The formation of an anabranching planform in a sandy floodplain by increased flows and sediment load

Anabranching rivers evolve in various geomorphic settings and various river planforms are present within these multi‐channel systems. In some cases, anabranches develop meandering patterns. Such river courses existed in Europe prior to intensive hydro‐technical works carried out during the last 250 years. Proglacial stream valleys, inherited from the last glaciation, provided a suitable environment for the development of anabranching rivers (wide valleys floors with abundant sand deposits). The main objective of the present study is to reconstruct the formation of an anabranching river planform characterized by meandering anabranches. Based on geophysical and geological data obtained from field research and a reconstruction of palaeodischarges, a model of the evolution of an anabranching river formed in a sandy floodplain is proposed. It is demonstrated that such a river system evolves from a meandering to an anabranching planform in periods of high flows that contribute to the formation of crevasse splays. The splay channels evolve then into new meandering flow paths that form ‘second‐order’ crevasses, avulsions and cutoffs. The efficiency of the flow is maintained by the formation of cutoffs and avulsions preventing the development of high sinuosity channels, and redirecting the flow to newly formed channels during maximum flow events. A comparison with other anabranching systems revealed that increased discharges and sediment loads are capable of forming anabranching planforms both in dryland and temperate climate zones. The sediment type available for transport, often inherited from older sedimentary environments, is an important variable determining whether the channel planform is anabranching, with actively migrating channels, or anastomosing, with stable, straight or sinuous branches. Copyright © 2017 John Wiley & Sons, Ltd.     

Verification of a coupled climate-hydrological model against Holocene palaeohydrological records

We have coupled a climate model (ECBilt-CLIO-VECODE) and a hydrological model (STREAM) offline to simulate palaeodischarge of nineteen rivers (Amazon, Congo, Danube, Ganges, Krishna, Lena, Mackenzie, Mekong, Meuse, Mississippi, Murray–Darling, Nile, Oder, Rhine, Sacramento–San Joaquin, Syr Darya, Volga, Volta, Zambezi) for three time-slices: Early Holocene (9000–8650 BP), Mid-Holocene (6200–5850 BP) and Recent (1750–2000 AD). To evaluate the model's skill in retrodicting broad changes in mean palaeodischarge we have compared the model results with palaeodischarge estimates from multi-proxy records. We have compared the general trends inferred from the proxy data with statistical differences in modelled discharge between the three periods, thereby developing a technique to assess the level of agreement between the model and proxy data. The quality of the proxy data for each basin has been classed as good, reasonable or low. Of the model runs for which the proxy data were good or reasonable, 72% were in good agreement with the proxy data, and 92% were in at least reasonable agreement. We conclude that the coupled climate-hydrological model performs well in simulating mean discharge in the time-slices studied. The discharge trends inferred from the proxy and model data closely follow latitudinal and seasonal variations in insolation over the Holocene. For a number of basins for which agreement was not good we have identified specific mechanisms which could be responsible for the discrepancy, primarily the absence of the Laurentide ice sheet in our model. In order to use the model in an operational sense within water management studies it would be useful to use a higher spatial resolution and a daily time-step.     

Late Devensian ice sheet characteristics: a palaeohydraulic approach

Glacial meltwater channels are incised into bedrock and diamicton along much of the length of the Mid-Cheshire Ridge. Detailed mapping of one such system near the town of Helsby reveals a dendritic channel network developed in the opposite direction to the regional ice flow during the last (Late Devensian) glaciation. The channels formed subglacially, under atmospheric and not hydrostatic pressure, presumably as the ice sheet downwasted during deglaciation. Morphological and palaeohydraulic evidence suggests that not all of the network was necessarily active contemporaneously. Former water levels in the channels can be estimated due to the presence of bar surfaces, giving a calculated palaeodischarge of at least 111 m³ s⁻¹. The ablation rates required to account for this large discharge are an order of magnitude greater than those obtained from theoretical calculations and those observed in modern glacial environments. This implies that some form of high-magnitude discharge, such as a seasonal flood event, must have taken place in this area during deglaciation. This picture of the Late Devensian ice sheet suggests that during recession the ice sheet was static, crevassed and relatively thin (<50 m). This study also shows that there is no simple relationship between meltwater channel direction and ice dynamics, and that care is required when using the former to make inferences about the latter. Copyright © 1998 John Wiley & Sons, Ltd.     

Facies architecture of asymmetrical branching distributary channels: Cretaceous Ferron Sandstone,Utah, USA

Distributary channel systems are an important component of deltaic systems, but details of their branching pattern, stream‐order, internal variability and relation with adjacent levée, bay and bayhead delta are rather poorly documented in ancient examples. Photomosaic and measured sections collected along a gooseneck‐shaped canyon in southern Utah allow direct mapping of the branching pattern of an ancient distributary system. The main channel belt is ca 250 m wide and narrows to ca 200 m downstream of the branching point. A subordinate channel belt, ca 80 m wide, branches off of the main channel, forming a distinctly asymmetrical branching pattern. Water discharge in the main channel is estimated to be 85 to 170 m³ sec^?1. Comparison with palaeodischarge estimates of trunk rivers mapped in previous studies suggests that the branching documented in this study probably is a fourth‐order split. The distributary channels are characterized by a U‐shaped geometry filled with medium‐grained, cross‐bedded sandstone, and are dominated by lateral accretion, suggesting limited lateral migration and moderate sinuosity. Tidally influenced facies and limited trace fossils indicate direct marine influence. The distributary channels erode into adjacent levée and underlying heterolithic bay‐fill deposits, and the marine influence suggests that they were deposited on a lower delta plain, rather than on a non‐marine floodplain. The subordinate channel fed a bayhead delta, suggesting that it was formed by a partial avulsion, rather than bifurcation around a mouth bar, as is more characteristic of terminal distributary channels. Channel‐floor drapes, bar‐accretion drapes and abandoned channel fills within the sandstone channel belts represent the most important heterogeneity from the perspective of reservoir characterization.     

Muddy lateral accretion and low stream power in a sub-recent confined channel belt, Rhine-Meuse delta, central Netherlands

The Hennisdijk fluvial system in the central Rhine-Meuse delta is an abandoned Rhine distributary that was active on a wide floodplain from 3800 to 3000 years BP . Cross-sectional geometry, lithological characteristics and planform patterns of the channel-belt deposits indicate lateral migration of the Hennisdijk palaeochannel. Channel-belt deposits are around 10 m thick and 200–400 m wide. A gravelly facies near the base of the channel-belt deposits represents channel-lag and lower point-bar deposits. The axis of the channel belt is dominated by a sandy facies (medium and coarse sand), showing an overall fining upward trend with multiple cycles. This facies is interpreted as lower and middle point-bar deposits. The sandy facies is capped by a muddy facies, which is 1–2 m thick near the axis of the channel belt and thickens to 5–6 m along the margins. It laterally interfingers with the sandy facies that occurs near the channel-belt axis, but it has sharp, erosive outer contacts marking the edges of the channel belt. The muddy facies comprises inclined heterolithic stratification (IHS) (fine/medium sand–mud couplets) in its upper part. The relatively thin muddy facies with IHS that occurs near the channel-belt axis is interpreted as upper point-bar deposits with lateral accretion surfaces, formed under marine influence. Along the margins of the channel belt the muddy facies consists of thick, fairly homogeneous, successions of mud with variable sand content, and fine sand. Based on facies geometry and position, this part of the muddy facies is interpreted as counterpoint deposits, formed along the upstream limb of the concave bank of a channel bend. Counterpoint accretion seems to have been associated with the confined nature of the channel belt, which was the result of low stream power (4·5–7·8 W m⁻², based on reconstructions of palaeodischarge and channel slope) and cohesive bank material, i.e. clayey floodbasin deposits with intercalated peat beds occurring next to the channel belt. In the literature, counterpoint accretion is mostly reported from alluvial valleys, where meandering is confined by limited floodplain width, whereas muddy lateral accretion surfaces are commonly reported from much wider marine-influenced floodplains. The present study shows juxtaposition of both forms of muddy channel deposits in a low-energy, wide coastal plain setting, where preservation potential is considerable.     

Discriminating between the roles of late Pleistocene palaeodischarge and geological‐topographic inheritance in fluvial longitudinal profile and channel development

This paper investigates the influences of palaeohydrology and geological‐topographic inheritance in shaping the channel of the lower River Suir, southeast Ireland. Results of acoustic surveys of the lower River Suir and Waterford Harbour reveal two scales of pseudo‐cyclic river bedforms. Longitudinal elevation profiles of the geological topography (undulating bedrock and till‐mantled bedrock) bounding the present floodplain swath reveal pseudo‐cyclicity in that terrain too. Spectral and statistical analyses are used to quantify the cyclicity of the long profile and geological‐topographic series. These methods show that the dominant cyclicity of the long profile reflects autocorrelation more than inheritance of cyclicity from the bounding geological topography. The cyclicity of the long profile mainly reflects a hydraulic control on pool‐spacing, although some cyclicity probably has been inherited from the geological‐topography. Channel‐forming palaeodischarge is estimated based on the dominant pool‐spacing revealed by spectral analysis, validated using relationships between meander wavelength, channel cross‐sectional geometry and hydraulically‐informed discharge reconstruction. The palaeodischarge estimates are in close agreement and are two orders of magnitude greater than present flood maxima. Significantly, these palaeodischarge estimates also agree closely with palaeodischarge calculated for the submerged Pleistocene palaeochannel that extends across the near‐shore continental shelf from Waterford Harbour. The pool‐sequence of the lower Suir and the submerged palaeochannel represent a former land‐system that was active during a period of low relative sea level during the last glacial. More broadly, the paper offers insights into the landscape evolution of formerly glaciated regions that experienced very wide discharge variability during and after the transition from glacial to interglacial regimes, in a context of complex relative sea level change. Copyright © 2017 John Wiley & Sons, Ltd.