The Drivers and Consequences of Change to the Physical Character of Waterholes on an Australian Dryland River

Abstract

<p>Waterholes are a critical feature of dryland rivers globally, providing geomorphic and ecohydrological complexity. Waterholes typically develop as a series of deep pools that can retain water for extended periods in the absence of surface water connectivity or groundwater inputs. They are often the only source of water in an otherwise arid environment and as such have high ecological, social, and cultural value. From an ecological perspective, waterholes provide refuge to aquatic biota in a drying environment ensuring both their immediate survival and their subsequent recolonisation of the broader river system once flows recommence. Waterholes provide an important water supply to local communities for stock and domestic use and recreation. They are also pivotal to the cultural practise and spiritual beliefs of Indigenous Australians. This study has shown that despite their significance in the landscape, waterholes are increasingly threatened by anthropogenic landscape change. The Barwon-Darling River, an alluvial, dryland river in south-east Australia, is a river where the impact of human activity on waterholes is of concern. This thesis is aimed at understanding (1) if and how the physical character of waterholes on the Barwon-Darling has changed since European colonisation and (2) understanding the drivers and consequences of change to waterholes on the Barwon-Darling River. </p> <p>In this study, a comparison of historical (1890s) and contemporary (2015) riverbed profiles revealed a substantial change to the depth and spatial distribution of waterholes post European colonisation. The trajectory and magnitude of change is spatially variable and closely aligned with the presence or absence of low-level weirs. These structures create artificially high-water levels immediately upstream of the weir structure, which has increased waterhole depths for approximately 40 % of the river. In contrast, waterholes located outside of the weir pool influence have experienced a significant decline in maximum waterhole depths (median decline of 1.6m). This has resulted in fewer deep waterholes, which is also associated with an increase in the distance between the remaining deep waterholes (i.e., those deeper than 4m). In some cases, those distance have more than doubled. A change to the depth of waterholes and their spatial distribution has serious implications for hydrological connectivity, water quality, habitat availability and for the long-term presence and persistence of waterholes in the landscape. </p> <p>The decline in waterhole depths observed on the Barwon-Darling has been attributed to an increase in the rate of sedimentation associated with anthropogenic landscape change. Anthropogenic landscape change has increased the delivery of sediment to the river whilst reducing the capacity of the river system to transport sediment. Although sediment can originate from a range of sources, this study focused on the potential contribution from alluvial floodplain gullies. Floodplain gullies were found to be a prevalent feature within the catchment, with their presence increasing over the past 50 years. Over 4000 gullies were recorded, impacting an area of floodplain of approximately 148 million m2 and with a combined gully length of 2364 km. The total volume of sediment originating from these gullies is approximately 168 million m3, which is magnitudes higher than previous estimates for gully erosion in the Barwon-Darling catchment. However, given the gullies in this study have been cut into floodplain alluvium it was assumed that a sizable proportion of the fine sediment exported from the gullies would have remained in suspension and therefore transported some distance from the original source gullies. This was supported through statistical analysis that showed no predictive relationship between the total volume of gully derived sediment delivered to a reach and the magnitude of change to waterhole depth in that reach. The less mobile coarse sediment fraction would, however, be more likely to contribute to the shallowing of waterholes and to the expansion of in-channel feature such as bars and benches.</p> <p>Like many dryland rivers, the flow regime of the Barwon-Darling River has been modified as a result of water resource development. The abstraction of water, interception of floodplain flows and storage of water within in-stream impoundments has substantially altered the frequency, magnitude, and duration of flows. These changes have compromised the capacity of in-channel flows to entrain and transport sediment, reducing opportunities for sediment to be conveyed throughout the system. The number of events capable of entraining sediment has declined from 23 to 48 % across all sediment calibres (i.e., coarse sand, coarse silt/fine sand, fine silt/clay), whilst the frequency and duration of flows conducive to sediment deposition have increased considerably. Overbank flooding has halved, limiting the opportunities for lateral sediment exchange with the floodplain. During periods of low flow, the longitudinal transport of sediment is limited by low-level weirs. Collectively, these modifications increase the likelihood of sediment being retained within the river channel and, as such, increases the opportunity for within-channel sedimentation and the in-filling of ecologically important waterholes.</p> <p>Fish populations were used to examine the influence of waterhole depth on dryland river ecology. In particular, the focus was on determining if access to deep water habitat within the broader waterhole setting would influence fish assemblage patterns. Waterhole depth is an ecologically important variable as it influences habitat availability, water quality, density-dependent biotic interactions, and waterhole persistence. As such, waterhole depth was expected to shape fish assemblage patterns in the Darling River, however, its affect was thought to be difficult to isolate given the association that water depth has on creating habitat complexity through the inundation of in-stream wood and other habitat features. As a result, this study investigated the independent and interactive effects of both waterhole depth and habitat complexity on fish assemblage patterns. The presence and size of instream wood was used as a surrogate for habitat complexity. The results suggest that waterhole depth does not directly influence fish assemblage patterns, nor does it have an interactive effect with habitat complexity. A significant relationship with in-stream wood was observed, but only when native fish assemblages were considered. The inclusion of exotic species had a homogenising influence due to the relatively even distribution of common carp across sites of varying complexity. Despite this outcome, depth remains a critical factor in dryland rivers as it ensures the presence and persistence of waterholes within the landscape and creates important fish habitat through the inundation of instream wood and other habitat features.</p> <p>The findings in this thesis enhance our understanding of the trajectory and magnitude of geomorphic adjustment that can occur on a dryland river as a consequence of anthropogenic landscape change. This study has shown that the threat to dryland river waterholes is not limited to any one issue but is instead the culmination of a broad range of disturbances within the catchment (i.e., land use on the adjacent floodplain, modifications to the flow regime, in-stream infrastructure, climate change). Often though, it is difficult to isolate the impact of individual activities given the large spatial and temporal scale at which human disturbance has occurred. As such, it is imperative to establish an integrated approach to managing sediment on dryland rivers, which could include landscape management through gully remediation and grazing management, the provision of environmental flows and the removal of redundant in-stream infrastructure. The results from this study have relevance to dryland rivers globally, highlighting the significance of these important and widespread rivers.</p>