Lake sediments can retain imprints of past floods, enabling reconstructions that span well-beyond instrumental time series. Time series covering thousands of years can document the natural range of flood variability, which is critical for understanding the potential causality between changing flood patterns and climate. Here, we analyzed sediments from Lake Sandvinvatnet in southwest Norway. Detailed environmental magnetic analyses of an 830 cm-long sediment core covering the last 6,500 years captured decadal scale trends in local flood frequency. Magnetic susceptibility (MS) assessments were carried out both on split cores and individual samples to track variability in sedimentary influx; the ratios of MS measured at 77 and 293 K (MS ratios) provided information on potential changes in source regions. The results suggested that sediments from the Buerdalen valley dominate the signal in the core, and the amount of ferromagnetic (high MS) carriers increases during flood events. These carriers were assumed to be transported from slope deposits in Buerdalen during rainstorm-triggered flood events. The reconstructed flood frequency, based on sediment layers with ferromagnetic carriers, showed high variability over the past 6,500 years, and the finding was validated by overlapping with known historical floods in the area. We observed periods with a high frequency of extreme floods (4,100–3,140 cal. yr BP) compared with intervals with a few or no extreme floods (6,050–4,100 cal. yr BP). Floods in this region are commonly a result of intense rain events during fall and snow and glacial melt during late spring and summer. The systematic frequency changes during the past 6,500 years suggest a certain persistency in the processes that cause floods, where mean trends in summer temperature and precipitation may have played a role.

湖泊沉积物可以保留过去洪水的痕迹，从而使重建工作跨越超出仪器时间序列的范围。涵盖数千年的时间序列可以记录洪水变化的自然范围，这对于理解洪水模式和气候变化之间的潜在因果关系至关重要。在这里，我们分析了挪威西南部桑德温瓦特湖的沉积物。在过去的6,500年中，对一个830厘米长的沉积岩心进行了详细的环境磁分析，捕获了当地洪水频率的十年尺度趋势。磁化率（MS）评估在分裂岩心和单个样品上均进行，以追踪沉积物涌入的变化。在77和293 K下测得的质谱比（质谱比）提供了有关源区电势变化的信息。结果表明，Buerdalen山谷的沉积物主导了岩心中的信号，洪水期间铁磁（高MS）载流子的数量增加。假定这些运载工具是在暴雨引发的洪水事件中从Buerdalen的斜坡沉积物运出的。在过去的6,500年中，基于带有铁磁载体的沉积物层的重建洪水频率显示出高变异性，并且通过与该地区已知的历史洪水重叠来证实这一发现。我们观察到极端洪水发生频率较高的时期（4,100–3,140 cal.yr BP），相比之下，只有极少洪水或没有极端洪水发生的间隔（6,050–4,100 cal.yr BP）。该地区的洪水通常是秋天期间强降雨事件和春末夏初的积雪和冰川融化的结果。