Seasonal variations in flooding inferred from lake sediments in Western Norway

Abstract. Holocene flood reconstructions from western and southern Norway indicate a non-stationary behaviour through time, with a major regime shift around 4000 years ago. Under the influence of global warming, flood frequency, seasonality, and magnitude are changing worldwide. The full range of natural variability in flood frequency and seasonality remains poorly constrained, underscoring the need for ultra-high-resolution records to place recent changes in a long-term context. This study presents an 8000-year flood record from Lake Vangsvatnet, Western Norway, that combines high-resolution computed tomography (CT) scanning, X-ray fluorescence (XRF), grain-size analysis, and radiocarbon dating to differentiate between seasonal flood deposits (spring snowmelt versus autumn rainfall). The 11-metre sediment core reveals distinct flood layers (n = 230), linked to varying hydrological conditions, and validated by historical discharge data (1892 CE–2016 CE). The record shows fluctuating flood frequencies, with peaks at 6000–5300 cal yr BP and 1400 cal yr BP to present. A seasonal shift from rainfall-dominated to snowmelt-dominated floods occurred around 3100 cal yr BP, coinciding with regional cooling. The last 500 years exhibit the highest flood frequency of the entire record. These findings highlight the sensitivity of flood regimes to climatic and, in the most recent centuries, human influence. Under future warming, reduced snowpack may diminish spring floods, while intensified autumn and winter rainfall could increase flood risks.