Death by a thousand (head)cuts? Alluvial gullies and geomorphic condition of the lower Darling Baaka

The lower Darling Baaka River, in far western NSW, is under stress from a range of local and catchment-scale pressures which are manifesting in marked declines in river health. A key biophysical threat to the Darling Baaka is the ubiquity of alluvial floodplain gullies which are thought to be significant source of sediment to the river causing within-channel habitat degradation and infilling of waterholes. Significantly elevated suspended sediment concentrations are a persistent water quality concern for the Darling Baaka. Understanding the key sources of this excess sediment is critical in order to guide management practices to reduce sediment loads in the river. Alluvial floodplain gullies are a poorly understood source of sediment to the river and have not been investigated in detail in the lower Darling Baaka. Other biophysical threats identified were a lack of river red gum recruitment (affecting bank stabilisation and habitat provision) and river regulation and impoundments/weirs (reducing geomorphic diversity in impounded reaches).

Geomorphic condition of the lower Darling Baaka was assessed using the River Styles framework in conjunction with novel mapping and analysis of geomorphic features including alluvial gullies (i.e. headcuts in the banks). Overall, the lower Darling Baaka and its major anabranches are in moderate to good geomorphic condition, although legacy impacts associated with historical overgrazing, desnagging and river regulation have left an imprint on the system, including elevated erosion rates and reduced geomorphic diversity. Mapping highlighted that alluvial gullies are abundant in the study area, though their density and size vary significantly. Two main types of gullies exist with differing formative processes and implications for sediment delivery to the Darling Baaka. Gullies formed in the black clay floodplains are generally associated with the waning stages of flooding and return flows to the river. In contrast, gullies that extend from higher elevated red, sandy soils are caused by relatively high energy runoff during localised storms.

Between Wilcannia and Weir 32, alluvial gully densities range from ~3.2 to 7 gullies/km of river. The downstream reaches of the Darling Baaka River have substantially lower alluvial gully densities ranging from ~0.8 to 1.5 gullies/km of river. Previous work on the upper Barwon-Darling showed densities ranging from ~3.1 to 3.8 gullies/km. Altogether, it is clear that alluvial gullies are now a ubiquitous feature of the Darling Baaka that require further investigation to understand the factors controlling their formation and development, as well as the impacts on sediment loads, water quality, and aquatic ecosystem function.

The Darling Baaka River Health Project was delivered by the NSW DCCEEW Science and Insights Division in partnership with the NSW EPA between September 2023 and June 2025. The project aimed to measure the health of the lower Darling Baaka and inform on the river recovery extensive flooding in 2023. The project integrated modelling, spatial analyses, and an intensive field sampling campaign to assess the overall health of the river from Wilcannia to Wentworth, including the Great Darling Anabranch.