Why are the worlds big rivers so different?

Importance of big rivers

Big rivers can be found in all of the world’s continents and in every region, across the sub- tropics, high-latitudes and equator. They dominate the earth’s surface and affect many of the people that live on it. In fact, the world’s 10 largest rivers drain 17% of all continental land. Much of the world therefore falls within a drainage basin of a big river.

Big rivers carry huge amounts of sediment, contributing significant amounts oceans and coastlines around the world. As big rivers undergo changes, they have the ability to influence large areas of land, coastlines and oceans. For example, the introduction of dams over the last century has resulted in 15% less sediment being delivered to the oceans by big rivers. This significant reduction in sediment input can lead to decreased beach replenishment and therefore increased coastal erosion. Big rivers matter because they have the ability to shape much of the world.

Defining ‘big rivers’

Activity 1: How big is ‘big’?
How would you define a big river? Using your existing knowledge of rivers, write down a list of criteria that you could use to objectively define a ‘big river’. You might want to consider:

• The length/width/depth required for a big river

• The amount of water discharged by a big river

• The area of land drained by a big river

• The number of big rivers in the world

• Examples of big rivers

Extension: Select one of the world’s big rivers and carry out research to find out exactly how big it is. Consider length, width, depth, discharge and its drainage basin. 

Defining a ‘big river’ is no easy task. First of all, we have to consider what criteria to use. Do we use width, length, depth, catchment area, sediment yield or discharge? Do we take a maximum or an average for each of these values? Do we require a river to meet a number of the criteria, or just one?

Big rivers are so difficult to define because they are so different. For example, the main Amazon (Brazil) channel is around 5 km wide above tidal influence. However, the Brahmaputra’s (Bangladesh) braided channel can reach over double that, at up to 12 km wide.

Rivers also widen and narrow over very short space intervals – in any given river you can go from a 10 km wide section of a river, very quickly down to a 1 km wide section nearby. Size is therefore not the best attribute for defining a big river.

Other variables include sediment yield, catchment area and discharge. However, these factors vary greatly between the world’s ten largest rivers as well. We therefore use a combination of two variables, defining a big river as: greater than 500m wide, with a discharge greater than 1000 cumecs (by comparison, the Thames is around 250 wide in central London and has an average discharge of around 70 cumecs).

Variety of channel patterns

What do the big rivers look like? Well, they vary. Some have single channels, others have multiple. Some are sinuous with many meanders, others are non-sinuous with a relatively straight course. Some are dominated by standing water bodies in the floodplain, while others are not. Ultimately, big rivers show huge variety in their channel pattern.

The world largest rivers are anabranching – they are a system of different channels that divide from the main channel and then then rejoin further downstream. These individual channels can have one of five different patterns:

• Straight: Non-sinuous channel. Relatively rare in nature, but more common in big rivers. At the large scale of big rivers, the local bedrock can provide strong banks to control the course of the channel. Rivers are not straight from mouth to source, but only for particular sections. Example: Columbia (Canada), Mekong (Cambodia), Yenisei (Russia)

• Meandering: Highly sinuous channel, with meanders migrating laterally across a floodplain. Can create scroll bars on the inside of the bends. The peaks (‘ridges’) and troughs (‘swales’) of these bars are formed as a result of deposition on the inside of meanders. Example: Ob (Russia)

• Braiding: Multiple and highly unstable channels, which can change over short periods of time. Can have a chaotic array of channels of different sizes and orders. Can be found in the lower course of big rivers, not just the upper course as we would traditionally expect. Example: Brahmaputra (Bangladesh)

• Wandering: A transitional phase between braiding and meandering. Channel has larger and more permanent islands than the shifting bars found in braided channels. Channel is also more permanent and sinuous than in a braided channel. Example: Yukon (USA)

• Anastomosing: Multiple stable channels with huge wetlands bordering some of the individual channels. The channels have levees, where sediment has built up on the banks. Can sometimes have crevasse splays, where the river breaks through the weak banks. Example: Niger (Mali)
  These five terms can be used to describe any river in the world, regardless of size

Changing rivers

So far we have learnt that big rivers are very different from one another and that they are very complex. This complexity is driven by the supply, exchange and transportation of sediment. So how do river characteristics change over time?

It is generally understood that three main factors that influence the changing shape and characteristics of rivers: gradient, water discharge, and sediment supply and load. People also agree on three secondary factors: tectonics, geology and existing landforms.

However, these factors fail to explain why big rivers are so different. After all, they all have low gradients, large discharge and very fine sediment load; yet they have a wide range of channel formations (straight, meandering, braided, wandering, anastomosing). Clearly, other factors must be shaping the world’s largest rivers.

Measuring rivers

To understand the factors responsible for shaping big rivers, data must be collected and analysed. But only recently have we had the technology required to take measurements of the world’s largest rivers. The only way to measure rivers up to 12 km wide and 100 meters deep is using a boat equipped with remote surveying instruments, including:

Acoustic Doppler Current Profiler (ADCP): Measures flow structure. By sending sound signals, which are then reflected back, this device records the velocity of water across the river channel. This gives an understanding of velocity at different points across the channel cross-section.

Multibeam Echo Sounder (MBES): Measures the depth and shape of riverbed. 512 different beams, fan out to emit a number of sound signals at once. Reflected back, these creates a 2D image of the channel cross section to a resolution of just 3mm. This allows us to see tiny undulations on river bed. Collecting readings whilst driving the boat up and down the river allows the 2D image to become 3D – since distance downstream adds a third dimension.

Parametric Echo Sounder (PES): Measures subsurface sediments, below the riverbed itself. Unlike the MBES this is able to penetrate soils, helping to understand the structure of the ground beneath and therefore enabling more accurate predictions.

Activity 2: Assessing research methods 
The equipment used to measure big rivers produces data that can be used to create 3D model to a very high resolution. However, this equipment isn’t used by everyone that collects data about rivers. Suggest why this equipment might not be suitable in all scenarios. You might want to think about:

• Any fieldwork that you might have carried out and the methods that you used

• The practical limitations of collecting data using a boat with sophisticated equipment

• Methods of data analysis required to make sense of large amounts of data  

Modeling river change

The computer models discussed in this lecture are some of the first of their kind in the world and reveal a little bit about how and why big rivers change. According to the model, there are three driving factors that influence big river channel patterns:

• Strength of banks: Resistant geology can result in strong backs, which allow sections of the river to remain narrow and relatively straight. If sand bars develop in the river, they will migrate downstream over time. Weak banks are more easily eroded, allowing the channel to widen and meander laterally

• Rate of vegetation growth: Controls how the floodplain develops. Fast vegetation growth will stabilise the river and result in less lateral erosion. The river will have just one or two channels that change relatively slowly. Slow vegetation growth results in high levels of sediment exchange. This results in a very wide river with many channels forming and shifting regularly

• Sediment mobility: The ratio between sediment transport via suspension and traction/saltation. Sediment in suspension moves through the river very quickly with little effect on riverbed. But sediment rolling (traction) and bouncing (saltation) along the riverbed can have much more of an impact in shaping riverbed features

Conclusion

The biggest rivers in the world are highly complex and are very different from one another. They also have much more variety than the smaller rivers that we are familiar with in the UK – such as the Thames, Trent, Ouse or Clyde.

Now that we have the technology to analyse the processes and the landforms of big rivers, we can begin to understand how and why they change. This allows research to ask questions about what will happen to them under climate change and about how they should be managed.

Glossary

Crevasse splay: A deposit of sediment at the edge of a river channel. It forms when a river bank or levee is breached, allowing water to flow onto the floodplain. This floodwater often fans out and deposits sediment as it loses energy.

Cumecs: A short-hand term for ‘cubic meters per second’ (m3/s) – a common measure of river discharge. The Thames has an average discharge of around 70 cumecs, whilst big rivers can have a discharge of over 1000 cumecs.

Levees: A natural embankment of sediment either side of a river in its middle or lower course. These can be strengthened through hard engineering.

Scroll bars: Found in the inside of meanders as a result of a river’s lateral movement across the floodplain. They are characterised by peaks (‘ridges’) and troughs (‘swales’), running parallel to the course of the river. These scars on the landscape act as evidence of an earlier river course.

Activity 3: Why are big rivers so different?
Using the information presented in this lecture, as well as your own geographical knowledge, provide written answers to the following questions:

• Suggest two reasons for big rivers operating differently to the smaller rivers (4 marks)

• Outline the main factors responsible for shaping big rivers (6 marks)

• Explain why some rivers meander while others are straight (10 marks)

• Critically evaluate the belief that the world’s big rivers are not that much different from one another (15)