Academic literature on the topic 'Barwon-Darling River'

AbstractPalaeochannels of lowland rivers provide a means of investigating the sensitivity of river response to climate-driven hydrologic change. About 80 palaeochannels of the lower Macquarie River of southeastern Australia record the evolution of this distributive fluvial system. Six Macquarie palaeochannels were dated by single-grain optically stimulated luminescence. The largest of the palaeochannels (Quombothoo, median age 54 ka) was on average 284 m wide, 12 times wider than the modern river (24 m) and with 21 times greater meander wavelength. Palaeo-discharge then declined, resulting in a younger, narrower, group of palaeochannels, Bibbijibbery (125 m wide, 34 ka), Billybingbone (92 m, 20 ka), Milmiland (112 m, 22 ka), and Mundadoo (86 m, 5.6 ka). Yet these channels were still much larger than the modern river and were continuous downstream to the confluence with the Barwon-Darling River. At 5.5 ka, a further decrease in river discharge led to the formation of the narrow modern river, the ecologically important Macquarie Marshes, and Marra Creek palaeochannel (31 m, 2.1 ka) and diminished sediment delivery to the Barwon-Darling River as palaeo-discharge fell further. The hydrologic changes suggest precipitation was a driving forcing on catchment discharge in addition to a temperature-driven runoff response.

The occurrence of a severe cyanobacterial bloom is described. This bloom affected almost 1000 km of the Barwon-Darling River, New South Wales, Australia, in November and December 1991 and was dominated by Anabaena circinalis Rabenhorst. This cyanobacterium was present in concentrations of around half a million cells per millilitre at some localities during its peak in mid November. Moderate to very high toxicity was demonstrated by mouse bioassay at many localities during this time. The bloom was attributed to very low flow conditions and high nutrient concentrations, especially of total phosphorus. However, warm water temperatures, elevated pH, reduced turbidity, and improved water transparency would also have been contributing factors. Very high ammonia concentrations were also observed during the bloom. The bloom declined during December and was eventually flushed from the river by increased flows following heavy catchment rainfall between mid December and early January.

Benefits of reintroduced large wood in river channels are largely based on studies at site scales in high-energy systems. By comparison, relatively little is known of the benefit of reintroduced large wood in low-energy systems at larger, reach scales. The present study assessed the effects of reintroducing large wood on fish assemblages along the Barwon–Darling River, Australia. Fish were sampled in replicated reaches subject to three treatments: six reference (wooded), six control (unwooded) and six managed (wood reintroduced) reaches. Sampling was conducted before and several months after wood addition, and then during a period following several large floods. Results demonstrate that reintroducing large wood had limited effects on fish. There were significant differences between treatments in fish length, but not in total abundance or species composition between treatments. Significant differences were detected in total abundance, species composition and fish length over time. There was an interaction recorded between treatments and time for fish length, but not total abundance or species composition. It is suggested that the lack of response by fish was because the physical character and position of the reintroduced wood pieces did not replicate ‘natural’ reference conditions. However, high variability in fish assemblages through time, likely in response to hydrological variation, reduced the power of the study to detect differences between fish over the shorter time period of the study (<5 years).

Many aspects concerning the association of riverine fish with in-channel habitat remain poorly understood, greatly hindering the ability of researchers and managers to address declines in fish assemblages. Recent insights gained from landscape ecology suggest that small, uni-scalar approaches are unlikely to effectively determine those factors that influence riverine structure and function and mediate fish-habitat associations. There appears to be merit in using multiple-scale designs built upon a geomorphologically-derived hierarchy to bridge small, intermediate and large spatial scales in large rivers. This thesis employs a hierarchical design encompassing functional process zones (referred to hereafter as zones), reaches and mesohabitats to investigate fish-habitat associations as well as explore patterns of in-channel habitat structure in one of Australia's largest dryland river systems; the Barwon-Darling River. In this thesis, empirical evidence is presented showing that large dryland rivers are inherently complex in structure and different facets of existing conceptual models of landscape ecology must be refined when applied to these systems. In-channel habitat and fish exist within a hierarchical arrangement of spatial scales in the riverscape, displaying properties of discontinuities, longitudinal patterns and patch mosaics. During low flows that predominate for the majority of time in the Barwon-Darling River there is a significant difference in fish assemblage composition among mesohabitats. There is a strong association between large wood and golden perch, Murray cod and carp, but only a weak association with bony herring. Golden perch and Murray cod are large wood specialists, whereas carp are more general in there use of mesohabitats. Bony herring are strongly associated with smooth and irregular banks but are ubiquitous in most mesohabitats. Open water (mid-channel and deep pool) mesohabitats are characterised by relatively low abundances of all species and a particularly weak association with golden perch, Murray cod and carp. Murray cod are weakly associated with matted bank, whereas carp and bony herring associate with this mesohabitat patch in low abundance. Nocturnal sampling provided useful information on size-related use of habitat that was not evident from day sampling. Both bony herring and carp exhibited a variety of habitat use patterns throughout the die1 period and throughout their lifetime, with temporal partitioning of habitat use by juvenile bony herring and carp evident. Much of the strong association between bony herring and smooth and irregular banks was due to the abundance of juveniles (<100mm in length) in these mesohabitats. Adult bony herring (>100mm length) occupied large wood more than smooth and irregular banks. At night, juvenile bony herring were not captured, suggesting the use of deeper water habitats. Adult bony herring were captured at night and occupjed large wood, smooth bank and irregular bank. Juvenile carp (<200mm length) were more abundant at night and aggregated in smooth and irregular banks more than any other mesohabitat patch. Adult carp (>200mm length) occupied large wood during both day and night. There is a downstream pattern of change in the fish assemblage among river zones, with reaches in Zone 2 containing a larger proportion of introduced species (carp and goldfish) because of a significantly lower abundance of native species (bony herring, golden perch and Murray cod) than all other zones. In comparison, the fish assemblage of Zone 3 was characterised by a comparatively higher abundance of the native species bony herring, golden perch and Murray cod. A significant proportion of the amongreach variability in fish assemblage composition was explained at the zone scale, suggesting that geomorphological influences may impose some degree of top-down constraint over fish assemblage distribution. Although mesohabitat composition among reaches in the Barwon-Darling River also changed throughout the study area, this pattern explained very little of the large-scale distribution of the fish assemblage, with most of the variability in assemblage distribution remaining unexplained. Therefore, although mesohabitat patches strongly influence the distribution of species within reaches, they explain very little of assemblage composition at intermediate zone and larger river scales. These findings suggest that small scale mesohabitat rehabilitation projects within reaches are unlikely to produce measurable benefits for the fish assemblage over intermediate and large spatial scales in the Barwon-Darling River. This indicates the importance taking a holistic approach to river rehabilitation that correctly identifies and targets limiting processes at the correct scales. The variable nature of flow-pulse dynamics in the Barwon-Darling River creates a shifting habitat mosaic that serves to maintain an ever-changing arrangement of habitat patches. The inundation dynamics of large wood habitat described in this thesis highlights the fragmented nature of mesohabitat patches, with the largest proportion of total in-channel large wood remaining unavailable to fish for the majority of the time. At low flows there is a mosaic of large wood habitat and with increasing discharge more potential large wood habitat becomes available and does so in a complex spatial manner. What results in this dryland river is a dynamic pattern of spatio-temporal patchiness in large wood habitat availability that is seen both longitudinally among different river zones and vertically among different heights in the river channel. Water resource development impacts on this shifting habitat mosaic. Projects undertaking both fish habitat assessment and rehabilitation need to carefully consider spatial scale since the drivers of fish assemblage structure can occur at scales well beyond that of the reach. Fish-habitat associations occurring at small spatial scales can become decoupled by process occurring across large spatial scales, making responses in the fish assemblage hard to predict. As rivers become increasingly channelised, there is an urgent need to apply research such as that conducted in this thesis to better understand the role that in-channel habitats play in supporting fish and other ecosystem processes. Habitat rehabilitation projects need to be refined to consider the appropriate scales at which fish assemblages associate with habitat. Failure to do so risks wasting resources and forgoes valuable opportunities for addressing declines in native fish populations. Adopting multi-scalar approaches to understanding ecological processes in aquatic ecosystems, as developed in this thesis, should be a priority of research and management. To do so will enable more effective determination of those factors that influence riverine structure and function at the approariate scale.

Rivers around the world are under increasing pressure from a variety of human activities. Effective management of riverine landscapes requires an ecosystem approach and one that recognises the complex interactions between their physical, chemical and biological components. Perceptions of pattern and process are central to our understanding of riverine landscapes. Pattern and process operate over multiple scales to produce heterogeneous mosaics of landscape patches that change over time. Hierarchical patch dynamics provides a useful approach to unravel pattern and process at multiple scales in riverine landscapes. This thesis adopts a hierarchical patch dynamics approach to investigate floodplain sediment and nutrient dynamics within the Barwon-Darling River in South Eastern Australia. The flow regime of the Barwon-Darling River is highly variable. As a result, it has a complex channel cross section featuring inset-floodplain surfaces that occur at multiple elevations within the channel trough. These surfaces formed the focus of this study. The texture of inset- floodplain surface sediments displays a patchy spatial distribution and one that did not reflect lateral or longitudinal gradients within this floodplain landscape. Rather a sediment textural patch mosaic was identified. Nutrient concentrations associated with the surface sediments of the inset-floodplains were also shown to vary significantly resulting in a nutrient patch mosaic. This spatial nutrient mosaic was enhanced by factors including the surface elevation of the floodplain surface. Sediment and nutrient exchange between the river channel and inset-floodplain surfaces was measured during several flows in 2001, 2002 and 2005. Pin and sediment trap data showed that significant quantities of sediment were exchanged between the river channel and floodplain surfaces during inundation with both cut and fill processes occurring. Patterns in sediment exchange appear to be related to local sediment supply and seasonal sediment exhaustion, rather than the top down geomorphic constraints considered. These material exchanges resulted in a change to the spatial configuration of the sediment textural patch mosaic. Distinct new sediment textural patches were created following inundation, while other patches were lost post inundation and other patches changed sediment textural character to move into pre-existing patches. Thus a truly dynamic sediment textural mosaic exists within this floodplain landscape. Nutrient concentrations associated with floodplain sediments also changed over time. While nutrient concentrations increased after the December 2001 flow event, they generally decreased after the March 2002 event, highlighting their dynamic nature over time. The spatial distribution of nutrient concentrations also varied over time, with a 40 percent change to the nutrient mosaic as a result of the March 2002 flow event. In addition to the influence of the changing physical template (sediment texture mosaic), nutrient concentrations were shown to be influenced by rainfall processes on non flooded surfaces, and also a number of top-down constraints and bottom-up influences operating over multiple spatial scales. Overall, the inset-floodplains studied in this thesis acted primarily as sediment and nutrient sinks, and were a source for dissolved nutrients. Nutrient exchange was associated with the exchange of sediments in this riverine landscape, over both inter-flow and decadal timescales. It was demonstrated that water resource development within the catchment reduced the number, magnitude and duration of flow events down the Barwon-Darling River and as a result reductions in the exchange of sediment, associated and dissolved nutrients between inset-floodplains and the main river channel were calculated. The greatest reductions were with the release of dissolved nutrients (42-25 percent) and the exchange of sediment and associated nutrients from high level surfaces (43 percent). Effective conservation and management of riverine ecosystems must occur at the correct scale. This study identified potential nutrient hotspots at several scales in the Barwon-Darling floodplain landscape that could be targeted by management. The low predictability of the location of nutrient hotspots at the inset-floodplain scale over time means that environmental flows should be targeted at high level surfaces (<25 000 MLD-1) that provide long term sources of carbon to the river channel. Conserving flows of this magnitude will also reinstate flow variability, an important facet of the Barwon-Darling River?s hydrology that has been changed by water resource development. The research presented in this thesis highlights the importance of not only considering pattern and process at multiple scales, but also the way in which these processes influence landscape patterns over time, leading to the identification of the appropriate scales that can best be targeted for the conservation of these systems.