Rising sea levels and human intervention in rivers are rapidly changing river-dynamics across the world. Read more below in today’s edition of Digging Deep.
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.