Contemporary changes of the channel pattern and braided gravel-bed floodplain under rapid small valley glacier recession (Scott River catchment, Spitsbergen)

Abstract

After the Little Ice Age, high-latitude proglacial river catchments have been observed to transition to paraglacial conditions through the activation of fluvial processes and changes in the river valley and channel forms. This project provides an analysis of contemporary changes in the morphology of the valley bottom and channel development of the proglacial Scott River catchment (NW part of the Wedel Jarlsberg Land, SW Spitsbergen) based on observations conducted during the 2009–2013 melt seasons. Almost half of the catchment (area of 10 km2) is occupied by a valley glacier, currently in the stage of rapid recession. The processes driving the development of the glacier-free valley floor are determined by (1) long-term factors such as geological structure, glaciotectonics, or the morphogenesis of the terminoglacial zone; (2) medium-term factors, including climatic changes and changes in the extent of the glacial terminus and hydrological regime; and (3) short-term factors such as weather-driven flooding events. Geological factors determined the development of two valley narrowings, and a clear division of the valley floor into three morphological zones. The upper gorge in the terminal moraine rampart distinguishes the wide (up to 700 m) upper section of the valley bottom dominated by an outwash plain, where the river system is fed by small tributaries. The lower valley gorge dissecting the elevated marine terrace separates the middle section of the braided river floodplain from the mouth section (alluvial fan). The geomorphological field mapping of the valley floor and riverbed was conducted using geodesic and hydrometric measurements with high-resolution survey techniques based on the global positioning system (GPS). The detail measurements of the Scott River valley and channel were carried out at representative cross-sections and longitudinal profiles in various parts of the catchment. Because of the dominance of the glacial alimentation regime, changes in the discharge rate and bedload transport regime are influenced by the glacial ablation rate, which determines the variability of channel morphology and the contemporary development of the valley bottom. The upper section of the valley is dominated by a multiple-channel braided system fed by sub- and supraglacial waters, while in the middle section the development pattern changes from a meandering single-thread channel to a multiple-channel wandering system to a multiple-channel braided system. The current valley bottom is almost entirely occupied by a braided river floodplain and multiple-channel river bed. Changes in the channel system are related to periods of snowmelt and the related flooding. After the retreat of the snow cover, the channels are highly stable, as their spatial pattern is usually transformed during several days of ablation or ablation-precipitation flood flows. In the river's mouth section, the channel returns to a single-thread meandering pattern before transitioning to a multiple-channel distribution system and finally to a single-thread straight channel. The river below the ebb in the storm bank develops a subaqual prodelta.