Water and sedimentary transport

Most of the water on Earth occurs in the large ocean basins. However, water is also produced by volcanic activity on the surface of the Earth and some of it actually disappears into space each year. As well as carrying sediments in suspension, water also carries large quantities of sediments in solution (e.g. dissolved components) and this sometimes exceeds the total amount of sediments carried by other processes.

The water table

The groundwater or water table is the surface between two zones of rock or soil below which the pores are completely filled with water. This is also called the saturated or phreatic zone.

Above the water table is the unsaturated or vadose zone. This zone is also called the zone of aeration because the pore spaces are filled partly by air and water.

The water table in any area is rarely stable, even for a few months. In periods of low rainfall, the water table falls, while in periods of heavy rainfall, it rises, eventually leading to flooding when all the pore spaces are filled with water.

Aquifers are permeable beds or rock masses (e.g. vesicular basalt) whereas aquicludes are relatively impermeable beds or rock masses (e.g. quartzite). In some areas, the water in the aquifer is very old (e.g. in the Great Artesian Basin of Australia, much of the water is believed to be older than 50 000 years) and the aquifer recharge may not balance the removal of water from the aquifer. This will eventually lead to the aquifer becoming depleted of water.

Sedimentary transport

Sediments are transported downhill by either water or air until they meet a depression in the surface of the Earth.

Sedimentary basins

Most sediments are eventually deposited in large depressions which we call sedimentary basins. As more sediment is deposited within a sedimentary basin, subsidence occurs (brought about by the weight of the sediments) and the basin becomes deeper, thus allowing even more sediment to be deposited. Sedimentary basins can form in a number of different environments, both on land (e.g. lakes) or sea (e.g. ocean basins).

How sediments are transported

Sediments can be transported in three different ways:

• Suspension load is when sediments are carried in suspension (usually fine-grained sediments that can be carried along easily by the flow)
• Bed load is when the forward force of the moving current acts more directly on the larger particles at the bottom as it pushes, rolls, and slides them along
• Saltation is more complex and usually affects sand-sized particles. Here, the particles are sucked up by eddies into the flow, travel with the flow for a while, and then fall back to the bottom