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”