Sensitivity,persistence and resolution of the geomorphological record of valley-floor floods in an alpine glacier-fed catchment,Leirdalen,Jotunheimen,southern Norway

Abstract

Coarse-grained flood deposits, including 52 discrete berms, bars, terraces, splays and sheets, were investigated along a 6.6 km stretch of the Leira river, in a catchment containing several alpine glaciers. These landforms were mapped, characterized in terms of clast size and reconstructed hydrological parameters, and dated using several techniques with differing temporal range and resolution. High-resolution lichenometric dating indicated that 60–65% of the deposits date from the 20th century whereas 5.7% are 300–600 years old. In contrast, low-resolution Schmidt-hammer exposure-age dating (SHD) indicated that 85% of deposits have an age <500 years whereas 13.5% are older deposits with age estimates of 1000–6000 years. Simple indicators of soil development and vegetation succession demonstrated extensive recent disturbance of most surfaces by floods. Optically stimulated luminescence (OSL) and radiocarbon dating of fine-grained flood layers and buried palaeosols confirmed the existence of flood deposits spanning the whole of the Holocene. Apparent discrepancies between dating techniques are explained by a complex pattern of overwashing and reworking, which is related to height of flood deposits above the active river channel, and downstream patterns of disturbance associated especially with tributary confluences. While, in general, these deposits are of low persistence and most are highly sensitive to disturbance, some elements of the valley-floor landscape have persisted throughout the Holocene. Comparison of this fragmentary geomorphological record of floods with the historical flood record shows that the surviving deposits are sensitive to recent multidecadal-scale glacier and climatic variations.