Hybrid River Landscapes in Mountain Regions: Anthropogenic Changes of Rivers and Flood Events in the W-Harz (Germany) from a long-term perspective

Mountain rivers have always been of crucial importance to human societies, as they serve as a source of drinking water, irrigation, and energy supply, but also pose dangers in the form of flooding. In prehistoric times, water in fluid and frozen form was the main natural processes shaping the landscape, and human impact were mostly minimal. Early agricultural societies began modifying rivers through irrigation channels, terracing, livestock grazing and last but not least dam constructions, which mostly changed river morphology and sediment load only at the local level. Nowadays, a large proportion of the rivers worldwide have been altered by hydraulic engineering projectsat a global level. In the Anthropocene Discourse, dams are considered one of the important indicators of the anthropogenic transformation of river landscapes.

Against this background, the present study investigates when and in what ways humans began to alter watercourses and flood regimes through water infrastructure, in particular by the constructions of dams and relataed infrastructure in mountain regions using the Harz Mountains as a case study. As highest upland region in Northern Germany, it is naturally prone to flooding, especially the more rugged regions in the W-Harz. Since historical times, floods have altered the landscape and led to extensive destructions for settlements in the mountains and their forelands. The high extent of damage caused by flooding was one of the reasons for the construction of six large dams in the W-Harz during the 20^th century. They have drastically reduced the occurrence of floods, thereby altering the natural flow dynamics of the rivers.

However, already in historical times, rivers were modified by the water management for mining activities and timber rafting. In the Early Modern Period, the Harz Mountains represented one of Europe's largest mining regions for the extraction of ores such as silver, copper, and lead. To supply the mines with energy, an extensive water management system with artificial ponds, ditches and underground waterways was constructed between the 16^th and 19^th centuries, the largest energy supply system for mining of its time, and since 2010, part of the UNESCO World Heritage in the Harz. The study provides an overview of anthropogenic changes or river systems and flooding events, examining their spatial and temporal distribution from a historical perspective with a special emphasis on the former mining district of St. Andreasberg in the catchment areas of the rivers Oder and Sieber. The study is based on field work carried out since 2016 in the Harz Mountains, the analysis of historical archives (reports, maps, photographs) and digital elevation models. In the context of the predicted increase in extreme hydrological events in Germany, knowledge of the historical development of rivers and the extent of anthropogenic influence is crucial for landscape management, nature conservation, risk management and the preservation of historical monuments in mountain regions.