Wednesday, August 25, 2021

In Praise Of The Estuary

Be sure to take some time out and watch his lovely documentary on the Mulki Estuary by Dakshina Kannada Wildlife team.

Email subscribers who can't see the embedded video can watch it here - The Mulki Estuary.

This estuary has formed at the confluence of the Sambhavi and Nandini rivers (the documentary wrongly calls it the Gurpur river) on the Karnataka coast. Estuaries are particularly rich ecosystems. The confluence of fresh and saline waters, changes in water temperature and turbidity, nutrient supply by the river and coastal upwellings, the rise and fall of tides, and a meeting and interference of shore parallel and river currents create conditions suitable for a thriving biosphere. The documentary brings these aspects out beautifully in its capture of coastal landforms and varied wildlife.

Estuaries are of great interest to geologists too. The present configuration of our coastline has developed relatively recently in the geologic past. Before 15,000 years ago, during the Ice Age, sea levels were  about 100 meters lower than present. The coastline would have been tens of kilometers to the west of today's shore, and rivers like the Sambhavi and Nandini would have carved a valley across this stretch of the continental shelf and would have met the sea at a far westward location. As the Ice Age ended and the earth warmed, the rising seas progressively inundated the continental shelf. The estuaries along the Indian coast as with other coastlines are really drowned river valleys.

Coastal environments like beaches, mud flats, sand bars, mangroves, lagoons, and marshes, are in a state of constant adjustment to the movement of tides and currents, sediment supply from rivers and their distribution and deposition, and the impact of vegetation in stabilizing these features. Shore currents and sediment supply have an outsize effect on the evolution of the coast. One interesting example is found at Mangaluru, just south of Mulki, where another estuary has formed at the confluence of the Gurpur and Netravati rivers. The map shows this coastal setting. 

A geological investigation by B. R. Manjunatha and K. Balakrishna has shown that the Gurpur river once flowed a shorter route to the sea (paleo-course outlined in blue). Starting a few thousand years ago, excessive sediment deposition blocked this channel and strong southerly currents began building a barrier spit, forcing the Gurpur river to turn south and flow parallel to the coast for a good 8 kilometers before eventually joining the Netravati river in a joint exit to the sea. The Netravati river too has shifted slightly northwards over time. A quick survey along the Karnataka and Maharashtra coast show many similar situations where the river makes an abrupt turn and flows parallel to long sand spits before entering the sea. Do these rivers too have a similar history of course change? Its an intriguing observation.

Besides such adjustments to the coast due to currents and sediment deposition,  fluctuations in sea level too will change the prevailing geography. A sea level rise will push beaches and lagoons landwards, while a fall will cause rivers and deltas to extend seawards and bury past shorelines.

The estuary being a low lying region and a sediment trap preserves this history of sea level change. Geologists love to drill and take out sediment samples from the estuary bed and associated marshes and mangroves, because they contain, in its changing sediment and biological composition, clues to earlier environmental shifts. Understanding how past sea level change affected the coastal system is of great value in tuning our expectations and our preparedness of how the ongoing sea level rise will impact our present day coastlines.

I must confess that this post was inspired not just by this documentary but by my own memories of an estuary. During my student days, I had visited the coastal town of Malvan at the end of a small trek along the west coast of Maharashtra. A friend suggested we spend the day boating and exploring an estuary south of town. It turned out to be a most relaxing and enjoyable experience.

Taking inspiration from these children, we too rowed vigorously and covered quite a bit of distance. 

Malvan then was a small sleepy place and this meeting of river with the sea even more so. Not a soul was in sight, as hour after hour the waters of the estuary gently lapped against our boat. A salty sea breeze kept us refreshed. As the sun beat down upon us we finally decided to take a break. In search of something to eat and drink we scoured the shore for a small settlement. Up ahead in the middle of the estuary was our savior. 

 A luscious looking coconut island beckoned.

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