6 min readBengaluruAug 13, 2022 01:08 PM IST
First published on: Aug 12, 2022 at 01:24 PM IST
The mouth of the river, where it meets the ocean, is the scene of much action. As the river reaches the ocean, it slows down and deposits sediment. If the river is carrying enough sediment, deposition can create new land, pushing the river mouth further into the sea to form a river delta. The delta, therefore, is the result of the equilibrium between the ocean and the river.
However, rising sea levels and human intervention in rivers are rapidly changing these dynamics. A study published last month, by geologists at the Cailfornia Institute of Technology, examines global delta systems and how rising sea levels and human disturbances to rivers (e.g. the building of dams) are likely to affect them.
The land created by the deposition of sediment by the river is known as a ‘lobe’. When rivers abruptly change course, a phenomenon known as ‘river avulsion,’ one lobe is abandoned for another. In other words, sediment deposition stops at the ‘old’ lobe and is diverted to another location, creating a new ‘active’ lobe. River avulsion is an abrupt event, and occurs at different rates in different settings: Coleman et al. (1998) reported avulsions taking place only once in a millennium in the Mississippi River, USA, while Ganti et al. (2014) showed they occur almost once in a decade in the Yellow River, China. The land created by the abandoned, inactive lobe inevitably retreats as the river-sea interaction is then tipped in favour of tidal action by the sea, submerging the land earlier created.
Austin Chadwick and his colleagues at Caltech first used mathematical models to predict land-losses on inactive lobes in deltas across the world. Then, they supplemented these models by laboratory-based experiments that simulated the river basin, sediment supply and rising sea levels. The entire process was filmed by overhead cameras. Water in the experiment was coloured with a fluorescent green dye, so that areas under and above water are easily distinguishable in ultraviolet light.
As the simulated sea level-rise was introduced in the experiment, the inactive shoreline – quite predictably – began to retreat. Importantly, ‘[the study] found that the amount of shoreline retreat was actually greater than expected based on traditional forecasting techniques, which do not account for the fact that rivers change course over time.
When rivers change course, they temporarily build land that later drowns after the next change in course. This process leaves less of the rivers sediment resources available to sustain the persistently dry and habitable land, leading to more extensive land loss,’ Dr Chadwick, the lead author of the paper, explained in an email communication with indianexpress.com
Based on their results, Chadwick et al. developed a new technique that accounts for rivers changing course during sea-level rise. Their revised forecasts suggest that deltas worldwide will require more sediment to sustain land through the coming century. The study acknowledges that in both the mathematical model as well as the experiment, the land abandoned by the river was not further eroded by waves or tides— an optimistic scenario which is not necessarily the case in reality.
In the event of rising sea levels, the study concludes that deltas are losing land at an unprecedented rate and that they will need more sediment influx than they have at present, in order to ‘sustain’ their dry-land area and maintain the delta plain. For most of the delta plains under their purview – the Mississippi, USA; Orinoco, Venezuela; Parana, Argentina; Rhone, France; and the Rhine, Germany – need anywhere between three to five times of the current sediment supply to prevent being faded into oblivion – either due to human disturbance on the river course or sea levels. The Danube delta system in Romania/Ukraine might even need ten times as much.
Naturally, not all deltas are expected to show the same response to rising sea levels: Chadwick et al. (2022) show that the deltas of Yellow and Brahmaputra have ‘enough sediment to maintain their current land area.’
However, not all is lost. One, nature has its own way of combating. As sea levels rise, it brings about a negative feedback mechanism in rivers. In such a scenario, while the rising sea level would certainly accelerate the rate of land loss, the negative mechanism would cause river avulsion to happen more rapidly. There is a ‘relative increase’ in the sediment input, and a reduction in the ‘time that lobes spend inactive.’ But, this can mitigate the impact of land only to a certain extent. The balance is tipped if sea levels rise faster than the avulsion frequency of rivers.
Two, just like human rerouting of river courses can dampen sediment supply in the delta, they can also be deployed to rescue the delta. According to a 2020 report, human engineered diversions have succeeded in reclaiming land back, especially in the case of the Yellow River delta. The study suggests that at least for the Mississippi and the Orinoco delta plains, similar interventions done consistently can yield the same results. Even partial diversions can, at least temporarily, increase the number of lobes active at a certain point in time and increase the overall land gain in the region.
The author is a research fellow at the Indian Institute of Science (IISc), Bengaluru, and a freelance science communicator. He tweets at @critvik
Studies suggest deltas are losing land at an unprecedented rate and will need more sediment influx. (Image Source: Pixabay) 
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