Rivers have a lot of energy and because they have energy, they do stuff. The obvious things rivers do with their energy is flow but, besides this, they also transport load, erode load and erode the channel through which they flow.
Erosion is the breaking down of material by an agent. In the case of a river, the agent is water. The water can erode the river’s channel and the river’s load. A river’s load is bits of eroded material, generally rocks, that the river transports until it deposits its load.
A river’s channel is eroded laterally and vertically making the channel wider and deeper. The intensity of lateral and vertical erosion is dictated by the stage in the river’s course, discussed in more detail here but essentially, in the upper stage of the river’s course (close to the source of the river) there is little horizontal erosion and lots of vertical erosion. In the middle and lower stages vertical erosion is reduced and more horizontal erosion takes place.
There are several different ways that a river erodes its bed and banks. The first is hydraulic action, where the force of the water removes rock particles from the bed and banks. This type of erosion is strongest at rapids and waterfalls where the water has a high velocity. The next type of erosion is corrasion1. This is where the river’s load acts almost like sandpaper, removing pieces of rock as the load rubs against the bed & banks. This sort of erosion is strongest when the river is transporting large chunks of rock or after heavy rainfall when the river’s flow is turbulent.
Corrosion is a special type of erosion that only affects certain types of rocks. Water, being ever so slightly acidic2, will react with certain rocks and dissolve them. Corrosion is highly effective if the rock type of the channel is chalk or limestone (anything containing calcium carbonate) otherwise, it doesn’t have much of an effect.
Cavitation is an interesting method of erosion. Air bubbles trapped in the water get compressed into small spaces like cracks in the river’s banks. These bubbles eventually implode creating a small shockwave that weakens the rocks. The shockwaves are very weak but over time the rock will be weakened to the point at which it falls apart.
The final type of erosion is attrition. Attrition is a way of eroding the river’s load, not the bed and banks. Attrition is where pieces of rock in the river’s load knock together, breaking chunks of rock off of one another and gradually rounding and shrinking the load.
When a river erodes the eroded material becomes the river’s load and the river will then transport this load through its course until it deposits the load. There are a few different ways that a river will transport load depending on how much energy the river has and how big the load is.
The largest of particles such as boulders are transported by traction. These particles are rolled along the bed of the river, eroding the bed and the particles in the process, because the river doesn’t have enough energy to move these large particles in any other way.
Slightly smaller particles, such as pebbles and gravel, are transported by saltation. This is where the load bounces along the bed of the river because the river has enough energy to lift the particles off the bed but the particles are too heavy to travel by suspension.
Fine particles like clay and silt are transported in suspension, they are suspended in the water. Most of a river’s load is transported by suspension.
Solution is a special method of transportation. This is where particles are dissolved into the water so only rocks that are soluble, such as limestone or chalk, can be transported in solution.
Capacity & Competence
Rivers can only carry so much load depending on their energy. The maximum volume of load that a river can carry at a specific point in its course is called the river’s capacity. The biggest sized particle that a river could carry at a specific point is called the river’s competence.
To transport load a river needs to have energy so when a river loses energy it is forced to deposit its load. There’s several reasons why a river could lose energy. If the river’s discharge is reduced then the river will lose energy because it isn’t flowing as quickly anymore. This could happen because of a lack of precipitation or an increase in evaporation. Increased human use (abstraction) of a river could also reduce its discharge forcing it deposit its load.
If the gradient of the river’s course flattens out, the river will deposit its load because it will be travelling a lot slower. When a river meets the sea a river will deposit its load because the gradient is generally reduced at sea level and the sea will absorb a lot of energy.
The Hjulström Curve
A Hjulström curve is a special type of graph that shows how a river’s velocity affects it competence and its ability to erode particles of different sizes. There’s a lot going on on the graph but it’s fairly easy to read once you get the hang of it:
There’s two curves on the Hjulström Curve, a critical erosion velocity curve and a mean settling velocity curve. The critical erosion curve shows the minimum velocity needed to transport and erode a particle. The mean settling velocity shows the minimum speed that particles of different sizes will be deposited by the river. The shaded areas between the curves show the different process that will be taking place for particles that lie in those shaded areas.
As an example, a river flowing at 10cms-1 will transport clay, silt and sand particles but will deposit gravel, pebble and boulder particles. Conversely, a river flowing at 100cms-1 will erode and transport large clay particles, silt particles, sand particles and most gravel particles. It will transport all but the largest of pebbles and will deposit boulders.
The easiest way to read the curve is to draw a horizontal line from the velocity you’re trying to read and seeing which shaded area it crosses the particle size you’re interested in in. This will tell you whether that particle is eroded, transported or deposited at that velocity.
There’s a few interesting things to note about the Hjusltröm Curve. The first is that clay sized particles don’t appear to have a mean settling velocity. This is because these particles are so fine that a river would have to be almost perfectly stationary in order for them to fall out of solution. In addition, the small particles seem to have an erosive velocity that’s the same as the velocity for larger particles. This is because smaller particles are cohesive, they stick together, making them harder to dislodge and erode without high velocities.