Controls on downstream variation in surficial sediment size of an outwash braidplain developed under high Arctic conditions (Kaffiøyra, Svalbard)

The changes in morphology and surficial sediments of an outwash braidplain developed under high Arctic conditions are closely related to geomorphic processes, tectonic conditions and climate variations, and are affected by the existence of permafrost. These factors control the changes in river channel slope, their morphology and active braidplain width, which influence the downstream variation in grain size of outwash surficial sediments. Interdependence between these parameters has not previously been considered in relation to the spatial distribution of surficial sediments of outwash braidplain developed in the forefield of a retreating subpolar glacier. In this study, controls affecting the surficial lithology of the Waldemar River outwash (NW Spitsbergen, Svalbard) were recognised and indicated multiple correlations between the downstream changes in sediment texture, the geomorphometry of distributary channels, and outwash surface slope. Due to this, the techniques of multivariate regression modelling, artificial neural network and multivariate adaptive regression splines were used. Results show that observed relations between the sedimentary and morphological features indicate the existence of thresholds for downstream variation in bed material size which are not constant. Moreover, the processes of sediment transport in an outwash braidplain are strictly related to Shields stress, which is proportional to changes in braidplain slope, median grain diameter and sediment sorting in the proximal and middle zones of the outwash, but is inversely proportional to median grain diameter in the distal zone, contrary to Shields' bed-material threshold entrainment model. The results of this study will help understand outwash fan and plain evolution in paraglacial landscapes under conditions of changeable rate and source of sediment supply and transport capacity.