The Shape of Water


Dr Anjani Ganase looks at the influence of rivers, the creation of deltas and the places where the sea comes in.

River deltas are formed where rivers meet the ocean. The dynamic water movement driven by the forces of waves, tides and river flow all shape deltas and result in unique wetland ecosystems that live within the narrow margins between the land and sea. As seasons change, as storms surge and tides turn, we also observe shifts in the distribution of soil, vegetation and aquatic life that utilise the confines of the delta. Rivers bring soil from inland, and the sediment settles out as the river slows where it nears the ocean. This is because of the flatter, even terrain that occurs when the river reaches sea-level that allows the river to widen as it is no longer is restricted by the river banks and can spill over a low lying flood plain. Apart from the river flow, ocean waves, tides and even the spin of the earth (coriolis effect - which is the deflection of water flow because of the spinning of the earth) drive the patterns of sediment settlement in a river delta.  


The Ganges-Brahmaputra delta as seen from space is over 40,000 square miles in size. Image courtesy Jacques Descloitres, MODIS Land Team

The shape of the spread is the result of the forces of tides and waves that brace the flow of the river water and relocate the sediment locally. The classic shape of a delta resembles the Greek symbol it’s named after, a delta or a triangle. It is also described as a (alluvial) fan made up of a series of channels constructed by the build up sediment in certain localities and erosion in other areas. However, not all deltas take this shape. One classic exception is the Mississippi delta that from space appears as giant bird’s claw as it extends into the Gulf of Mexico. The flow of the river out to sea pushes into this shape, which has become more prominent over time, owing to the net increase in the sediment along these channels. While the pattern of outflow from the Mississippi river has been this way in recent time (last 600 years), and largely fixed in this pattern owing to development; over the last 10,000 years, the Mississippi has frequently changed course within a 300 km range, accounting for its wide expanse.
 
The Mississippi Delta sticks in the Gulf of Mexico like a bird’s foot (2001). NASA image created by Jesse Allen, using data provided by the University of Maryland’s Global Land Cover Facility.

The ecology of river deltas and the fertility of the soils that collect along deltas have been known by man for centuries, and deltas have been used as agricultural lands that supported many civilizations. These include the ancient civilizations that surround the Nile delta in Egypt; the Tigris and the Euphrates that flow into the Persian Gulf, and the Ganges-Brahmaputra delta in Bangladesh. Some of the largest deltas around the world include the Amazon delta in Brazil, the Ganges-Brahmaputra delta in Bangladesh, the Huong–he in China and the Mississippi River in the USA. As comparison of the delta sizes, the Ganges-Brahmaputra delta is about 40,000 square miles in area, which is about the size of Iceland. The sediment load discharged from the delta has been carried a great distance, over 2,500 km from the origin of the Ganges River, the Gangotri Glacier in the Himalayas. 

 
At night, city lights outline the Nile delta from space. Astronaut photograph ISS025-E-9858 was acquired on October 28, 2010. ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center

In our marine backyard, the Orinoco river delta, which flows out to sea just south of Trinidad and Tobago, is larger than the area of Trinidad; and its discharge is felt all around Tobago. For a delta this size, it makes sense that the Orinoco river discharge has such an influence on our marine environment and is responsible for significant sediment collection in the Gulf of Paria. Any disturbances upstream of the Orinoco will not just affect the dynamics of the Orinoco delta but the water that flows around Trinidad and Tobago.

With the development of human civilizations around deltas, changes in land and water use in turn affect the structure of deltas. In a study published this year, a group of scientists were able to model the impact of human activities, namely damming and deforestation on the changes in the shape of 11,000 river deltas worldwide using historical satellite imagery. They found prominent trends with regard to the changes in sediment deposition patterns in the deltas that resulted in either land gain or loss. Additional gains in land around deltas were related to deforestation. Deforestation upstream destabilises sediment and results in an increase in sediment flux in the river and consequently greater deposits at river deltas. Conversely, the damming of rivers upstream often resulted in the reduction of sediment flow downstream that results in the land loss or alternatively a shift to tide driven deltas where the incoming tides infill delta channels.

While Trinidad and Tobago do not have any major deltas, we are influenced by the South American rivers, the Amazon and the Orinoco. We can also see the influence of river outlets on both Trinidad and Tobago: consider the wetlands of the Caroni, and the many rivers that drain into Nariva. Tobago’s Main Ridge gives rise to several rivers, many of which have been named for the bays into which they drain: Bloody Bay River, King’s Bay River, Roxborough, Goldsborough, Castara, and others which become a stream in the dry season.  Let the giant deltas teach us this: the natural dynamics that occur at river mouths should be allowed to persist in order to sustain healthy wetland ecosystems. There must be active management of upstream activities to reduce deforestation and maintain water quality. Awareness of river flow should be an element of coastal development planning to reduce the risk of flooding.

References
Nienhuis, J. H., Ashton, A. D., Edmonds, D. A., Hoitink, A. J. F., Kettner, A. J., Rowland, J. C., & Törnqvist, T. E. (2020). Global-scale human impact on delta morphology has led to net land area gain. Nature577(7791), 514-518.





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