Where does all the sediment go?

We're all familiar with the sediment on beaches - it makes great sandcastles! But what happens to the sediment that gets carried deeper into the ocean?

As rivers flow across the landscape they pick up and transport an incredible amount of sediment, ranging from large pebbles and boulders to very find sand and mud. As the rivers head down towards the sea, most of the very large material has been dumped out upstream. However, an enormous volume of sediment (around 8 billion tons every year) still makes it down to the sea. This is especially noticeable after storms and floods, when the heavy discharge of sediment from rivers can turn the coastal waters yellow or brown. But what happens to the sediment once the river reaches the sea?


Left: Flooding on the Rhone River in November 2002 caused high sediment discharge into the Gulf of Lions, as shown on this satellite image (NASA/ORBIMAGE).


Estuaries

When rivers reach the sea, the river flow slows down considerably, and loses energy. This means that the river is no longer able to carry sediment, and large quantities are dumped at the mouth of the river (estuary). What happens next depends a lot on the physical conditions in the estuary. If the currents, tides and waves are very strong, they will wash the sediment away from the river mouth, along the coast or out to sea. However, if these forces are quite weak, or if the river is carrying a very heavy sediment load, sediment can build up at the river mouth, forming features like lagoons and barriers. Eventually, the sediment buildup might be so great that a delta will form.


Deltas

If conditions are right, the river sediment is deposited on the seabed at the river mouth, creating a fan-shaped platform of sediment called a delta. As more and more sediment is brought down to the sea by the river, so the delta grows. Channels often form on the top of the delta, carrying more sediment to the very front of the delta where it piles up, causing the delta to grow outwards (prograde).

One of the most famous deltas is the Mississippi Delta, which covers a huge area on the margin of the Gulf of Mexico. The delta has formed at the mouth of the Mississippi-Missouri river system which discharges an average of 16,800 cubic meters of water per second, said to be the sixth largest river discharge in the world. The Mississippi Delta is so big that it extends right out over the continental shelf, allowing sediment to be transported over the delta directly onto the continental slope.

The Mississippi Delta is an example of a birds-foot delta, so called because of the way it looks when viewed from above, with lots of channels dissecting the delta. You can see from the photograph to the right that much of the top surface of the delta is exposed above sea level, allowing vegetation to grow on the sediment. Sediment records have shown that the Mississippi Delta has changed shape and position many times over the past centuries, but it is the only real example of a bird's foot delta in the world.



The Mississippi Delta, USA. Top satellite image by NASA/ORBIMAGE. Lower photo by John Simmons, © The Geological Society of London

Currents

If a delta doesn't form, the river's sediment is washed away from the estuary by currents, waves and tides. Most coastlines are affected by alongshore currents which pick up sediment and transport it along the coast. On many coastlines, longshore drift is a real problem caused by currents continually moving material along the coast. Many beaches around the UK have coastal protection methods to try to stop sand and pebbles being washed away. Where longshore drift is very active, a spit - a long, narrow strip of pebbles and sediment - may form. Chesil Beach in Dorset (left) is a good example of a large spit.


Left: Chesil Beach in Dorset stretches 29km along the coast of southern
England and has been formed by alongshore transport of pebbles.

Left on the shelf...

Most of the sediment transported to the oceans today comes to rest on the continental shelf less than 6km from the shoreline. Very little sediment is transported further than 30km from the shore. So, most of the sediment sitting on the outer continental shelf is old, dating back about 18,000 years to the last ice age when sea levels were much lower. It has been estimated that 80% of all the sediment in existence on Earth is sitting on the continental shelf!

But plenty of sediment still reaches the deep sea. As sediment piles up on the edge of continental shelf, it becomes unstable and avalanches down the continental slope towards the deep ocean basins. However, in many places the continental shelf is cut by deep canyons and channels which funnel sediment from the nearshore areas down to the continental slope...

Canyons


The Grand Canyon, USA. Photo: US National Park Service

Imagine the Grand Canyon...under the sea! Fantasy? Far from it - in many places of the world, currents have carved a network of steep-sided ravines and gorges into the seafloor. These canyons typically have steep walls and are V-shaped in cross section. Most of them start on the continental shelf, often at or near a river mouth, and can extend all the way to the base of the continental slope. For example, a large canyon connects the Congo River to the base of the continental slope, so all the river sediment is transported straight to the deep sea. The length of canyons varies immensely, but the longest known submarine canyon is located in the southern Bering Sea and measures 370km in length.

Sediment can move easily down the canyons - either by gently creeping downhill, or cascading down in a giant underwater avalanches, known as debris flows and turbidity currents.

Where the canyon ends, usually at the base of the continental slope, a submarine or deep sea fan forms. These features are rather like deltas - they are formed when the sediment transported down the canyon suddenly reaches the flat seafloor and settles out in a big fan-shaped pile. Some canyons then continue their journey across the fan and over seafloor as deep sea channels, hundreds of metres deep and several kilometres wide. In many places, several canyons join together at the base of the continental slope and continue as a single deep sea channel snaking its way across the depths of the ocean.

Down to the abyssal plain...

By the time sediment reaches the deepest part of the ocean basin, the abyssal plain, it is mainly composed of very fine particles. Most coarse sediment is left behind on the continental shelf and slope - the low energy environment of the abyssal plain means that most of the time there isn't enough energy to carry the larger particles this far. Exceptions occur when large, powerful turbidity currents break onto the edge of the abyssal plain, carrying sediment from further upslope. The map below shows that compared to continental shelves and the areas of seafloor adjacent to major rivers, sedimentary cover on the abyssal plains in very thin. In fact, if you look really closely you will see that the areas of thinnest sediment cover are around the mid-ocean ridges. Because the oceanic crust here is very new, there has not been much time for sediment to settle here.


Above: Map showing how the thickness of marine sediments varies around the world's oceans. Blue = thin sediment; red=thick sediment. Click to see a larger version. Source: NGDC

Above: This cartoon shows the different parts of the submarine sediment transport system on a typical ocean margin



Find out more about marine geology:

Underwater landscapes
The ocean basins
Where does all the sediment go?
Mud, mud, glorious mud...
What is a turbidity current?

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© NOCS
February 2007