Knudsen Ó

Marren PM

Russell AJ

Recent work on active, inactive and Quaternary proglacial outwash plains or sandur, subjected to various magnitude and frequency regimes has yielded new insights into controls on sandur evolution. This paper presents a model of sandur development for varying magnitude and frequency regimes. Proglacial rivers are dominated by meltwater discharge fluctuations that vary on a number of timescales. At the smallest scale, diurnal variations are controlled by daily temperature changes. Daily trends are superimposed onto a seasonal cycle. Glacier margin fluctuations control meltwater release over periods of years to decades. Ice-sheet build up and decay controls meltwater discharge over centuries – millenia. High magnitude – low frequency floods are an additional source of meltwater input to the proglacial environment. High magnitude glacial floods can have a number of causes. The largest floods in the glacial environment are generated by the sudden release of meltwater from ice-marginal, subglacial and supraglacial lakes, or by the rapid melting of ice by subglacial volcanic eruptions. These floods are frequently referred to by the Icelandic term j\uf6kulhlaup. Braided rivers on sandur dominated by low magnitude – high frequency flows display a number of structures associated with variable discharge. However, the preservation potential of many structures is low. The dominant sedimentary signature in the Quaternary record is either of repeatedly reworked bar surfaces and numerous channels and scours infilled with fine sediment, or stacked bar core deposits associated with aggradation during summer peak discharge repeated over a number of years. The impact of flood events is dependent on discharge compared to normal summer peak discharge, and the recurrence interval and recovery time between floods. On large sandur the history of channel occupancy (largely controlled by glacier margin fluctuations) can thus act as a major control on flood impact. In flood hardened systems, large j\uf6kulhlaup bars persist between floods and control the braided channel pattern. Superficial reworking of j\uf6kulhlaup bars may occur between floods, but is only likely to be preserved in aggrading rivers. In rivers previously unaffected by j\uf6kulhlaups, large j\uf6kulhlaup fans can develop, characterized by ice-block pitted bar surfaces, large-scale bedforms and antidune structures. J\uf6kulhlaup fans are frequently topographically higher than ‘normal’ rivers, and are therefore well preserved. Interaction between the varying magnitude and frequency regimes is largely controlled by glacier margin fluctuations. Glacier advance encourages aggradation whilst retreat encourages incision, but also creates accommodation space. The model presented here predicts the landform and sedimentary record for specific magnitude – frequency regimes and glaciological conditions, and provides a tool for interpreting the sedimentary record of formerly glaciated areas

Discharge magnitude and frequency as a control on proglacial fluvial sedimentary systems

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