Magnitude and frequency in proglacial rivers: a geomorphological and sedimentological perspective

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Abstract

Proglacial fluvial sedimentary systems receive water from a variety of sources and have variable discharges with a range of magnitudes and frequencies. Little attention has been paid to how these various magnitude and frequency regimes interact to produce a distinctive sedimentary record in modern and ancient proglacial environments. This paper reviews the concept of magnitude and frequency in relation to proglacial fluvial systems from a geomorphic and sedimentary perspective rather than a hydrological or statistical perspective. The nature of the meltwater inputs can be characterised as low-magnitude–high-frequency, primarily controlled by ablation inputs from the source glacier, or high-magnitude–low-frequency, primarily controlled by ‘exceptional’ inputs. The most important high-magnitude–low-frequency inputs are catastrophic outburst floods, often referred to by the term jökulhlaup (Icelandic for glacier-burst). Glacier surges are an additional form of cyclical variation impacting the proglacial environment, which briefly alter the volumes and patterns of meltwater input. The sedimentary consequences of low-magnitude–high-frequency discharges are related to frequent variations in stage, the greater directional variability that sediment will record, and the increased significance of channel confluence sedimentation. In contrast, the most significant characteristics of high-magnitude–low-frequency flooding include the presence of large flood bars and mid-channel ‘jökulhlaup’ bars, hyperconcentrated flows, large gravel dunes, and the formation of ice-block kettle hole structures and rip-up clasts. Glacier surges result in a redistribution of low-magnitude–high-frequency processes and products across the glacier margin, and small floods may occur at the surge termination. Criteria for distinguishing magnitude and frequency regimes in the proglacial environment are identified based on these major characteristics. Studies of Quaternary proglacial fluvial sediments are used to determine how the interaction of the various magnitude and frequency regimes might produce a distinctive sedimentary record. Consideration of sandur architecture and stratigraphy shows that the main controls on the sedimentary record of proglacial regions are the discharge magnitude and frequency regime, sediment supply, the pattern of glacier advance or retreat, and proglacial topography. A model of sandur development is suggested, which shows how discharge magnitude and frequency, in combination with sandur incision and aggradation (controlled by glacier advance and retreat) can control sandur stratigraphy.

Introduction

Proglacial fluvial sedimentary systems (sandur, plural: sandar) receive water from a variety of sources and have variable discharges that occur over a range of magnitudes and frequencies. The primary control on water input is the source glacier but atmospheric precipitation and downstream tributary inputs also play a significant role. Proglacial rivers are typically braided (Maizels, 1995), and many models of braided river sedimentation have been developed using proglacial rivers (e.g. Boothroyd and Ashley, 1975, Miall, 1977). Much recent research has focused on the impact of high-magnitude–low-frequency flood events (Maizels, 1993b, Maizels, 1997, Russell and Knudsen, 1999a, Russell and Knudsen, 1999b, Russell et al., 2001b). Less emphasis has been placed on studying how the various magnitude and frequency regimes interact. An important issue is how the sedimentology and sedimentary architecture of proglacial fluvial systems responds to long histories of discharge inputs of varying magnitude and frequency. Unravelling the discharge characteristics of fluvial systems from Quaternary glaciers and ice-sheets can provide information on a range of factors. Studies which include both the actual magnitude of discharge and the magnitude and frequency characteristics of discharge can provide understanding of rates of deglaciation, volume and timing of meltwater input into the proglacial environment, prevailing palaeoclimate, and the occurrence and character of surges and floods.

Determining the magnitude and frequency regime of modern proglacial environments can provide the understanding necessary for management of modifications to the fluvial system. For instance, it is important to understand the rates and timing of meltwater and sediment inputs to proglacial rivers that are being used for hydroelectric schemes. It is necessary to recognise that these inputs will change if glacial floods or surges occur, and that the volume and character of meltwater and sediment inputs will fluctuate in response to climate change. Similarly, floods and surges, glacier margin fluctuations, and proglacial aggradation and incision will affect engineering structures such as roads and bridges.

This paper reviews the evolution of the magnitude and frequency concept in rivers in general (Section 2.1), before considering magnitude and frequency in proglacial rivers in particular (Section 2.2), and the significance of extreme events (Section 2.3). The characteristics of discharge in glacial regions are considered in more detail in Section 3. The range of sedimentary features formed by discharges of varying magnitude and frequency in proglacial rivers are reviewed in Section 4, and a suite of criteria for identifying the various magnitude and frequency regimes in the sedimentary record are identified and discussed in Section 4.5. The interaction of the various magnitude–frequency regimes to produce the sedimentary record of proglacial regions is considered in Section 5, and a provisional model of relating magnitude and frequency to alluvial architecture is produced. The approach of this paper towards the magnitude–frequency concept is geomorphological and sedimentological rather than hydrological or statistical, although underlying hydrological factors are discussed in 2 Magnitude and frequency in fluvial systems, 3 Proglacial rivers.

Section snippets

Background to concepts of magnitude and frequency in fluvial systems

A persistent issue in geomorphology has been the relative importance of small, frequent events versus infrequent larger events (Wolman and Miller, 1960, Wolman and Gerson, 1978). Various solutions to this problem have been presented, largely depending on what is meant by ‘geomorphological effectiveness’, but also on the environment which is being studied. For example, the study of Wolman and Miller (1960) defined ‘work’ in fluvial systems in terms of suspended sediment transport, and concluded

The proglacial setting

The proglacial environment is distinctive in that is dominated geomorphologically by glacial meltwater (Röthlisberger and Lang, 1987). The wide range of features which can form as meltwater and sediment interact in the ice-marginal and proglacial environment is great, and consequently, the terrestrial ice-contact environment is extremely complex (Ashley and Warren, 1997). Although features such as ice-contact lakes may act as stores for water and sediment in the immediate proglacial zone, the

Proglacial braided channels

The chief sedimentological implication of low-magnitude–high-frequency discharge is that regular sediment reworking will take place (Maizels, 1995, Maizels, 1997). As discussed above, proglacial rivers are generally braided, and low-magnitude–high-frequency flows controlled by seasonal and diurnal variations in meltwater discharges are therefore likely to form a fluvial architecture consistent with established models of braided rivers (Boothroyd and Ashley, 1975, Miall, 1977, Miall, 1985,

Quaternary proglacial outwash literature

It was suggested earlier that the fluvial sedimentary record might provide a means of unravelling the magnitude–frequency issue in proglacial environments. Application of the criteria outlined in Section 4.5 provides a method of determining the magnitude and frequency regime of discharges associated with deposition of a Quaternary sandur deposit. At present very few studies have attempted to address these issues in relation to the Quaternary sedimentary record.

Quaternary successions dominated

Conclusions

This paper has demonstrated that there is a link between glacial meltwater discharge magnitude and frequency and the character of the proglacial fluvial sedimentary system. Proglacial rivers deserve to be treated separately in the magnitude and frequency debate. Low-magnitude–high-frequency discharges from proglacial rivers are largely cyclical, on a range of scales, from diurnal variations, to the build up and decay of large ice sheets. Proglacial rivers are also subject to

Acknowledgements

Parts of this paper are based on material previously presented in my PhD thesis, which was funded by the School of Earth Sciences and Geography, Keele University, and supervised by Dr Andrew J. Russell. Many additional insights have been gained through fieldwork in Iceland, funded through Earthwatch grants awarded to Dr Russell and by the Keele University 1996 Iceland Expedition. Earthwatch volunteers provided invaluable assistance with fieldwork between 1998 and 2003. I am grateful for

Philip M. Marren received a degree in Geology with Geographical Science from the University of Luton (UK) in 1994 and a PhD in Physical Geography at Keele University (UK) in 2000. His PhD work reconstructed the magnitude and frequency regimes of Quaternary proglacial rivers in eastern Scotland. Postdoctoral research at Keele concentrated on the sedimentology of jökulhlaups and surges in Iceland. He is presently working as a postdoctoral research fellow at the University of the Witwatersrand

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    Philip M. Marren received a degree in Geology with Geographical Science from the University of Luton (UK) in 1994 and a PhD in Physical Geography at Keele University (UK) in 2000. His PhD work reconstructed the magnitude and frequency regimes of Quaternary proglacial rivers in eastern Scotland. Postdoctoral research at Keele concentrated on the sedimentology of jökulhlaups and surges in Iceland. He is presently working as a postdoctoral research fellow at the University of the Witwatersrand (South Africa), investigating the geomorphology and sedimentology of wetlands. His main research interests are in fluvial and glacial geomorphology and sedimentology.

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