Long Term Hydromorphological Changes in Two Swiss River Floodplains: Analyses of Historical Maps

Abstract

Freshwater ecosystems undergo greater changes than even the most affected terrestrial environments, primarily due to human modifications such as river channelization or dam construction. To counteract these impacts, freshwater restoration and re-naturalization initiatives are being implemented internationally. However, digital tools such as aerial and satellite imagery lag behind many natural and human-induced changes. As a result, historical maps serve as an essential resource for understanding more natural riverine environments and compositions. Recognizing their value, historical landscape data are being integrated into river restoration and conservation efforts, providing a crucial reference point for assessing past modifications to guide future improvements. Considering the potential of historic maps, this study quantified nearly a century of spatio-temporal changes in the near-natural River Sense and hydropower-regulated Sarine in Switzerland from 1879 to 1987. Using Switzerland's historical Siegfried maps and Old National Maps, 40 km of the Sarine and 46 km of the Sense were analyzed through geoprocessing techniques. Hydromorphological metrics, such as wetted channel width or braiding index, were derived to assess roughly 100 years of changes in the rivers' complexity, connectivity, and habitat composition. The Sense River, one of Switzerland's most natural landscapes, served as a baseline comparison to the Sarine River, which has been modified by hydropower infrastructure. The multi-decadal data derived from Swiss historical maps was compared to a similar study where smaller sections of the two rivers were analyzed from current and historical aerial images, offering an overlap of roughly four decades of historical maps and historical aerial imagery. Results showed abrupt narrowing in channel width and a decrease in network complexity in the Sarine, reflecting anthropogenic modifications. In contrast, the Sense exhibited more stable channel widths and dynamic habitat variability, despite some early modifications. The study underscores the value of historical maps for understanding long-term ecological changes and supports the need for restoration activities, such as artificial flooding, to enhance riverine biodiversity and resilience.