River deltas and delta cities

Published in The Freeman (Philippine Star), Dec. 22, 2011

In the olden days, there are only 2 factors which determine the location of cities: military and economic. Feudal cities and empires have to defend their settlements from marauding attacks of others in the course of human history. But economics was also a potent factor in where cities evolved. Hence, cities almost always are built either at the mouth of rivers or where there is a safe harbor, especially in coastal locations or in island countries.

Why at the mouth of rivers? – a bit of facts in river hydrology. Rivers carry rainwater from a catchment area to the nearest body of water. Their lengths and shapes vary considerably, but all always carry sediments – bits of broken rock, sand or soil particles, scoured and carried by the riverwater flowing downstream. These are heavier than water but are carried as sediments by sheer force of the waterflow. When the flow slows down, the sediments are deposited in the riverbed and banks and when enough are accumulated, these form new land areas, sand bars, and “deltas” which are relatively very fertile, being accumulation of rich soil sediments. And since these are always saturated by water, fertile lands in a region is almost always besides rivers and streams. And since river flow is slowed down to a virtual standstill when a river empties into a sea or lake, the mouths of rivers are oftentimes the most fertile land where settlements evolved. These include, such cities as Tokyo, Jakarta, Hong Kong, New York, New Orleans, London, and in the Philippines, Manila, Cotabato, Butuan, Cagayan de Oro, and Iligan, the last two being the area of devastation caused by Typhoon Sendong early Saturday morning.

These cities are oftentimes called “delta cities.” When a river meets the sea, huge volumes of soil sediments are deposited at the mouth of the river forming fertile islands which oftentimes form part of the socio-economic system of a city. They are usually triangular, though they maybe other forms depending on the river and coastal flows, and that’s the reason why they are called “deltas” – which comes from the fourth letter of the Greek alphabet, which is simply a triangle. The very famous delta in the old days was the Nile River delta, in Egypt. There are large and small deltas as there are large and small rivers, but these have one thing in common – rich, fertile areas where cities grew fast due to agricultural abundance and easy navigation from inland econoies through the river systems.

Here in our country, deltas sometimes are formed a bit inland inside the cities but still along the rivers. These areas in our cities should be have been off limits to habitation since these are formed out of soft soil sediments AND prone to regular flooding, but informal settlement often mushroom in these “islands.” And every now and then, disaster strikes, like the flooding caused by Sendong last weekend, and the destruction is oftentimes more severe in these areas. Last week, Filipinos may not have an inkling where Isla de Oro was, but I’m sure everyone now knows where it is and what happened to it, especially in the severity of the destruction. Maybe we should put more time in learning from the past. Isla de Oro is today, what Isla Verde was 20 years ago. Or do we even remember Isla Verde? – which was similarly devastated by the great Ormoc flood of 1991. Check through satellite images and compare how Ormoc has long since upgraded its river flood control infrastructure. Flood control is good but I agree with President Aquino that Isla de Oro should not be inhabited. More than that, I hope we can go check other river deltas all over the country – these too should be off limits to habitation. I know this is only just a small part of the solution and future disaster prevention, but I think we have to start first on the most vulnerable.


Leave a Reply

Your email address will not be published. Required fields are marked *