Spatial heterogeneity in the paraglacial response to post-Little Ice Age deglaciation of four headwater cirques in the Western Alps
Résumé
This paper aims to understand how the paraglacial response to recent glacier retreat varies between four cirques in the Western Alps. Post‐Little Ice Age glacier retreat has created extensive forelands where a variety of gravitational and fluvial process operate on both till‐floored and rock‐floored cirques. These processes may affect transitions from subglacial to proglacial landsystems, by reworking sediment and reorganising drainage. Landsystems achieve a state of preservation once no more adjustment is possible due to buffering by channel network evolution, channel armouring, and sediment exhaustion. We find no consistent trajectory of change across all studied sites: paraglacial responses differ from the classical valley–glacier model, involving variable slope‐channel coupling. Proglacial drainage networks on till surfaces have become more integrated by reducing their low‐order bifurcation ratios, unlike streams locked into rock channels. Reasons for diverse and site‐specific behaviour include cirque floor width, gradient, and surface materials (bedrock, fine till, and/or blocky till). At some cirques, these restrict the downstream diffusion of a paraglacial “signal” of fluvial‐transported sediment. At others, increased sediment flux originated from the erosion of terminal moraines. A high proportion of glacial material generally remains within the glacier foreland, due to proglacial basin sediment traps, inefficiency of fluvial networks, armouring of floors by coarse tills, and rock‐controlled channels. The millennial‐timescale preservation potential of most recent primary glacial deposits and within‐cirque paraglacial landforms appears to be high