Geomorphology concerns itself with the features we can observe on the surface of the earth and the forces and processes that produce such features and landforms. One of the powerful forces that shapes the surface of the earth and interacts with it is [water], especially moving water in the form of rivers.

When studying rivers geographers use a range of terms and as you learn to define these terms you will also gain insight into a great deal that is known about rivers.

Drainage systems

A. Why studying drainage systems and rivers may be important

Watch this video of Dr. Al Duda talking about Integrated River Basin Management to get a sense of how rivers impact many people in many different waysIRBM

In an internet article Prof. Nelson at Tulane University makes the following points:

A stream is a body of water that carries rock particles and dissolved ions and flows downslope along a clearly defined path, called a channel. Thus, streams may vary in width from a few centimeters to several tens of kilometers. Streams are important for several reasons:

Streams carry most of the water that goes from the land to the sea and thus they are an important part of the water cycle.
Streams carry billions of tons of sediment to lower elevations and thus are one of the main transporting mediums in the production of sedimentary rocks.
Streams carry dissolved ions, the products of chemical weathering, into the oceans and thus make the sea salty.
Streams are a major part of the erosional process, working in conjunction with weathering and mass wasting. Much of the surface landscape is controlled by stream erosion, evident to anyone looking out of an airplane window.
Streams are a major source of water, waste disposal, and transportation for the world's human population. Most population centers are located next to streams.
When stream channels fill with water the excess flows onto the land as a flood. Floods are a common natural disaster. (Source: https://www.tulane.edu/~sanelson/eens1110/streams.htm)

B. An overview of some important concepts to do with origins

First, we need to understand where the water in rivers and streams comes from. Water can fall directly into rivers through precipitation such as rain or snow. Water can also flow into rivers from the surrounding dry land. When it rains or snow melts, the resulting water seldom remains where it lands. It moves under the influence of gravity to lower ground. This is called '''surface runoff'''.

However, sometimes the runoff is slowed down by the presence of vegetation or soft porous soil. Water seeps into the air spaces in the soil. (Remember when you studied soil erosion in grade 9?) The water percolates down through the soil to lower and lower levels until it reaches '''impermeable''' rock.

When there is no more space for water we say the ground is saturated. The part of the ground beneath the surface that is saturated is called the '''saturation zone'''. In the ground above the saturation zone there are still spaces for more water to seep in if it were to rain again. This area is called the '''aerated zone'''. The upper limit of the saturated zone which is the boundary between the saturated and the aerated zones we call the '''water table'''.

Look at the following diagram that illustrates much of this detail - Groundwater diagram

In most of South Africa, as you probably recall, the winters tend to be very dry with little or no rainfall and most of the rainfall is in the summer. It makes sense then that the amount of water sinking below the ground's surface in summer will be much more than in winter and the saturation zone will be bigger. As a result, the water table in the wet season will be closer to the surface or higher than it is in the dry season. In other words in areas with distinct wet and dry seasons, like South Africa, we can speak of two water tables, a wet season water table, and a dry season water table.

This water under the ground below the water table we call '''groundwater'''. Sometimes it meets an obstacle in the form of impermeable vertical rock and it is pushed towards the surface and bubbles out as a spring. This water will then flow downhill into a river as a form of '''indirect runoff''' similar to surface runoff which is also indirect runoff.

At times the river has eroded so much of the land that the bed or channel is actually deeper than the surrounding water table and groundwater will then feed into the river there. This is called '''base flow'''

C. An overview of some important concepts to do with structure

In the following video Drainage Sytems in South Africa, the instructor uses a diagram of a drainage basin and very clearly explains the important terms and concepts you need to know about the structure of a drainage system. In the top right-hand corner of the video screen there is also a tab to click onto in order to download a comprehensive set of notes. This wiki will add a few additional details that have been omitted in the video and notes. This image also summarizes most of these concepts Drainage basin

D. River types

When building settlements near a river or undertaking developments on rivers such as bridges or dams it is important to know whether the amount of water in the river is likely to be constant there or to vary. For example, if one were to build close to a dry or very shallow river or to build a low bridge over it this could have disastrous consequences if the river flows much more strongly during the wet season. Similarly, if a river is observed shortly after an unusual period of heavy rains, it might seem to be a good source of irrigation for crops nearby, but if this is actually an unusual episode and the river is usually dry this would be a problem. There are many ways of categorizing rivers, but most geographers agree on four basic types: [Episodic rivers] only flow on rare occasions of exceptional rainfall. [Periodic rivers] flow strongly during wet seasons, but may dry up or become very shallow during the dry part of the year. [Permanent rivers] flow strongly throughout the year because they are fed by groundwater as well as rainfall throughout the year. They are also sometimes called perennial rivers. Some rivers appear and behave like permanent rivers although they are in dry regions and may actually be above the water table and be receiving no groundwater. This is because they are strongly fed by tributaries further upstream that receive high rainfall or constant groundwater and are flowing permanently. These are called [exotic rivers]. Episodic rivers normally have their deepest point above the wet season water table. Periodic rivers have their deepest point between the dry and wet season water tables and permanent rivers have their deepest point below the dry season water table.

E. Drainage patterns

The video mentioned above deals with drainage patterns in some detail. Drainage patterns can be defined as the pattern of a river system when seen on a map. It gives a useful clue as to the underlying rock structure and the surrounding slope of the land. Conversely, on a contour map, it is possible to predict the likely drainage pattern that will occur. The commonest type of drainage pattern is a [dendritic pattern] found where there is an even slope and the underlying rock is fairly uniform or homogenous. On the other hand ground with a similar even slope but where the underlying rock is hard igneous rock with frequent faults will present as a [rectangular] rather than a dendritic pattern.

There are two types of drainage pattern that are not dealt with in the video.

A [radial] pattern is rather like the spokes of a wheel radiating out from a single central high point. An isolated conical mountain or a volcano would be the typical sort of site where radial drainage would be found because water from the top of the mountain flows down almost equally on every side.

The opposite of a radial drainage pattern occurs when a number of rivers or streams from different directions all flow down towards a central low point or depression. This is called a [centripetal] drainage pattern. It is quite rare because there are very few such depressions on the earth's surface. In southern Africa, the best-known example of this would be the Okavango Swamp in Botswana.

The following images show how the drainage patterns are related to underlying rock structure and slope. Drainage patterns.Drainage including parallel pattern.

F. Drainage density

Drainage density refers to how quickly water that falls on the ground finds its way into streams and back to the sea. Water that precipitates in the form of rain or snow could either stay on the ground and seep into the soil slowly becoming groundwater or it could easily and quickly flow across the surface of the ground, collect into streams and eventually feed into larger rivers. In such cases, the surface runoff is much greater. When there are many smaller streams joining together and carrying water away we say there is a HIGH drainage density and not much of the water remains to seep into the earth. A LOW drainage density means water reaches rivers more slowly and is carried away from the area less quickly so that more will seep into the soil.

Calculating drainage density

Geographers calculating drainage density in an area by measuring the combined length of all the streams in the area (in km) and dividing this by the area (in square km). So if in an area of 20 sq. km there are many small streams totaling 25 km in length then the drainage density would be 1,25 but if there are very few streams and their total length is only 5 km then the drainage density would be 0,25. it is important to know the drainage density of an area because it affects the potential use of the area. Well-drained soil suits particular types of crops and plants (e.g.palms) whereas less well-drained soil favours other crops. It also affects the type of building, development, and human activity that can take place there.

What factors contribute to the drainage density?

Rainfall - High rainfall areas have more water to discharge and are likely to already be more saturated so they will have a higher drainage density
Relief -Steep slopes cause water to run over the surface faster allowing less to seep into the ground and increasing the likelihood of streams forming.
Resistance of underlying rocks - Harder rock that is not as easily eroded is less likely to form channels for streams or rivers
Permeability - If the underlying rock or soil is more porous and allows water to infiltrate more easily there will be less runoff and a lower drainage density.
Vegetation cover- Water flows more easily and quickly over bare ground resulting in a higher drainage density whilst vegetation slows down the flow of water allowing more to seep into the ground.

G. Stream order and density on topographic maps

Look at the section from a topographic map shown here Bell Twins map Why are some of the streams shown as broken lines and others as solid lines? The broken lines indicate periodic streams whereas the solid lines are permanent streams. How can you determine whether the streams are flowing away from the point called The Mitre? Look at the spot heights and the contour patterns and you will soon see that The Mitre is high ground and the source of the streams. The stream exiting from view in the top left-hand corner of the map has been fed by a number of tributaries. When we analyse these we see that it is a third-order stream. The diagram below shows how this is calculated Stream order diagram. Note that there are several different methods of classifying stream order such as Strahler, Horton, Shreve, and others. In the South African curriculum, we use the Strahler method which is the one shown in the image. This video also explains stream ordering Stream order

H. Stream flow or discharge

The steeper the slope down which water is moving the faster it will flow. If a river is moving over fairly level land the water will flow quite slowly. We call the rate at which the water is flowing the velocity and it is usually expressed in metres per second (m/s).

When planning for the use of a stream for any purpose it is important to have an accurate idea of how much water is moving through the stream channel in a given period of time. This is called the stream discharge In most countries, it is measured in cubic meters per second. The following equation defines stream discharge mathematically:

Q = V x W x D

where Q is the discharge, V is the velocity, W is the average width, and D is the average depth of the flow. Stream discharge varies over both time and space. Discharge normally increases downstream as more water enters the stream channel from runoff and groundwater flow.

Why are width and depth important? Imagine two containers each holding a litre of water. Container A is a narrow bottle with a narrow neck and a small opening. Container B is a wide, flat dish. Which container can be emptied first? Obviously the wide, flat dish because there is more space for the water to pass through and less resistance or frictional drag. For this reason, the flow will be strongest at the surface and the middle of a stream and less at the bed and the banks.

The overall pattern of the flow can fall into one of three types: Laminar flow - water flow in the stream is not altered in its direction. Water flows as parallel molecular streams. The surface appears to be smooth and calm and there is little mixing of different layers at different depths so that the surface temperature can be quite a bit warmer than the temperature lower down. Turbulent flow - water flows as discrete eddies and vortices. This is caused by channel topography and friction, obstructions such as rocks. There is much mixing of different water layers and of water with air causing frothy "white" water. Helical flow - spiral flow in a stream. This is caused by channel shape. Meandering channels cause this type of flow. Helical flow has an important role in sediment transport and deposition, and in the creation of point bars. The following image shows the difference between laminar and turbulent flow diagrammatically Stream flow The helical flow is illustrated diagrammatically in the following images: Helical flow cross section Helical flow river bend The type of flow pattern plays an important role in the type of animal and plant life found in the stream at that point. Flora and fauna that prefer smooth calm water (water lilies or mosquito larvae for example) will not be found in turbulent water and those that thrive in turbulence will shun water with a laminar flow.

Fluvial processes

A. Introductory remarks

Fluvial comes from the Latin word for "river". Rivers are not fixed or static objects; they are constantly moving and changing. Between the point where a river starts as a small trickle to its mouth which may be several hundred metres wide a great deal happens and the river grows and matures as it goes through different stages in much the same way as you and I grow and mature. Along the way rivers can change the surrounding landscape and be changed by it and they can even be taken off course and "captured" or they can almost become stuck and then be rejuvenated. In our life journey, we can also influence our surroundings and be influenced by them and the people in them. We, too, can go "off-course" and be captured by those with wrong intentions or when we become stagnant in our spiritual zeal we can become renewed or revived. When we study rivers we can also glean valuable insights into deep spiritual truths about our human condition.

B. River profiles

Transversal profile

If you could start at one point on the bank of a river and draw a line along the river bed to aa point immediately opposite the start point on the other bank you would have a cross-section of the river at that point. This is called a transversal profile. Obviously this profile will change from place to place along the river's course. Near the source of the river it the stream tends to flow down a steep gradient and the profile is deep and narrow because the river is able to erode quickly but over a limited area. further down the river slows down as the land becomes flatter, but the volume of water is greater so the river erodes its banks and widens and flattens the bed. Near the mouth, the river becomes wide and shallow and begins to deposit sediment.

Longitudinal profile

The longitudinal profile of a river follows a line from the source to the mouth. Since most rivers originate on high grouhd and flow down to a shallow flat coastal plain the typical longitudinal profile of most rivers is concave. The lowest point that a river can normally erode to is sea level so this is called the ultimate or permanent base level. (There are some rare exceptions like the river Jordan which flows into the Dead Sea that is below sea level) Along its course a river may sometimes encounter points where it cannot erode further such as a waterfall and this is then called a temporary base level.

C. River grading

A fast flowing river that has surplus energy allowing it to erode rapidly is called an overgraded river. A slow-flowing river that has so little energy that it deposits its load is called an undergraded river. When a river has just enough energy to flow evenly without significant erosion or deposition it is said to be an ungraded river.

D. Features of the landscape on rivers

The moving water in a river has energy that allows it to erode the land over which it moves. Four important factors affect the impact and energy of the river. Velocity is the speed at which the water is moving and the faster the water moves the greater the energy it has and the more it is able to erode. Slow-moving rivers lack energy and tend to deposit the material they are carrying. Volume refers to the amount of water that is moving, The greater the volume the more energy there is which is why floods cause so much damage. Gradient. Water moves much faster over steep ground than over flat ground so gradient affects energy. Friction Friction slows down a river so when a river moves over rough or uneven ground it loses energy.

Waterfalls

When a river crosses a layer of hard resistant rock that it cannot erode then meets much softer rock the softer rock downstream is eroded more quickly and this causes the bed of the river to drop away sharply resulting in a waterfall. the plunging water erodes away a deep plunge pool at the base of the waterfall and cuts away into the soft rock that is supporting the hard layer causing an overhang. Over time the overhang breaks off and collapses and the waterfall retreats back upstream. The following diagram illustrates this process Waterfall daigram

Rapids

Rapids (also called cascades) appear as a series of small step-like waterfalls and occur when the river crosses a portion of uneven ground consisting of mixed hard and softer rock.

Meanders

In the mid-course of a river the landscape often flattens considerably and the river loses velocity and begins to weave from side to side as it finds its way around obstacles. As the water moves around a bend it has a greater velocity on the outside of the bend and erods more there whil depositing material on the inside where it moves more slowly resulting in steep cliffs on the outer bank and slip-off slopes on the innner bank. This process is illustrated in the following diagram Meander diagram

Oxbow lake

Over time the neck of land between adjacent bends in a river meander can become very narrow and during a flood the river my cut across the neck and from a new channel resulting in the old meander being cut off and forming an oxbow lake as shown in this diagram Oxbow lake formation

Floodplain

In the lower reaches of the river, the volume of rivers is often increased because many tributaries have added their flow to the river and when some of the tributaries receive extra inflow the river can flood and expand beyond its banks. When it does so it deposits much of the soil and material it is carrying and over time this results in an extended fairly flat plain of highly fertile soil beside the river. This flood plain is reflected on a topographic map by an area where the contour lines are far apart. In the following picture you can clearly see the deposited soil on the river floodplain Flood plain Because most of the deposited soil contains a lot of topsoil the flood plains tend to be very fertile and popular for farming, but there is also an increased risk of intermittent flood damage. A typical example of this is the Ganges plain in Bangla Desh which is densely populated and produces most of the country's rice and other food but is frequently devastated by major floods causing loss of life and property. The following diagram illustrates a flood plain Flood plain

Natural Levee

Rivers that often overflow their banks deposit layers of soil and material on the banks and the banks become built up and stabilise when vegetation starts to grow there. The following diagram shows how natural levees are formed Natural levees. The implication of natural levees is that they progressively raise the level of the river above the surrounding land thereby increasing the extent of the damage that happens when the river does flood its banks. Natural levees must be distinguished from artificial or manmade levees that are built to prevent flood damage

Braided stream

When a river crosses a very flat area it may be moving so slowly that it has difficulty going around obstacles and may then break up into a network of smaller interconnected streams called a braided stream. Here is a very good example of a braided stream Braided stream

Delta

When a river flows into the sea or a large body of still water it often forms a delta because it deposits the soil it has been carrying and this is not washed away by the water it flows into. On the other hand, if there are strong sea currents that will wash the sediment along the shore then a delta will not form. The sediment builds up and blocks the flow of the river forcing the river to break up into a number of smaller streams that empty into the sea. These are called distributaries. Soil that is deposited eventually becomes permanent land where there was no land. There are different types of delta depending on the shape of the land that is formed.

1Arcuate delta

In an arcuate delta the new land extends out into the sea in an arc. The most famous example of an arcuate delta is the Nile delta as shown here Nile delta

2Bird's foot delta

Where the sea bed is shallow near the river mouth then deepens suddenly you can find a Brid's foot delta forming when a limited number of distributaries form their own mini-deltas. This can be seen in the satellite image of the Mississippi River delta Bird's foot delta

3Estuarine delta

This forms when river sediment is deposited in the submerged river mouth and there is a mixture of fresh and salt water Estuarine delta

4Cuspate delta

This builds new land out to sea, but there are no distributaries and the river mouth remains open. The sediment is distributed fairly equally on either side of the mouth by complimentary sea currents. Cuspate delta

The following video also explains simply how deltas are formed Delta formation video

E. River rejuvenation

At the end of this section you will be able to define river rejuvenation and describe the causes and effects of rejuvenation as well as identify typical landforms associated with rejuvenation and explain the link between river rejuvenation and river grading.

Rejuvenation comes from the Latin word for youth and it means literally to be made young again.

In the early stages of a river’s “life cycle” the river usually flows rapidly over steep ground and erodes deeply. At this stage we say the river is OVERGRADED.

As the river comes closer to the mouth the ground becomes flatter and the river moves more sluggishly and lacks the energy to erode as much and starts, instead, to deposit sediment. At this stage it is said to be UNDERGRADED.

However, if for any reason the river were again at the lower stages to suddenly gain more energy and again flow strongly we would say it has been REJUVENATED.

What causes a river to suddenly gain energy and become overgraded or rejuvenated? This can happen because of either of two main reasons:

(a) There is an increase in the volume of the river resulting in more energy. This can be caused by climate change producing more rainfall and greater run off into the stream or by water from another stream diverting into the stream through river capture.

(b) There is an increase in gradient causing the stream to flow more rapidly again. This can be caused by eustatic change which results in the sea level dropping. This happens, for instance, during ice ages when there is less water in the sea because of the freezing of the ice cap. It can also be caused by isostatic change which results in the land being raised higher, usually through tectonic activity. This diagram illustrates the concept of eustatic change:Eustatic change.

River rejuvenation leads to the appearance of several distinctive land forms along rivers that can be distinctively recognized on maps as well.

Incised meanders

The increased grading of the river can cause it to erode much more deeply on either side of a meander making the sides of the valley steeper. Incised meanders can be easily recognized on maps because the contours are close together with no sign of a flood plane. The image below shows the steep sides of a an incised meander very clearly Incised meander Notice in the diagram below the difference between an incised meander (A) on the left and a normal meander (B) with a floodplain on the right. [https://media.springernature.com/original/springer-static/image/prt%3A978-3-540-31060-0%2F9/MediaObjects/978-3-540-31060-0_9_Part_Fig5_HTML.jpg

River terraces

When an undergraded river floods it deposits material in sheets across the floodplain and if floods occur regularly these layers build up. if the river then becomes overgraded it erodes rapidly and deeply into the raised banks. The alternating flood deposits and subsequent erosion results in a series of terraces. The following diagram shows this process clearly: River terrace formation The following image shows actual river terraces River terraces. This short video is also helpful in explaining how terraces are formed: River terrace formation video

Knickpoints

These are parts of a river where there is a sharp change in channel slope, such as a waterfall or lake. On a map a [knickpoint] often reflects as a sharp change in contours shown by close contours at the knickpoint. The following diagram explains knickpoints schematically:Knickpoints The following image shows a typical knickpoint and one can clearly see how the depth of erosion is far greater below the knickpoint than above it Knickpoint picture

River capture

Rivers are separated from each other by natural barriers of rock called watersheds. When one of the streams erodes through a watershed and connects with another stream the water in one of the streams will be diverted into the other and this is called river capture or stream piracy. The river that has been diverted is called the captured stream and the river that has gained extra volume is called the captor stream. The point at which the diversion takes place is referred to as the elbow of capture and it is a type of [knickpoint]. This typically occurs in the upper reaches of a river in the headwaters. The following diagram illustrates the process:River capture diagram.

Why river capture occurs

1 The rock on one side of a watershed may be softer and more easily eroded than on the other side, what is called unequal rock resistance.
2 Climate change resulting in increased rainfall may cause one stream to gain extra volume.
3 Human intervention such as dams can lead to river capture.
4 Geological or tectonic activity such as earthquakes can result in river capture.

Superimposed and antecedent drainage

Rivers usually follow the elevation of the land and flow downhill and around obstacles. Sometimes a river may seem to "break the rules" and flow right through a barrier of high ground. because this is so contrary to normal patterns we talk of it being "discordant". There are two major reasons why a river could flow in a discordant fashion:

1 The river may have established its course long before the surrounding high ground was present. Because of geological uplifting such as tectonic folding or faulting underlying layers of rock were forced up in the form of new high ground, but the river course was so well established that the river eroded through the high ground and continued to run along the same course as before. This is called an antecedent drainage pattern.
2 The river may be more recent in origin but it manages to erode softer surface cover until it reaches harder rock lower down. The river is able to erode through some of this harder rock as well leaving high ground on either side. This is called asuperimposed drainage pattern.

Both of these patterns present as steep, narrow gorges or valleys.

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Geomorphology - Rivers

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