The River Widens

I’m almost as much interested in the motion of matter on the surface of the Earth as I am in the motion of rocks in space. I’ve got a good model of the latter, but not the former. And lots of things that happen on the surface of the Earth puzzle me. More or less everything, in fact.

Rivers rather fascinate me. I got interested in them when I started sitting by the river outside the pub I used to sit inside until the smoking ban drove me outside. I count it as one of the only benefits of the smoking ban that it drew my attention to the river Otter in Devon. Eventually I visited its headwaters in the hills, and its mouth where it discharged into the sea.

I can understand why rivers flow: they’re rolling downhill.  I can also understand that as they flow downhill they scour away material from the riverbed beneath them, and carry it downhill to the sea, and deposit it in the sea. And as they scour away material, they form river valleys where almost all the water is retained in a single river, with tributaries descending into it from the surrounding hills. There was one such little tributary that entered the river Otter right next to the pub, and formed one of the sides of the pub grounds.

What I don’t understand it why rivers which have many tributaries at their headwaters should fan out at their mouths into deltas with multiple distributary rivers. Why don’t they just flow straight into the sea? The river Otter does actually flow straight out into the sea, but I think that this is only because it has been blocked with a mole built across its mouth that forces it through a narrow exit. I think the Otter used to have a delta long ago, and the mole is wholly artificial, and man-made.

The sort of delta I’m thinking about is the Nile delta, shown at right, flowing from south to north into the Mediterranean sea. Egypt is mostly a very dry, sandy country, and nothing grows anywhere except along the banks of the Nile, or along the banks of the distributaries that fan out at its mouth.

The Nile starts fanning out into the delta at Cairo, some 180 km from the sea. At Cairo the river is about 400 metres wide, and maybe 10 metres deep, and it has a flow rate of 2,800 cubic metres of water per second. Given that width x depth x water velocity = flow rate, the velocity of the river is 0.7 m/s or 2.5 kph.

According to the BBC, deltas form in this manner:

Formation of a delta:

  • A river carrying sediment reaches the sea or a lake.
  • It loses energy and deposits material.
  • The sediment may be sorted as the heaviest material is deposited first.
  • Over time, more and more sediment is added.
  • If the tides are strong enough the sediment will be washed away. If not, it will build up a land mass (delta) at the mouth of the river.

Does that mean that the sea used to reach as far inland as Cairo, and that the entire delta has been built up as the river has entered it and deposited sand and stones on the sea floor? If so, then Cairo lay within a conveniently delta-shaped bay, into the tip of which the river Nile conveniently happened to flow. And the same must be true of any number of other river deltas.

Is this likely? It seems to me to be rather improbable. It seems more likely that the original coastline fell roughly along a line connecting the other two points of the delta (shown dotted in red), and there never was a delta-shaped bay waiting to be filled with alluvial sand.

So how else might the delta have been formed?

Perhaps it’s that rivers don’t actually suddenly reach the sea at a single point, and come to a stop, but they instead transition gradually from river to sea. As the moving river (orange) flows down towards immobile sea (blue), there is a transition zone of height H and length L in which moving river water gradually becomes mixed with motionless sea water. And at any point in this transition zone, the speed of the river depends upon how much of it is made up of motionless sea water and moving river water. So the river gradually slows down.

And as it slows down, the river must widen if it is to continue to maintain its overall flow rate.

If so, the Nile must enter its transition zone at Cairo, and begin to slow (and widen) at that point, and keep on steadily widening until it disappears completely in the sea at the coast. The Nile delta is not formed by the deposition of sand in the sea, but by the widening of the river in the transition zone between the two. As it widens, the river may also split into separate rivers, which would in their turn slow and widen and split into further separate rivers.

Can we check this? According to this link, Cairo is 23 metres above sea level.  But Cairo is built well above the Nile, most likely because the Nile used to regularly flood to a height of 7.6 metres. I estimate from the buildings behind the bank on the photo at right that the sloping river banks are between 8 and 12 metres high. And since we know the river is about 10 metres deep, it means that the riverbed is 18 – 22 metres below Cairo. So it looks like the Nile will indeed enter its transition zone somewhere near Cairo.

The height of the Nile floods was measured by a nilometer:

In the center of the pit a marble, octagonal column with a Corinthian capital that rises from its depths surmounting a millstone. At the top there is a wooden beam spanning the Nilometer. To measure the Nile flood, this column is graded and divided into 19 cubits (a cubit is slightly more than half a meter, and hence, it was capable of measuring floods up to about 9.2 meters). The flood that this Nilometer measured was both important to the rulers of Egypt as well as the whole population. An ideal flood filed the Nilometer up to the sixteenth mark and less than this could mean drought and famine. On the other hand, if the measurement exceed the 19 cubits, a catastrophic flood was at hand. In the days prior to the expected flood, this column would be anointed with saffron and musk in order to help induce a good water level.

Another test would be to see what the ground beneath the delta was composed of. If it was composed of alluvium to a depth of many metres (whatever the sea depth once was), then my explanation is wrong. If it’s right, the delta rivers will be flowing on rocks no different from those upstream, and only islands in the delta will be composed of alluvial sands.

Much the same argument might be employed to explain why some rivers have widening estuaries. In these estuaries, the water will not be motionless, but flowing slowly towards the open sea. The river once again has to widen as it slows, but this time as a single body of water, rather than a set of discrete distributaries.

Roxy Music: “The river widens, growing stronger through the years”


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14 Responses to The River Widens

  1. waltc says:

    On yesterday’s Israeli army blog (sorry I missed it), it’s not just a question of divisiveness and morale, it’s a question of mental and physical efficiency. Recall this article that I believe you once helped to translate (and which only covers a portion of the evidence). It’s not for nothing that soldiers smoke and add to it, if wounded, smoking eases pain.

  2. Smoking Lamp says:

    Interesting post. Rivers and deltas are fascinating places. I grew up near New York harbor so waterways have always been an interest. Later on I grew to love the Mississippi delta.
    I saw an interesting article today. “SMOKERS ARE THE LAST NICE PEOPLE ONLINE: Taking a cigarette break on the smoking internet” at The writers would have found the same here and a few other places we know. The antis must be seething.

    • Joe L. says:

      Thanks for the link, SL. That article is mostly accurate with regards to the positive interactions of smokers with one another (we smokers truly are very kind and supportive of one another; but I think that can probably be said for members of all groups of persecuted minorities throughout history). However, I feel like the author takes a patronizing tone and depicts smokers as some kind of circus freak show, especially near the end. Take this paragraph, for instance:

      There are many cigarette reviews on YouTube. A lot of them (all of them, if I am honest), are mesmerizingly odd. Some of the people doing them make me feel sad, and worried about their quality of life or their undoubtedly troubled past. They are a bit twitchy and strange, making wild claims about the government’s suppression of the free circulation of cigarettes throughout Europe.

      The author sets this tone in the opening paragraph, displaying her bias by denigrating smokers with some obligatory unsourced parroting of antismoker propaganda:

      The online people-who-love-to-smoke community is one of the most supportive and kindly corners of the internet I have ever encountered, especially for a group entirely preoccupied with the abetting of a habit that is the leading cause of preventable death worldwide. I’m not sure if this is a particularly illuminating observation, whether it says something profoundly terrible about humanity and where it is headed, or whether this is something we should find solace in.

      The author, who blindly believes that smoking is the “leading cause of preventable death worldwide,” finds it shocking that smokers can actually be friendly and supportive. She is one of many who has been brainwashed to believe that all smokers are rude, selfish assholes.

      I feel like anything positive this article might convey about smokers is severely overshadowed by this constant, not-so-subtle smoker-bashing. Part of me wonders if this was intentional, in order to draw attention to and discredit online pro-smoking communities such as this one.

      • nisakiman says:

        And yet it would seem that she is a smoker herself, since when talking about the airport smoker’s website, at one point she says:

        …I only came across it six months ago when trying to figure out if I could smoke in Doha airport…

        It’s quite possible that without paying lip service to the anti-tobacco mob, she wouldn’t have got the article published. Because it does appear that just about every article about smoking, whether or not it is pro-smoking, must contain one or two Tobacco Control soundbites denigrating either smoking or smokers, or both..

    • Frank Davis says:

      I’ve added to my blogroll. Could be useful for some people.

      • nisakiman says:

        Good idea. I’ll do likewise. It’s also worth noting that the guy who runs the site asks for feedback from people so that he can expand the database. So all you travellers out there, take notes, take pictures and let him know!

    • Emily says:

      I agree with Joe L.’s comments and also note that the article makes no mention of smokers’ or tobacco rights and people who actively campaign for them. It is all about smokers being “nice” on the internet which I suppose is the purported aim of the article, but it seems very superficial and insubstantial. The author mentions the word “aggrieved” at least twice when she does broach the fact that smokers might occasionally (rightfully) complain about their treatment.

  3. Mick Walker says:

    As the tide goes in and out, the area where the flow stalls moves with it.I think this has a lot to do with delta formation?

    My windows look onto a shallow sandy beach. When the tide goes out I get a miniature speeded-up model of river formation and meandering. Every day is subtly different. It’s endlessly fascinating!

    • Frank Davis says:

      There’s very little tide in the Mediterranean, because it’s an almost entirely enclosed sea. But some deltas are called Tidal Deltas, though I’ve yet to see an explanation of what effects tides have. But you’re right that what I call the transition zone will be moving around. At the moment I can’t figure out what effect that might have.

      Which reminds me of something I came across yesterday:

      All the tides we experience on Earth are generated in the Great Oceans as a result of their response to the tide raising forces generated by the gravitational attraction between the Earth, the Moon and the Sun.

      If the natural resonance period of the body of water matches the frequency of the tide raising forces then it will respond accordingly and a tide is generated. For example, the Atlantic has a natural period of resonance in the order of 12.5 hours and so responds vigorously to the twice daily (semi-diurnal) tide raising forces. The Pacific on the other hand has a natural period of resonance closer to 25 hours and so responds vigorously to the daily (diurnal) tide raising forces. There are, of course, exceptions to both in unique locations in either ocean but to understand why you would need to consult any authoritative textbook on tidal theory.

      I suppose everything has a resonant period. I wonder how you work out what they are.

      • nisakiman says:

        You’re correct that tidal movement in the Med is insignificant, but there is is a fluctuation of at least a metre in sea level depending on barometric pressure.

    • Frank Davis says:

      Did this get a mention?

      Sunday on CBS’s “Face The Nation,” Secretary of State Rex Tillerson said President Donald Trump would be open to remaining in the Paris climate accord.

      The Guardian said:

      His maiden address was unlike any delivered by a US president, and when it was over a sense of incoherence and menace hung in the air.

      But that’s what I expect the Guardian says about everything he ever does. In response to my own question, it reported:

      He did not even bother to mention climate change, generally seen as the greatest threat to the planet at the UN, but viewed as a liberal hoax by much of Trump’s political base – a view he has encouraged over the years.

      • RdM says:

        But, watching Al Jazeera tonight, Macron said no, no negotiations.

        So it seems he’s sucked in, just another puppet.

        As for “climate change”, the buzzword for what might have been initially and sensibly discussed between the two poles of CAGW Alarmists & CAGW Skeptics, except that the alarmist propaganda has seemed so overwhelming, Trump was on the right track and supported by sensible scientists to get out of or refuse the Paris climate “accord” scam.

        I hope he doesn’t waver, or if he does, hear again the sensible side and swing back…

        The site might seem a bit overwhelming, their audience is people who are already reading lots, and not only does each linked article have lots of comments, but you have to scroll down the main page quite a way to even see the sidebar on the right to other blogs… fairly grouped between warmists and alarmists, and in between…

        Yet here’s a straightforward essay on the benefits of CO2 & science you might find worth a read:

        And again, has continually updating comment on headlines

  4. Pingback: On The Beach | Frank Davis

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