Philip M. Marren

Morphology and sedimentology of a giant supraglacial, ice-walled, jökulhlaup channel, Skeiðarárjökull, Iceland: implications for esker genesis

Abstract This paper examines the sedimentary infill of a spectacular, 500-m-long, 100-m-wide ice-walled supraglacial channel, excavated into the snout of Skeiðararjokull, Iceland during the November 1996 jokulhlaup. The ice-walled channel developed in an area of the glacier, which was extensively fractured during the jokulhlaup. Sculpting of the ice-walled channel into the active snout of Skeiðararjokull suggests that the presence of stagnating glacier ice is not a prerequisite for the development of ice-walled channels. The ice-walled channel occupied an inter-lobate location, which acted as a focus for meltwater during the November 1996 jokulhlaup. The geometry of the supraglacial ice-walled channel system acted as a major control on the morphology and sedimentology of jokulhlaup deposits, through the tremendous spatial variability of resultant flow conditions. Maximum calculated jokulhlaup powers and shear stresses for the supraglacial ice-walled channel reached 40,000 W m −2 and 5000 N m −2 , respectively, with associated mean flow velocities between 7 and 11 m s −1 . Within the main ice-walled channel, Ground Penetrating Radar and outcrop exposure provide evidence of an ∼8-m-thick progradational and aggradational gravel macroform succession. The supraglacial ice-walled channel system is therefore analogous to a bedrock-confined fluvial system. This study provides a new analogue for the interpretation of ice-contact glaciofluvial deposits associated with former ice margins in Iceland and other areas subject to high magnitude discharges. Former supraglacial ice-walled channels resulting from tunnel collapse and ice margin break-up during high magnitude jokulhlaups will be associated with extensive coarse-grained, heavily kettled proglacial outwash surfaces. It is clear that the relationship between the characteristics of former ice-walled channels labeled as eskers and the prevailing glaciological and hydrological conditions needs to be modified in light of our knowledge of a modern flood-related large-scale supraglacial channel and its sedimentary infill. Such re-evaluation may provide a valuable new insight into former ice margin positions, modes of glacier retreat, and the role of high magnitude floods within the sedimentary record of former proglacial areas. This study therefore improves our understanding of the meltwater magnitude and frequency regime of former glaciers.

Fluvial–lacustrine interaction on Skeiðarársandur, Iceland: implications for sandur evolution

Abstract A complex assemblage of fluvial and lacustrine sediments is identified in a major section through proglacial sediments on Skeiðararsandur, south Iceland. High magnitude–low frequency events are identified in both the fluvial and lacustrine sediments. However, much of the sedimentary succession is composed of ‘normal’ low magnitude–high frequency sediments. Retreat of Skeiðararjokull following a surge event created a topographic basin occupied at various times by both fluvial and lacustrine depositional systems and also subjected to a high magnitude jokulhlaup. Previous studies have examined the processes and events occurring on Skeiðararjokull in isolation. This paper improves the understanding of the way in which processes with varying magnitude and frequency regimes interact and contribute to the sedimentary record of proglacial environments.

GPR derived architecture of November 1996 jökulhlaup deposits, Skeiðarársandur, Iceland

Abstract Skeiðarársandur in southeastern Iceland, with an area of > 1000 km2, is the world’s largest active proglacial outwash plain. In July–August 2000, a total of over 10 km of ground penetrating radar (GPR) profile data (at 50 MHz and 100 MHz) was collected from a variety of proglacial outwash sediments across the Gígjukvísl channel region of the Skeiðarársandur plain. GPR-profile results and their corresponding facies interpretations are presented for the flood deposits of a single supraglacial outwash fan and its associated source-proximal ice-walled canyon created entirely by the November 1996 jökulhlaup event. By combining the GPR data with ground surveying, photogrammetry and detailed sedimentary outcrop evidence, this study adds a new perspective to the large-scale analysis of single, high-magnitude flood events and the sedimentary record of former, ice-proximal outwash plains. The GPR derived architectures point to a higher degree of sediment reworking than predicted by previous sedimentary models and may provide a useful analogue for the study of sedimentation within similar bedrock fluvial and alluvial fan feeder systems.

Sedimentology of proglacial rivers in eastern Scotland during the Late Devensian

This paper reconstructs the characteristics of rivers which deposited proglacial fluvial sediments in east-central Scotland during the Late Devensian. Sediment depositional architecture and geometry, together with the relationship between high-stage and falling and low-stage depositional elements, were used to relate the proglacial sediments to the glacial meltwater discharge regime. The proglacial river systems studied were dominated by ‘normal’ ablation controlled discharge, rather than by high magnitude flood events. Consequently there is a great deal of spatial and vertical variability. Deposition occurred during short intervals of rapid aggradation, so that relatively fine-grained falling-stage sediments, as well as coarser, bar-core sediments are well preserved. Models relating the characteristics of the final deposit to the nature of the river are presented. These emphasise the role of stage changes and aggradation rates in controlling sediment architecture in braided fluvial deposits.

High and Dry: Comparing Literature Review Approaches to Reveal the Data that Informs the Geomorphic Management of Regulated River Floodplains

While the impacts of regulation on river systems have been investigated, the main focus of the research has been on ecological and in-channel impacts. An understanding of the impacts of regulation on floodplain geomorphology is needed in order to inform management actions, in particular environmental flows. Concurrently with a traditional literature review on the effects of river regulation on floodplain geomorphology, we undertook a systematic review of the evidence using causal criteria analysis. The causal criteria approach tested evidence from multiple studies against a ‘checklist’ of criteria, constructing an argument for causality by combining a number of pieces of evidence into a single coherent argument. The causal criteria analysis only found support for one hypothesized cause-effect linkage in the conceptual model. In contrast, after discussing many of the conceptual model linkages, the traditional review provided a series of eight hypothesised scenarios to highlight the different possibilities for floodplain geomorphic change resulting from regulation. The difference was partly as a result of the time taken to enter studies into the Eco Evidence Database, but mainly because of the quality of evidence from geomorphic studies. Low statistical rigour in study designs meant that many papers were inadmissible as evidence. Thus, the causal criteria analysis made transparent the quality of data used in a traditional literature review that might subsequently be used to justify costly management recommendations.

The origin and development of the Nyl River floodplain wetland, Limpopo Province, South Africa: trunk-tributary river interactions in a dryland setting

The Nyl River floodplain wetland, one of South Africas largest floodplain wetlands and a Ramsar site of international conservation importance, is located in an area of long-term and still active valley sediment accumulation. Creation of accommodation space for sedimentation has previously been attributed to tectonic controls, but new investigations reveal that a more likely cause is progradation of coarse-grained tributary fans across the narrow river valley downstream of the main area of floodplain wetland. Obstruction of trunk river flow and sediment transfer by these tributary fans has led to backponding and upstream gradient reduction and to accumulation of valley fills up to ∼35 m thick. Chronological control for the timing and rate of sediment accumulation is limited, but we hypothesise that a semi-arid to arid climate, characterised by asynchronous trunk–tributary activity that results in marked discontinuities in downstream water and sediment transfer, is likely to have been a key control. These interpretations are supported by other studies of dryland rivers globally and the findings add to our growing understanding of the controls on the origin and development of southern African wetlands, particularly by demonstrating how the combination of a particular physiography and a dryland climate can impart some distinctive geomorphological characteristics.

The Potential for Dams to Impact Lowland Meandering River Floodplain Geomorphology

The majority of the worlds floodplains are dammed. Although some implications of dams for riverine ecology and for river channel morphology are well understood, there is less research on the impacts of dams on floodplain geomorphology. We review studies from dammed and undammed rivers and include influences on vertical and lateral accretion, meander migration and cutoff formation, avulsion, and interactions with floodplain vegetation. The results are synthesized into a conceptual model of the effects of dams on the major geomorphic influences on floodplain development. This model is used to assess the likely consequences of eight dam and flow regulation scenarios for floodplain geomorphology. Sediment starvation downstream of dams has perhaps the greatest potential to impact on floodplain development. Such effects will persist further downstream where tributary sediment inputs are relatively low and there is minimal buffering by alluvial sediment stores. We can identify several ways in which floodplains might potentially be affected by dams, with varying degrees of confidence, including a distinction between passive impacts (floodplain disconnection) and active impacts (changes in geomorphological processes and functioning). These active processes are likely to have more serious implications for floodplain function and emphasize both the need for future research and the need for an “environmental sediment regime” to operate alongside environmental flows.

7. Icelandic jökulhlaup impacts

Iceland contains the worlds largest and best-documented active glacial outwash plains or sandar that have been studied since the 19th century. Vigorous subglacial volcanic activity and the presence of numerous ice-dammed lakes, make Iceland the prime location for the study of glacier outburst floods or jokulhlaups and their geomorphological and sedimentological impact. Increasing attention is being focused on large jokulhlaup channels related to both modem and ancient processes. Jokulhlaup impact within Icelandic bedrock channels has so far received little attention despite the fact that such channels are abundant as sandar in Iceland. Despite clear descriptions within Icelandic literature of jokulhlaup impact on glacier margins, there have been attempts to link jokulhlaup feeder system dynamics with processes and products in the proglacial outwash system. This chapter presents the latest research on the impacts of jokulhlaups generated by three main mechanisms: volcanic, ice-dammed lake drainage, and glacier surge. This chapter identifies and discusses the main advances in the understanding of jokulhlaup impact in Iceland.

Interaction of ENSO-driven Flood Variability and Anthropogenic Changes in Driving Channel Evolution: Corryong/ Nariel Creek, Australia

ABSTRACT Understanding the relative contributions of climatic and anthropogenic drivers of channel change are important to inform river management, especially in the context of environmental change. This global debate is especially pertinent in Australia as catchments have been severely altered since recent European settlement, and there is also strong evidence of cyclical climate variability controlling environmental systems. Corryong/Nariel Creek is an ideal setting to further study the interaction between climate and anthropogenic changes on channel evolution as it has experienced both significant periods of flood and drought, controlled by the El Niño Southern Oscillation (ENSO), and extensive anthropogenic changes. Since European settlement the floodplain has been completely cleared, the riparian zone almost entirely invaded by willows, and every reach of the channel has experienced some form of direct channel modification. Through the combined analysis of channel evolution, climate changes and anthropogenic history of the river it was found that both the ENSO-driven climate and anthropogenic drivers are significant, although at different scales of channel change. Significant straightening in response to land clearing in the early twentieth century occurred before any records of direct channel modifications. Following this, most river management works were in response to instabilities created in the clearing period, or to instabilities created by flooding triggering a new phase of instability in reaches which had already undergone stabilisation works. Overall, human activities triggered channel instability via land clearing, and management works since then generally exacerbated erosion during high flows that are driven by climate fluctuations. This research raises the interesting question of whether rivers in Australia have become more responsive to the ENSO cycle since the clearing of catchment and riparian vegetation, or whether the past response to climate variability was different.