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Glacially-induced scaling relations in mountain drainage basins Brardinoni, Francesco

Abstract

In glaciated British Columbia, Canada, Quaternary climate changes are responsible for profound spatial reorganization of earth surface processes. These changes have left a landscape characterized by topographic anisotropy associated with a hierarchy of glacial troughs. The evolution of formerly glaciated landscapes is examined by considering a set of scaling relations and assessing their departures from known unglaciated trends. Ultimately, the magnitude of these departures should provide a measure of the state of landscape recovery (transience) from glacial disturbance. The set of scaling dependences studied includes slope-area relations, for assessing geomorphic process domains; landslide magnitude-frequency (LMF) and yield-area relations, for evaluating landslide-driven sediment dynamics; and the spatial organization of channel-reach morphology. In addition, along channel long-profiles the scaling between drainage area and channel cross-sectional variates (downstream hydraulic geometry), coarse grain-size fraction, and stream power indices are examined. The methodological approach couples extensive field surveys, GIS-based topographic analysis, air photo interpretation, and multivariate statistical analysis. Slope-area analysis reveals generalized process-form disequilibrium with a mismatch between topographic signatures and currently active geomorphic process domains. At the landscape scale of "source" colluvial channels, the glacial/paraglacial signature commonly overrides that produced by contemporary debris flows. Along the axis of former ice flows, relict glacial cirques introduce a "hanging" fluvial domain at contributing areas as small as 8*10⁻² km² and produce complex channel long-profiles similar to those observed for rivers responding to tectonic forcing. The concept of process domains appears to hold; however, some major glacially-forced modifications in the alluvial-colluvial transition are observed and the definition of a depositional colluvial sub-domain is proposed. Direct spatial scale linkages and generalized departure from unglaciated scaling relations are observed at all levels of investigation. Glacial macro-forms, by imposing local channel gradient and degree of colluvial-alluvial coupling, dictate the spatial distribution of process domains, which in turn affect LMF relations, landslide yield, channel-reach morphology, downstream hydraulic geometry, and stream power. The combination of glacial and post-glacial fingerprints and the effects of ongoing earth surface processes generate a complex landscape whose glacial signatures may persist until the onset of the next ice age.

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