicative for a limnic environment and a high groundwater table. At the same time, adjacent stratigraphy shows drainage channel deposits (core LOR 20A, unit 3), which reflects anthropogenic controlled inflow via a channel (Renngraben). Our results based on a relative elevation model fit well with the historical records, that inflow for the anthropogenic channel was via the Old Weschnitz (topographically higher than the lake area) and that the artificial Landgraben-canal (topographical lower than the lake area) was overflowed with a bridge. It is a good example of how humans started as fluvial and water related agents at least for 500 years in the Weschnitz floodplain.