How are erosion, tranport & deposition related to changes in discharge?

The Hjulstrom graph shows the relationship between the velocity of a river and the size of particles that can be eroded, transported or deposited.Velocity increases as discharge rises and generally this enables a river to pick up larger particles from the bed or banks of the channel. Similarly, as velocity and discharge reduce, then particles are generally deposited according to their size, largest first. However, Hjulstrom's research showed three interesting relationships.

1- Sand particles are moved by lower pick up or critical erosion velocities than smaller silts and clays or larger gravels. The small clay and silt particles are difficult to pick up because they tend to stick together. They lie on the river bed and offer less resistance to water flow than larger particles. Much more power flows of water are required to lift them into the water.

2 - Once picked up, particles can be carried at lower velocities than those required to pick them up. However, for larger particles there is only a small difference between the critical erosion velocity and the settling velocity. Such particles will be deposited soon after they have been picked up.

3 - Smaller particles - clays & silts - are only deposited at very low velocities. Some clay particles may never be deposited on the river bed and can be carried almost indefinitely. This explains why such deposits occur in river estuaries. Here the fresh water of the river meets the salt water of the sea, causing chemical settling of the clays and silts to occur and creating extensive areas of mudflats. The process is called flocculation. The coagulation of the clay and silt particles causes them to sink more rapidly.

The type, source and character of the load of the river depend upon the nature of the drainage basin, its location and, increasingly, on human activity. The Hjulstrom graph categories the type of the river load by size. Dissolved load consists of soluble materials carried as chemical ions so there are no measurable particles.

Larger particles only form part of the load of the river during and immediately after extreme events that lead to significant increases in stream discharge. Such temporal changes occur following prolonged heavy rainfall, after flash floods or significant snow melt. In these circumstances, the competence of the river increases and allows larger particles to be carried. Boulders and cobbles form part of the load in the upper course because rivers seldom have the capacity to transport these particles great distances.

In general, the further downstream the river travels, the smaller the particles making up the load. This is partly the result of attrition - when particles are rounded and smoothed by this process they are also broken down into smaller pieces.

Total sediment yields tend to increase with distance downstream, due mainly to increases in both average discharge and velocity in the lower reaches of the river. Here, the river possesses a greater capacity, so it is able to transport more material.

Spatial variations in load can be seen when comparing rivers located in different parts of the world. The Mississippi River has a vast drainage basin, roughly one-third the size of the entire size of the USA. On average every year it transports some 136 million tonnes of load in solution, 340 million tonnes in suspension and 40 million tonnes by saltation. Other major sediment bearing rivers are located in Asia and South America, for example the Yangtze in China and the Amazon in Brazil. In other parts of the world i.e. Australia, sediment yields are much smaller.

Spatial variations in load are influenced by the following factors:

• size of the drainage basin - large drainage basins with many tributaries have a greater potential for transporting sediment, particularly in their lower courses, than do small drainage basins. In the UK, a relatively small country, the largest drainage basin - the River Severn - covers a much smaller area than the largest continental drainage basins.
• relief - in drainage basins with low relief, where there is a small difference in altitude between the source and the base level, the energy available for erosion and transport is limited. Such rivers have low loads compared with rivers that have upper reaches in areas of high relief.

• rock type - in drainage basins where the underlying geology consists of relatively soft sandstones & clay particles, the sediment transported consists mainly of sand or clay particles. Where the rock is limestone, more material will be transported as dissolved load because this rock type is soluble. Moving water does not easily erode resistant igneous rocks, such as granite and basalt. Therefore the total sediment yield in the river basins of igneous rock may be low whereas in drainage basins where there underlying rock is softer, sediment yield may be higher.
• percipitation - low loads are generally found in the drainage basins with low rates of percipitation. In such areas, less water is available as runoff compared with the drainage basins with high percipitation. Seasonal differences in sediment yield occur in some drainage basins, particularly those in areas where the climate has wet and dry season and where the snow melt in the spring adds to normal runoff from percipitation.

• human activity - this can both increase and decrease the sediment yield. In areas of the world where deforestation is occurring rapidly, there have been marked increases in the load carried by rivers. This is mainly caused by the increased soil erosion, which occurs because vegetation that protected the soil from the actions of moving water on its surface has been removed. There is also reduced water uptake by trees and other plants in deforested areas. The result is that soil is washed into the river and adds to the suspended load.

Many farmers use nitrates and phosphates as fertilisers. These substances can enter rivers by through flow and overland flow and are then transported in solution.

Major dams have been constructed on some rivers, e.g. the Aswan dam on the river Nile and the Hoover dam on the Colorado river. Such dams trap sediment, significantly lowering downstream sediment yields.

A fast-flowing river, at bakfull, has the competence to carry a large load. The particles erode the river bed and banks by abrasion creating distinctive features such as waterfalls, potholes and gorges.

If the volume of water in the river falls quickly, the load is deposited because of the fall in competence. When this occurs depostional features such as levees, floodplains and deltas are created.

In some sections of the river both erosion and deposition occur. This is particularly noticeable on a meander bend, where suspended load carried by the river erodes the outside edge of the bend by abrasion and load is deposited on the inside of the bend to form a pointbar.