Processes and Changes in Discharge

How are erosion, transport and deposition related to changes in discharge?
The Hjulstrom curve
The Hjulstrom graph shows the relationship between the velocity of a river and the size of particles that can be eroded, transported or depositied. Velocity increase as discharge rises and generally this enables a river to pick up large 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, Hjulstom’s research showed three interesting relationships:

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 (entrain) because they tend to stick together. They lie on the river bed and offer less resistance to water flow than larger particles. Much more powerful flows of water are required to lift them into the water.

Once entrianed (picked up), particles can be carried at lower veloicites 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 entrained.

The smallest particles – clays and silts – are only deposoted 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 meets the salt water of the sea, causing chemical settling of the clays and silts to occur and creating extensive areas of mudflats. This process is called flocculation. This coagulation (clustering of the clay and silt particles cuases them to sink more rapidly.

Use the video above and the picture of the Hjulstrom curve to help you understand it
River load
The type, source and cahracter of the load of the river depend upon the nature of the drainage basin, its location and increasingly, on human activity. The Hjulstrom curve categorises the type of river load by size.
Large particles only form part of the load of a river during and immediately after extreme events that lead to significant increases in stream discharge. Such temporal changes occur following prolonged heavy rainfall (particularly if the ground is saturated) after flash floods or after signhificant snowmelt. In these cirucmstances, the competence of the river increases and allows larger particles ot be carried. Boulders and cobbles form part of the load in the upper course because rivers seldom have the capacity to transport these partiles great distances.
In general, the further downstream the river travels, the smaller the paticles 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.
The amount of sediment carried by a river 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.
Factors affecting the load of a river
Size of the drainage basin – large drainage basins with many tributaries have a greater potential for transporting sediment, particularly in lower course, than do small drainage basins. In the UK, a relatively small country, the largest drainage basin – that of the River Severn – covers a much smaller area than the largest continental drainage basins.

Rock type – in drainage basins where the underlying geology consists of relatively soft (and easily eroded) sandstones and clays, 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 sedmient yield in rver basins of igenous rock may be low whereas in drainage basins where the underlying rock is softer, sedmient yield may be high.

Relief – in drainage basins with low relief, where there is a small difference in altitude between the source and base level, the energy available for eroision and transport is limited. Such rivers have low loads compared with rivers that upper reaches in areas of high relief.

Precipitation – low loads are generally found in drainage basins with low rates of precipitation. in such areas, less water is available as runoff compared with drainage basins with high precipitation. Seasonal differences in sediment yield occur in some drainage basins, particularly those in areas where the climate has wet and dry seasons and where snowmelt in the spring adds to normal runoff from preciptation.

Human activity – this can both increase and decrease sediment yield. In areas of the world where deforestaion is ocucring rapidly, there have been marked increases in the load carried by rivers. This is mainly caused by increased soil erosion, which occurs because the vegetation that proected 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 washed into the river and adds to the suspended load.
Many farmers use natrates and phosphates as chemical fertilisers. These substances can enter rivers by throughflow and overland flow and are then tranpsorted in solution.
Major dams have been constructed on some rivers for example the Aswan dam on the River Nile and the Hoover dam on the Colorado River. Such dams trap sediment, significantly lowering downstream sediment yields.
The effects of channel load on landforms
A fast-lowing river, at bankfall, has the competence to carry a large load. The particles erode the river bed and banks by abrasion, creating distinctive features such as potholes, waterfalls and gorges.
If the volume of water in the river falls quickly, the load is deposited because of a fall in competence. When this occurs, depositional features such as levees, floodplans 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 outisde edge of the bend by abrasion and load is deposited on the inside of the bend to form a point bar.