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Floodouts, drainage breakdown and wetland formation in a losing river in Eastern Australia part of Vignettes:Vignette Collection
The Macquarie River of eastern Australia is an inland draining perennial system whose lower reaches flow across a large, low gradient alluvial fan-plain without tributary input (Figure 1). Failure of the Macquarie to sustain its channel form leads to channel breakdown and floodout and the formation of extensive near-perennial wetlands the Macquarie Marshes. The Macquarie Marshes are an important Ramsar-listed waterbird habitat. The increasing abstraction of water for agriculture, as well as periodic drought and future climate change predictions all threaten the successful breeding of the birds. In addition, the inherently dynamic nature of the fluvial system (with channel avulsion and marsh abandonment) poses management challenges where conservation reserves are restricted within a broader alluvial landscape largely utilised for agriculture. Causes of channel breakdown Channel breakdown (the loss of ability to maintain a channelized course) and the resulting floodout (distribution of water and sediment across the unchannelised floodplain) is a common phenomenon of losing streams (which decrease in discharge in a downstream direction) in arid and semi-arid environments. The Macquarie is a losing stream in its lower reaches: the upper catchment of 26 000 km2 enters a broad alluvial fan-plain near Narromine where it has a maximum annual average discharge of 1.2 106 ML yr-1 (Figure 3). On the plain there are no tributary inputs but substantial losses to the floodplain and distributary channels during floods, evaporation, groundwater recharge (<2% of total discharge at Narromine) and increasingly to abstraction for irrigated agriculture. At the point of first floodplain marsh formation the average annual discharge is only 0.4 106 ML yr-1. The location of the channel breakdown region is determined by the channel hydrology: the threshold of stream power below which the channels cannot be easily eroded and in-channel sedimentation occurs. Sediment load is also critical, determining bank strength, tendency for sediment to deposit in the channel and to build up the banks. However, at the local scale, the complete breakdown into marshes is influenced by the degree of topographic confinement produced by surrounding alluvial ridges of palaeochannels (Figure 2): whether overbank flows can spread widely on the floodplain and whether those flows return to the channel or are dispersed within the marshes to infiltrate and evaporate. Stages of channel change and breakdown There are two stages in the channel breakdown: development from a single meandering channel to a distributary pattern of straight channels and then further reduction in channel size and continuity, increasing overbank flow even under low flow conditions and the formation of permanent and semi-permanent wetlands on the extensive floodplains (Figure 2). Several distinctive channel types are identified on the lower Macquarie, each playing a different role in the process of channel and floodplain evolution. At the upstream limit of floodplain marsh formation the trunk stream (Figure 4) reaches maximum sinuosity and minimum width/depth ratio and changes character from a dominantly laterally-adjusting channel to one with both vertical and lateral accretion features and avulsion. Some of the past avulsions have lead to relocation of the channel (abandonment of a reach of old channel). Some avulsions have created an anastomosing river (simultaneous operation of old and new channels and rejoining) while some have created a distributary pattern (old and new operate simultaneously but do not rejoin). Some avulsions are partial or incomplete, creating 'breakaways' (local term) - long-lived small distributary channels formed in the mid or upper bank of the trunk stream. All of these new 'distributary' channels are relatively straight and become shallower and smaller downstream. They are inherently unstable: their low stream power and more-or-less perennial flows encourage aquatic and riparian vegetation which further slows flow, enhances in-channel deposition, channel contraction and overbank flow leading to the formation of top-of-bank marsh channels to distribute water onto the floodplain (Figure 5). Marsh channels have very small capacity, usually have indistinct bed and banks and are fully vegetated. They may terminate or merge with gilgai depressions to form a reticulate network of marsh channels covering the floodplain (Figure 6). Overall, this sequence is one of channel breakdown and 'floodout' (loss of channelized flow). Marsh 'life cycle' and sediments New distributary channels are initiated by avulsion and incision into the floodplain surface and then gradually aggrade until a new avulsion results in abandonment, typically over a timescale of approximately 100 years. Avulsion and channel breakdown at the local scale are driven by the characteristic nature of the dense in-channel reed vegetation which chokes the streams and leads to in-channel aggradation, reduced fluvial efficiency and frequent overbank flows that ultimately lead to the development of new channels. Floodplain development occurs as a series of spatially discrete sediment lobes, each of which are deposited in marshes immediately downstream of the channel breakdown. The wetlands trap almost all of the sediment delivered to them. Simultaneously, upstream of the breakdown sediment from overbank flows is trapped by dense vegetation on the banks and leads to the build-up of low levees (10-50 cm). Lobe accretion is terminated by channel avulsion through the low levees which line the channels. The lower Macquarie is an almost entirely suspended load transporting river with up to 80-90% clay content and minor silt and fine sand. Despite this, low suspended sediment concentrations result in slow rates of floodplain accretion (Figure 7). Because of the semi-arid climate and low sedimentation rates the marsh sediment is nearly all clastic with only around 5% organic matter (LOI).
The continental dunefield of Australia part of Vignettes:Vignette Collection
Dunefields cover a large portion of the Australian continent in areas with favourable topography, climate and sand supply. The main areas of dunes lie in a diagonal band across the continent from the Mallee dunefield in the southeast to the Great Sandy Desert in the northwest (Fig. 1). These two dunefields with the Strzelecki Desert, Simpson Desert and Great Victoria Desert dunefields constitute the largest distinct sand seas. In addition there are many smaller dunefields surrounding these sand seas and isolated dunes over an even broader area. Climatic controls The major dunefields lie within the arid and semi-arid climate zones, although some smaller dunefields occur in areas which are presently sub-humid or humid and densely vegetated. The broadly concentric pattern of rainfall distribution over the Australian continent is the product of summer monsoonal rainfall over the tropical north decreasing towards the south and winter westerly frontal rainfall in the south decreasing to the north. Australia as a whole has low rainfall and extensive arid areas but does not presently experience hyper-aridity. The driest part of the continent, at the northern end of Lake Eyre, still averages over 100 mm of rain each year and is largely vegetated to a degree which is sufficient to stabilise dune crests (Hesse & Simpson 2006). The anticlockwise whorl formed by the longitudinal dune orientations has long been recognised and its general similarity with the anticlockwise winds of the sub-tropical high pressure system noted. Nevertheless, dune orientation is expected to reflect the long-term resultant sand drift direction (RDD) at any point and the subtropical ridge has a pronounced seasonal cycle of movement north and south over the southern half of the continent such that the dunefield does not represent the synoptic wind flow of any single period or season. At the centre of the whorl at approximately 26° S there is a distinct double centre to the pattern of dunes; one centred over Lake Carnegie (123° E 26° S) and the other NE of Mt Connor (132.5° E 25.5° S). Comparison of the dune map with the modern annual resultant drift direction determined by Kalma et al. (1988) shows that there are regions in which there is close agreement and areas of divergence and even opposed wind and dune directions. There is reasonably close agreement (within 15°) between RDD and dune orientation in large parts of the eastern and northeastern dunefields. However, dunes over most of the Great Victoria, Eyre, southwestern Mallee, Great Sandy and Gascoyne are oblique (up to 75° divergent) and there are areas of the Gascoyne, northern Great Sandy and along the axis of the dune whorl in central Australia where dunes are opposed or even reversed with respect to the modern RDD. Dune types and wind patterns Within the dunefield there are distinct distributions of the main types of dunes. Fig. 2a-2h shows Landsat images of major dune morphological types. Transverse dunes (Fig 2h) (excluding playa shoreline features) are rare in the Australian dunefield. This is likely because the combination of unimodal wind climate and high sand supply which favour transverse dunes is rare in the Australian deserts. Single-crested longitudinal dunes (Fig 2a) are the most common type in Australia and dominate in all dunefields except the Great Sandy where multi-crested longitudinal dunes dominate (Fig 3c, 2b). Longitudinal dunes are favoured by seasonally bi-directional winds and low sand supply, characteristic of Australia. Around and between the dual centres of the whorl, network dunes (Fig 2f) are common where dunes change orientation over relatively short distances (Fig 3b). The network dunes are short, linked linear segments varying from no detectable preferred orientation to dunes with a dominant set of longitudinal dunes but with connecting segments of different orientations. At the centre of the whorl, particularly in the west, there are areas of isolated mounds (Fig 3a, 2g): mostly small dunes separated by sandless plains but often with a regular arrangement. These are in some ways similar to star dunes, reflecting a highly variable wind regime, but are formed in areas of very low sand availability. Within the longitudinal dunefields there are areas of parabolic dunes (Fig 3d, 2e), especially towards the margins of the dunefield in areas where longitudinal streamlines are parallel and show little curvature, in the Mallee, Eyre and Wiso dunefields and in patches of the extreme southwest Great Victoria dunefield and through the southwest. In general, the distribution of dunes at this broad scale conforms with expectations of dune morphological response to wind direction (Wasson & Hyde 1983; Werner 1995). Topographic controls on sand availability and dunefield location Topography has a strong influence on the location of dunefields and dunes at all scales: it limits the supply of sand and erodible sediment in low-lying areas, the erosivity of runoff and the topographically-induced acceleration or deceleration of wind. At the broadest continental scale topography has an immediately obvious relationship with the supply of erodible sand. There are rare instances of small dunefields in upland areas associated with localised sand sources but most dunefields occur in valleys, piedmonts, coastal plains or lowland basins from sedimentary accumulations. In the eastern half of the continent there is a clear topographic distinction between erosional uplands and sedimentary basins (Fig. 4). With a few exceptions dunefields occur in the lowland basins and are absent from the Great Dividing Range of the east coast and the extensive low hills of the inland. The Mallee Dunefield occupies the western portion of the Cenozoic Murray Basin. The Cenozoic Lake Eyre Basin underlies both the Simpson and Strzelecki dunefields which are separated by the low structural ridge of the Gason Dome on which Sturt's Stony Desert is found (Fig 4). In the western half of the continent, the older cratonic crust has less dramatic relief and lacks Neogene basins. The extensive Great Victoria and Great Sandy dunefields occur on landscapes of subtle but distinct ridge and valley topography (Fig. 5). While the topography is often low enough that sand dunes can climb over and cover the ridges between valleys, it is clear from the topographic dependency that the valleys are the source of the sediment and that aeolian processes have not only re-shaped the valley floors but also blown sand out to cover much of the ridges between (Fig 5). In this respect they are very different from the eastern dunefields or even most dunefields of Africa or Asia of comparable extent which occupy geological and topographic basins (Breed & Grow 1979). Lithological limitations on sand supply Large low-relief areas of the arid zone are dune-free and these are related to low sand supply because of substrate or catchment lithology or sediment starvation. Examples are found in the Nullarbor Plain (limestone), Georgina Basin (shale/mudstone), Carpentaria basin (clay soils and hardpans) and parts of the Darling Riverine Plain and Murray-Murrumbidgee Riverine Plain (clay soils). The Yilgarn Block of southwestern Australia appears to have been a poor source of sand for dune formation in its subdued landscape or surrounding areas during the Cenozoic, possibly due to prolonged, deep weathering and duricrust formation. The generally low sand supply has contributed to the characteristic dune morphologies: longitudinal, network and mound. Climate change and the evolution of the dunefields The topography and extensive palaeodrainage network of the western half of the continent date to the late Mesozoic to Early Cenozoic. It is thought that this relict landscape was the product of advancing aridity through the Neogene leading to loss of effective fluvial transport. Subsequent evolution towards an arid landscape saw the development of saline groundwater windows, playas and aeolian landforms by at least 1 million years ago (Fujioka et al. 2009). Because of this trend to aridity and reduction of sediment supply, most of the dunefields today receive little or no new supplies of sand from fluvial sources but rework old coastal, fluvial and lacustrine sediments from relict shorelines, terrestrial basin deposits, piedmonts and valley floors. The repeated expansion of arid conditions in glacial stages of the Quaternary glacial cycles has left dunefields extending from the most arid parts of the continent into the semi-arid zone and isolated patches in presently humid areas. Growing geochronological evidence points to dune formation beginning in the mid-Pleistocene and accelerating as the climate has become increasingly arid in subsequent glacial cycles. However, the dunes appear to have suffered little modification of orientation after their initial formation, possibly due to stabilisation by pedogenesis and vegetation, and their orientation preserves the alignment due to sand drift direction at that time. There are substantial areas of large divergence between sand dune orientations and modern resultant sand drift direction which point to past altered circulation patterns: areas of onshore wind flow in the Great Sandy, Gascoyne and Eyre dunefields, and areas along the central axis of the dune whorl at 26° S. Some combination of latitudinal shift in response to hemispheric temperature gradients, altered land-sea temperature contrast and regional variations in proximity to the coastline as sea-level fell may explain this regional pattern of variation. There is no simple, continent-wide displacement of dune orientations and therefore simple latitudinal shift of circulation patterns, as has been frequently postulated, is not an adequate explanation of the observed patterns. The trend towards aridity experienced since the Miocene and modulated by the Milankovich cycles in the Quaternary has seen progressive landscape change, spread of aeolian landforms, development of playa lakes and loss of fluvial drainage networks. The dunefield exhibits many features reflective of the evolution of the climate as well as related changes in sediment supply.