Riverland communities are closely connected to the River Murray and rely on it for economic, social, cultural and recreational well-being. Over time, the health of the River Murray has been significantly affected by river regulation, de-snagging, overuse of water resources and drought. A re-snagging project has begun to rebuild in-stream woody habitat to restore the ecology of the River Murray and support local fish populations. What are snags?
‘Snags’ are in-stream woody habitat that may be made up of tree root masses, trunks or limbs. Snags naturally form when trees fall into the river following strong winds, flooding or drought. Snags create variation in stream flow required by native fish, and provide habitat for a range of aquatic wildlife including native fish, macro-invertebrates and crustaceans. Snags are favoured habitats for native fishes providing suitable conditions for breeding, feeding, shelter and rest. They also play a critical role in providing a place for bacteria, algae and micro-organisms to grow.
A natural snag in the Murray River (Image courtesy of Emma Pink) Why re-snag the River Murray?
Historically, the South Australian River Murray was de-snagged to aid boat navigation, reduce flood damage and beautify the river. In the 1990s, this practice was abolished as it became clear that snags play an important role in creating riverine habitat.
Habitat degradation due to historical removal of snags is thought to have greatly contributed to declines in native fish populations, which have reduced by more than 90% across Australia since European colonisation. Many Australian species now experience restricted habitats which are reduced to small and highly fragmented populations, leaving them increasingly vulnerable to extreme environmental events.
Without snags, there is no natural way to replenish native fish populations.
The paddle steamer ‘Grappler’ removes snags from the Murray River in approximately 1860 (Image courtesy of the State Library of South Australia [PRG 1258/1/1396]) What are the benefits of re-snagging?
Additional snags provide more in-stream woody habitat, which supports bigger populations of aquatic life and increases life in the River Murray
Increased habitat for bacteria, algae, micro-organisms, native fish, invertebrates and crustaceans
Indirectly support water birds and predatory marsupial populations
Increased food sources for native fish
Support native fish breeding by providing additional surfaces for eggs to be attached to
Protect native fish populations by providing a place of refuge from larger predators
Provide a food source for larger predators that like to ‘sit and wait’ for their food
Snags that are close to each other support a bigger population of native fish which will be more resilient to changing climate conditions
Snags will help sustain native fish populations into the future for recreational fishing
How do we know that it will work?
Monitoring at the introduced snags, located adjacent to Banrock Station, has found a range of different native species using new snags in the same way as natural snags.
Native fish have already been observed using the new structures including:
juvenile Murray cod
Multiple re-established snags in the River Murray (Image courtesy of Jon Ortlieb) What work has been done?
This project has introduced 47 snags along the River Murray.
In 2020, 23 new snags were added including:
13 snags downstream of Lock 3 near Overland Corner
10 snags downstream of Lock 4 near Bookpurnong.
In 2019, the pilot project added a total of 24 snags at 2 sites:
4 snags at the initial test site downstream of Lock 4 near Bookpurnong
20 snags downstream of Lock 3 near Banrock Station wetland.
Re-snagging efforts show multiple snags adjacent to Banrock Station wetland (Image courtesy of Tumi Bjornsson) How were snag habitats rebuilt?
Rebuilding this important habitat was a complex process requiring the careful selection of tree type, size, shape, position and river location to maximise ecological benefits.
Sourcing the right type of wood can be a barrier to re-snagging efforts. The tree must have been recently felled so that the snag will sink, rather than float away. Identifying approved and necessary tree removals is key to re-snagging efforts.
Snag materials were sourced through the Katarapko Floodplain project which was completed in June 2020. Trees removed for construction works were re-introduced back into the environment in the form of snags for in-stream habitat. All 47 re-established snags were sourced in this manner.
Construction of snags near Overland Corner (Image courtesy of Tumi Bjornsson)
This re-snagging project forms part of the $155 million South Australian Riverland Floodplains Integrated Infrastructure Program (SARFIIP) to improve the health and resilience of Riverland floodplains. SARFIIP is funded by the Australian Government through the Murray–Darling Basin Authority and implemented by DEW in partnership with SA Water.