How human impacts on geomorphology echo through the centuries: explaining the declining flow capacity in Australia's most important river, the Murray.

The River Murray is Australia’s largest and most important river.  It carries water for irrigation and city supply from the mountains in the east, semi-arid west.  The river crosses a series of low-angle alluvial fans (distributive fluvial systems) in its 1500 km path.  The Barmah Choke is a narrow section of the River Murray’s channel, formed by a Holocene river avulsion, that restricts flow capacity from the upper to lower Murray River.  The flow capacity of the Choke has declined nearly 20% since the 1980s, which imposes a constraint on regulated flows supplied down-valley, and is constraining economic development downstream.  Whilst there could be multiple geomorphic factors explaining the decline, we demonstrate that the most likely cause is a sheet of coarse sand shallowing the river, much of which originated from historical gold mining and catchment erosion in late 1800s.  The sheet of sand is reducing depth diversity, filling pools up to 5m deep, and degrading the ecological and cultural values of the river.  The whole length of the river through the Barmah Forest has shallowed over the last 30 years, aggrading by 1.9 m at the upstream end and 0.70 m in the most downstream section of the Choke (about 10% decrease in area).  The total volume of sand stored between Yarrawonga and Barmah is over 20 million m³, and the average total annual sand load transported into the 82km long Choke ranges between 130,000 m³ in a normal flow year to 500,000 m³ in a flood year (which translates into 2 – 9 cm of aggradation per year).  Surprisingly, overall, long-duration regulated irrigation flows have decreased the rate of sand transport through the Choke.  Decreasing downstream flow capacity across the Barmah Fan is a natural characteristic of this type of distributive fluvial system, but the sand sheet is accelerating the rate of decline.  Without intervention, conveyance through the Choke will inevitably continue to decline until the river avulses into a new channel.  A range of solutions are explored, but all are extremely expensive.