Gerald C. Nanson

A genetic classification of floodplains

Abstract Floodplains are formed by a complex interaction of fluvial processes but their character and evolution is essentially the product of stream power and sediment character. The relation between a streams ability to entrain and transport sediment and the erosional resistance of floodplain alluvium that forms the channel boundary provides the basis for a genetic classification of floodplains. Three classes are recognised: (1) high-energy non-cohesive; (2) medium-energy non-cohesive; and (3) low-energy cohesive floodplains. Thirteen derivative orders and suborders, ranging from confined, coarse-grained, non-cohesive floodplains in high-energy environments to unconfined fine-grained cohesive floodplains in low-energy environments, are defined on the basis of nine factors (mostly floodplain forming processes). These factors result in distinctive geomorphological features (such as scroll bars or extensive backswamps) that distinguish each floodplain type in terms of genesis and resulting morphology. Finally, it is proposed that, because floodplains are derivatives of the parent stream system, substantial environmental change will result in the predictable transformation of one floodplain type to another over time.

ANABRANCHING RIVERS: THEIR CAUSE, CHARACTER AND CLASSIFICATION

Anabranching rivers consist of multiple channels separated by vegetated semi-permanent alluvial islands excised from existing floodplain or formed by within-channel or deltaic accretion. These rivers occupy a wide range of environments from low to high energy, however, their existence has never been adequately explained. They occur concurrently with other types of channel pattern, although specific requirements include a flood-dominated flow regime and banks that are resistant to erosion, with some systems characterized by mechanisms to block or constrict channels, thereby triggering avulsion. The fundamental advantage of an anabranching river is that, by constructing a semi-permanent system of multiple channels, it can concentrate stream flow and maximize bed-sediment transport (work per unit area of the bed) under conditions where there is little or no opportunity to increase gradient. On the basis of stream energy, sediment size and morphological characteristics, six types of anabranching river are recognized; types 1–3 are lower energy and types 4–6 are higher energy systems. Type 1 are cohesive sediment rivers (commonly termed anastomosing) with low w/d ratio channels that exhibit little or no lateral migration. They are divisible into three subtypes based on vegetative and sedimentary environment. Type 2 are sand-dominated, island-forming rivers, and type 3 are mixed-load laterally active meandering rivers. Type 4 are sand-dominated, ridge-forming rivers characterized by long, parallel, channel-dividing ridges. Type 5 are gravel-dominated, laterally active systems that interface between meandering and braiding in mountainous regions. Type 6 are gravel-dominated, stable systems that occur as non-migrating channels in small, relatively steep basins. Anabranching rivers represent a relatively uncommon but widespread and distinctive group that, because of particular sedimentary, energy-gradient and other hydraulic conditions, operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges.

Episodes of vertical accretion and catastrophic stripping: A model of disequilibrium flood-plain development

This paper describes flood plains formed episodically by vertical accretion along high-energy, laterally stable channels, in southeastern Australia. Overbank deposition gradually builds a flood plain of fine-textured alluvium over a period of hundreds or thousands of years, following which catastrophic erosion by a single large flood, or a series of more moderate floods, strips the flood plain to a basal lag deposit from which it slowly reforms. This periodic destruction appears due to the progressive development of large levee banks and flood-plain surfaces of highly variable relief. As the levees and flood plain grow, overbank flow is gradually displaced from the broad flood plain into the main channel and flood-plain backchannels, with a resulting concentration of erosional energy. Eventually, high flows greatly exceed erosional thresholds, and wholesale scour of the channel boundary and flood plain occurs. Vertical-accretion flood plains at different stages of development result in a wide range of bankfull recurrence intervals, even along the same river. Some of these flood plains are so infrequently flooded that they can be mistaken for terraces formed under a prior flow regime. The almost random but catastrophic nature of this flood-plain erosion means that sediment supply and transport are highly variable and probably impossible to predict. This model of flood-plain formation is seen as only part of a continuum of alluvial environments ranging from vestigial, coarsegrained, traction-load flood plains along high-energy rivers in narrow gorges to extensive low-gradient flood plains in which alluvial stratigraphy is dominated by fine-grained overbank deposition.

Wetting and drying of Australia over the past 300 ka

Alternately dry and wet climatic episodes across central and eastern Australia during the past 300 ka have greatly affected Australia9s rivers, lakes, and dune fields. Evidence of widespread climate and flow-regime changes has been provided by 75 thermoluminescence (TL) dates and 18 U/Th dates from alluvial and eolian sediments. Fluvial conditions dominated part of the last two interglacials (stage 5 and 7), resulting in large sand loads in rivers in the present Simpson Desert and southeastern Australia. During the last interglacial, fluvial activity in central Australia peaked at ∼110 ka (stage 5 pluvial), probably ∼5-10 ka behind world temperature and sea-level maxima. Following the last late interglacial wet phase, aridity associated with dune building spread from central Australia toward its margins, achieving greatest intensity during the last glacial maximum. A less widespread wet phase, identified at about 55-35 ka (stage 3 subpluvial), is associated with high lake levels and paleochannel activity in southeastern Australia. This TL record of variable continental aridity in Australia correlates well with global changes, including the variable eolian dust flux into central China, the northern Pacific Ocean, and Antarctica.

Why some alluvial rivers develop an anabranching pattern

Anabranching rivers have been identified globally, but a widely accepted and convincing theoretical explanation for their occurrence has remained elusive. Using basic flow and sediment transport relations, this study analyzes the mechanisms whereby self-adjusting alluvial channels can anabranch to alter their flow efficiency (sediment transport capacity per unit of stream power). It shows that without adjusting channel slope, an increase in the number of channels can produce a proportional decrease in flow efficiency, a finding particularly relevant to understanding energy consumption in some braided rivers. However, anabranching efficiency can be significantly increased by a reduction in channel width, as occurs when vegetated alluvial islands or between-channel ridges form. The counteracting effects of width reduction and an increasing number of channels can cause, with no adjustment to slope, an otherwise unstable system (underloaded or overloaded) to achieve stability. As with other river patterns, anabranching can be characterized by stable equilibrium or accreting disequilibrium examples. Copyright 2007 by the American Geophysical Union.

A statistical analysis of bank erosion and channel migration in western Canada

Mean lateral-migration rates for 18 meandering river channels in western Canada are explained statistically in terms of hydraulic and sedimentological variables. The volume of sediment eroded from the outer bank of a meander bend is shown to be largely a function of river size and grain size of sediment at the base of the outer bank. These variables explain almost 70% of the volumetric migration rate for these relatively large, sand- and gravel-bed streams. It would appear that bank erosion and channel migration are essentially problems of sediment entrainment which is dependent on total stream power and sediment size. Vegetation on the outer bank is seen to have little significant effect in controlling channel migration. Further refinements of the type of data used here should permit the development of an accurate predictive model of regional channel migration. To this effect, it is most important to develop a precise relationship between bank resistance and the size of sediment at the base of the outer bank.

The last full glacial cycle in the Australian region

Abstract A broad regional reconstruction of climate in the eastern half of Australia through the last full glacial cycle is attempted incorporating a diversity of evidence from a wide range of environments. The critical climatic feature over much of the continent has been variation in precipitation. The majority of studies indicate that cool phases have been dry, and warm phases, wet. Recent, well-dated sequences combined with the refinement and reinterpretation of some existing records strongly suggest, however, that temperature and effective precipitation have not been in phase. Instead, wettest conditions appear to correspond with the interstadials within the latter part of Stage 5 of the marine oxygen isotope stratigraphy rather than with the Last (full) Interglacial or with the Holocene. Driest conditions most likely occured during the transition from the peak of the Last Glacial to the Holocene. The use of this period, however, as a model for cyclical climatic variation for the late Quaternary in Australia is limited to some extent by the likely impact of Aboriginal people on the landscape.

Hydraulic geometry and maximum flow efficiency as products of the principle of least action

Basic flow relationships have previously been seen to be insufficient to explain the self-adjusting mechanism of alluvial channels and as a consequence extremal hypotheses have been incorporated into the analyses. In contrast, this study finds that by introducing a channel form factor (width/depth ratio), the self-adjusting mechanism of alluvial channels can be illustrated directly with the basic flow relations of continuity, resistance and sediment transport. Natural channel flow is able to reach an optimum state (Maximum Flow Efficiency (MFE), defined as the maximum sediment transporting capacity per unit available stream power) with regard to the adjustment of channel form such that rivers exhibit regular hydraulic geometry relations at dominant or bankfull stage. Within the context of MFE, this study offers support for the use of the concepts of maximum sediment transporting capacity (MSTC) and minimum stream power (MSP). Furthermore, this study indicates that the principle of least action is able to provide a physical explanation for the existence of MFE, MSTC and MSP. Potential energy is minimized and consequently sediment transport is maximized in alluvial channels. Copyright

Chronology of Murrumbidgee River palaeochannels on the Riverine Plain, southeastern Australia

Four major periods of palaeochannel activity have been identified on the Murrumbidgee sector of the Riverine Plain of southeastern Australia. On the basis of stratigraphic information the channels reveal a picture of changing flow conditions during the last full glacial cycle. The ages of the periods were determined from nearly 40 thermoluminescence dates on surficial fluvial and aeolian sediments. These are named the Coleambally phase, which occurred from 105 to 80 ka (the mid- to latter part of Oxygen Isotope Stage 5), the Kerarbury phase from 55 to 35 ka (Stage 3), the Gum Creek phase from 35 to 25 ka (late Stage 3 to early Stage 2) and the Yanco phase from 20 to 13 ka (late Stage 2). The present flow regime was established by about 12 ka (Stage 1). The first two phases correlate with episodes of enhanced fluvial activity in northern and central Australia and with reduced dust activity globally. The phases in Stage 2 appear to be associated with seasonal snow melt and increased peak flows in periods flanking the Last Glacial Maximum. Source-bordering aeolian dunes associated with the Coleambally, Kerarbury and Yanco phases were found, however, the TL dates show that some have undergone aeolian reworking. Thermoluminescence dating and fluvial stratigraphy have revealed a detailed picture of Late Quaternary climate and flow regime changes that has the potential to extend to identified deposits stratigraphically older than those described here.

The role of vegetation in the formation of anabranching channels in an ephemeral river, Northern plains, arid central Australia

As the distribution and abundance of vegetation in drylands is often controlled by the greater availability of water along river channels, riparian vegetation has the potential to influence significantly dryland river form, process and behaviour. This paper demonstrates how a small indigenous shrub, the inland teatree (Melaleuca glomerata), influences the formation and maintenance of anabranching channels in a reach of the ephemeral Marshall River, Northern Plains, arid central Australia. Here, the Marshall is characterized by ridge-form anabranching, where water and sediment are routed through subparallel, multiple channels of variable size which occur within a typically straight channel-train. Channels are separated by channel-train ridges — narrow, flow-aligned, vegetated features — or by wider islands. By providing a substantial element of boundary roughness, dense stands of teatrees growing on channel beds or atop the ridges and islands influence flow velocities, flow depths and sediment transport, resulting in flow diversion, bank and floodplain erosion, and especially sediment deposition. Ridges and islands represent a continuum of forms, and their formation and development can be divided into a three-stage sequence involving teatree growth and alluvial sedimentation. 1 Teatrees colonize a flat, sandy channel bed, initiating the formation of ridges by lee-side accretion. Individual ridges grow laterally, vertically and longitudinally and maintain a geometrically similar streamlined (lemniscate) form that presents minimum drag. 2 Individual ridges grow in size, and interact with neighbouring ridges, causing the lemniscate forms to become distorted. Ridges in the lee of other ridges tend to be protected from the erosive effects of floods and survive, whereas individual teatrees or small ridges exposed to flow concentrated between larger ridges, tend to be removed. 3 Ridges lengthen, and coalesce with downstream ridges, eventually subdividing the channel-train into well-defined anabranches. This sequence turns a channel, initially obstructed with dense and chaotic stands of teatrees, into a well-organized system of ridge-form anabranches. In the moderate- to low-gradient Marshall River, which is colonized by an abundance of within-channel vegetation and subject to declining downstream discharges, this helps to minimize flow resistance, thereby maintaining an efficient water and sediment flux. Copyright