Sediments
Different types of sediments are carried into waterways in different ways. River bank erosion and general catchment run off produce lots of sand, silt and clay. Large amounts of sand can get into a waterway along a coast or lake shore, and wind blowing onto nearby dunes and intertidal sand banks can carry fine sand into a waterway. Catchment run off also carries organic matter (such as leaves and bark) to waterways.
Sediment comes mainly from:
Sediments found in waterways are usually a mixture:
Sand particles are generally larger than mud particles. They don't stick together well and are chemically inert. How sand particles behave in a waterway depends on their size and shape. Smaller particles can more easily float (be `brought into suspension') and stay suspended in the water, especially if the water is turbulent.
These are formed from a variety of minerals. They are generally small in size, stick together well, and tend to be electrically charged because of their chemical make-up. How they behave depends on their physical, mineralogical and chemical characteristics. Compared with sands, muds are more easily moved by the flow of water.
How much nutrient there is in a waterway depends on lots of different processes. The most important are rainfall, how the nutrients are taken up by plants, and whether nutrients are released from sediment.
Sediments can hold or release nutrients, and this affects how much nutrient there is in a waterway. Phosphorus usually settles into sediments quite easily, but phosphorus can mix with water if the wind whips up the water. This turbulence can bring up phophorus in particles from the bottom, or carry dissolved phosphorus through the water.
When phytoplankton die in winter, they sink to the bottom and decay. The decomposition uses up oxygen. If there is less oxygen, phophorus is more likely to be released from the bottom sediments. This phosphorus is taken up by other algae and they start to grow. Once the algae blooms, especially if the water is stratified, there is less oxygen at the bottom, which continues to encourage the release of phosphorus from the sediments. Most native animals and plants do not like these kinds of conditions and either move away, fall ill or die.
How sediment moves depends on the size of the particles and the speed and direction of the water flow. Sediment can move in three distinct ways:
If the force of the flowing water is strong enough, sediment will be pushed along the bottom of the waterway, rolling and sliding. If the water flow increases, the particles will start to hop along the bed, making more or less regular leaps, called saltations. If the water speeds up even more, the particles will be lifted from the bed and carried in the water.
The transport of particles by rolling, sliding and saltating is called bed load transport, while the transport of suspended particles is known as suspended load. Usually the heavier sand particles from the bed of the estuary make up the bed load. The finer silt and clay particles which come into a waterway from the catchment land, together with the smaller particles from the bed of the estuary, make up the suspended load.
Currents, especially in estuaries, move most of the sediment around. Currents can be caused by fresh water flowing in, or by the tides. The faster the current, the more `shear stress' (the ability to cut matter from other matter) and turbulence on the bed of the waterway. More sediment will be picked up and moved the faster the current flows.
Bedload transport changes easily if there is a change in water speed. Water speed can change because of the tides changing (usually by the incoming tide pushing upstream and slowing the flow of water in a river) or because the water enters deeper water and slows down. The tides bring in sands which form shoals and tidal deltas in the lower reaches of an estuary.
A lot of sediment can be carried during a flood or peak river flow. Shallow sand shoals are deeply scoured during floods and the sand is carried downstream into the sea.
Waves caused by the wind (rather than the ocean's swell) affect how sediment is carried around an estuary. Wind waves in estuaries are generally not very big and quite short, but they can still cause a lot of bank erosion. An estuary is often shaped by the prevailing wind.
The movement of sand in and out of the entrance of an estuary is a complex process. The tides, waves and fresh water all play a part. Ocean waves breaking at an estuary's entrance pick up a lot of beach and bottom sand and carry it into the estuary with the tides.