Live Oral Presentation in person as part of National Virtual Conference AFSS Conference 2020

Is there a link between gully erosion and sedimentation in the waterholes of the Barwon-Darling River? (#10)

Marita Pearson 1 , Michael Reid 1 , Darren Ryder 1 , Cara Miller 1
  1. University of New England, Dubbo, NSW, Australia

Waterholes or deep pools capable of retaining water in the absence of flow are a critical feature of dryland rivers, which are increasingly being threatened by accelerated rates of sedimentation.  Sedimentation causes the infilling of waterholes leading to a decline in geomorphic complexity, habitat quality and waterhole persistence.  In dryland rivers the source of sediment is often not well understood and is often assumed to come from upstream sources at some distance to the waterholes of concern.  To address this assumption, this study quantifies the frequency of alluvial gully erosion and the magnitude of gully derived sediment contributing from the adjacent floodplain to a dryland river in south-east Australia.  Alluvial gullies along the Barwon-Darling River were observed at 3-4 gullies per kilometre, with a median gully volume of 3873 m3.  The number of gullies has increased on average by 44 % since the late 1960s, which is attributed to the hydrological influence of low-level weirs.  The volume of gully derived sediment contributed to the river also appears to be linked to these large, static weir pools.  However, more work is required to investigate the combination of factors driving gully expansion and to determine if these factors vary from those driving gully formation.  A predictable relationship was not observed between sediment contribution and change in waterhole depth.  Instead, the distance of a waterhole from its closest upstream tributary had a greater influence on changing waterhole depths.  However, it is suggested that the processes driving sedimentation are operating at a spatial and temporal scale larger than explored in this study.  Consequently, the depth of present-day waterholes may have been impacted by sedimentation, but its effect may be masked by alternative sources of sediment, artificially high-water levels created by low-level weirs or by a spatial and temporal lag between sediment delivery and deposition.