The sedimentary response to a pioneer geo‐engineering project: Tracking the Kander River deviation in the sediments of Lake Thun (Switzerland)

Human activities such as river corrections and deviations, lake‐level regulations and installations of hydropower plants affect and often strongly modify natural processes in lacustrine systems. In 1714, the previously bypassing Kander River was deviated into peri‐alpine Lake Thun. This pioneering geo‐engineering project, the first river correction of such dimensions in Switzerland, doubled the water and sediment input to the lake. In order to evaluate the sedimentary consequences of the Kander River deviation, the lacustrine sediments were investigated using a combined approach of high‐resolution (3·5 kHz) reflection seismic data and sediment cores (maximum length 2·5 m). The significance of this study is increased by the possible hazard represented by ammunition dumped into the lake (from 1920 to 1960) and by the recent installation of a gas pipeline on the lake floor in 2007/2008. The first 130 years after the river deviation were dominated by an extremely high sediment input, which led to the frequent occurrence of subaquatic mass movements. Slope failures primarily occur due to rapid sediment accumulation, but were occasionally triggered in combination with earthquake‐induced shocks and lake‐level fluctuations. After 1840, mass‐movement activity and sedimentation rates decreased due to a reduced sediment input as the Kander River adjusted to its new base level and, to a smaller degree, by further engineering of the Kander River bed and gravel withdrawal at the Kander Delta. A further consequence of the Kander River deviation is that the shores around Lake Thun have been more frequently affected by flooding due to the increased water input. In the time span from 1850 to 2006, six historically and/or instrumentally documented flood events could be correlated to flood turbidites in the sediment cores. This study demonstrates the significant usefulness of lacustrine sediments, not only in archiving natural hazards and human impact but also in assessing the consequences of future anthropogenic interventions on lacustrine systems.