5. Journey of a River

JOURNEY OF A

RIVER

From Source to Mouth

DRAINAGE BASIN• The land based part of the

hydrological cycle is called the Drainage Basin System. A drainage basin is the name given to the area of land which is drained by a river. When water reaches the surface there are a number of routes which it may take in its journey to reach the river. These are shown in the diagram opposite.

DRAINAGE BASIN FEATURES• Drainage Basins have a number of distinct

features which you need to be able to name and identify. The edge of a drainage basin is characterised by the highest points of land around the river, this is known as the watershed. The point at which a river starts is called its source. As the river continues to flow down stream it may be joined by smaller rivers called tributaries. The point at which these smaller rivers join the main river is known as a confluence. As the river continues its journey, eventually reaches the sea - the point where the river flows into the sea is known as the river mouth.

• Finding out more.....

Check out this great animation on watersheds

THE JOURNEY OF A RIVER

RIVER COURSE• Having understood the basics of a

Drainage Basin we now need to consider the journey that a river within a Drainage Basin takes from its beginning to its end. The path the river follows from its source to mouth is known as the river's course. When studying rivers we often divide it into 3 main sections, the upper course; middle course and lower course. Each part of the river has distinctive features which form and the characteristics of the river and its surrounding valley change downstream (

UPPER COURSE OF RIVER• In the upper course of a river,

water flows quickly through a narrow channel with a steep gradient; as it does so it cuts downwards. This vertical erosion results in a number of distinctive landforms including the steep sloping v-shaped valley through which the river flows in its upper course.

So how does a v-shaped valley form?

• 1. Vertical erosion (in the form of abrasion, hydraulic action and solution) in the river channel results in the formation of a steep sided valley2. Over time the sides of this valley are weakened by weathering processes and continued vertical erosion at the base of the valley3. Gradually mass movement of materials occurs down the valley sides, gradually creating the distinctive v-shape.4. This material is then gradually transported away by the river when there is enough energy to do so.

• V-shaped Valley - a valley which resembles a 'v' in cross section. These valleys have steep sloping sides and narrow bottoms.

Interlocking Spur - spurs are ridges of more resistant rock around which a river is forced to wind as it passes downstream in the upper course. Interlocking spurs form where the river is forced to swing from side to side around these more resistant ridges.

Load - collective term for the material carried by a river

RIVER EROSIONRiver erosion is the wearing away of the land as the water flows past the bed and banks. There are four main types of river erosion. These are:

• Attrition - occurs as rocks bang against each other gradually breaking each other down (rocks become smaller and less angular as attrition occurs)

• Abrasion - this is the scraping away of the bed and banks by material transported by the river

• Solution - chemicals in the river dissolve minerals in the rocks in the bed and bank, carrying them away in solution.

• Hydraulic Action - this is where the water in the river compresses air in cracks in the bed and banks. This results in increased pressure caused by the compression of air, mini 'explosions' are caused as the pressure is then released gradually forcing apart parts of the bed and banks.

• Here is a great little animation by a teacher from Somerset (Noel Jenkins) showing the main processes of river erosion - make sure you learn them!

UPPER COURSE OF RIVER

• An other feature found in the upper course of a river, where vertical erosion is dominant, is a waterfall. The highest waterfall in the world is the Angel Falls in Venezuela (see picture right) which have a drop of 979m. Other particularly famous examples include Niagara Falls (North America), the Victoria Falls (on the Zambia / Zimbabwe border) and the Iguazu Falls (South America).

• Although much smaller in scale, there are many waterfalls in the upper course of UK rivers (e.g. Thornton Falls, Yorkshire - above), but how do they form?

WATERFALL FORMATION• The labelled diagram of a cross section

through a waterfall below, shows the formation process (click on diagram for a larger version).There are also number of excellent animations on the internet which can help you visualise how a waterfall forms. Try out the following:

1. A good step by step animation of waterfall formation showing all the main stages involved (Wycombe High School)2. This simple but excellent animation showing an aerial view of waterfall formation clearly shows the development of a gorge as the waterfall retreats upstream! (Cleonet)3. An animation of waterfall formation from a different 3-dimensional perspective. Look carefully at how the plunge pool is enlarged during the formation process.

The upper course begins in the hills. Here the river is smaller and usually has a rapid, tumbling flow that cuts a narrow channel through rocky hills or mountains. It may roll large boulders along in its swift current.

• The river can create waterfalls where it carves out layers of soft rock and leaves a cliff of hard rock standing.

• It forms rapids along sloping rocky beds

• Over thousands or millions of years the river erodes, or wears away, soil and rocks and carves a canyon or a deep, V-shaped valley.

MIDDLE COURSEHaving studied the characteristics of a river in its upper reaches we now need to follow the river as it enters its middle course. Here the river channel has become much wider and deeper as the channel has been eroded and the river has been fed by many tributaries upstream. Consequently, despite the more gentle gradient the velocity of flow may be as fast as in the uplands. As well as changes in the river channel, its surrounding valley has also become wider and flatter in cross-section with a more extensive floodplain. One of the most distinctive features of the river in the middle course is its increased sinuosity. Unlike the relatively straight channel of the upper course, in the middle course there are many meanders (bends) in the river.

• When the river descends to lower levels, it runs more slowly over the gently sloping land of its middle course. Its current no longer has the force to carry stones or gravel.

• This material drops to the riverbed, where it forms bars of sand or gravel or builds islands.

• Everything is continually changing shape as the river deposits or erodes material. Often the river will alter its course. River currents swing toward one bank or the other, gradually undercutting the banks and widening a V-shaped valley into a U shape.

MIDDLE COURSE• As a river continues its journey towards the sea, the valley cross

section continues to become wider and flatter with an extensive floodplain either side of the channel.

• By the time it reaches the lower course the river is wider and deeper and may contain a large amount of suspended sediment.

MIDDLE COURSE- FLOODPLAIN FORMATION

• When the river floods over the surrounding land it loses energy and deposition of its suspended load occurs. Regular flooding results in the building up of layers of nutrient rich alluvium which forms a flat and fertile floodplain

• As the river flows downstream it reaches the still gentler slope of its lower course. It drops more of its load than it did upstream and begins to build up its bed instead of tearing it down .

• The valley has been eroded into a wide plain. The river swings in great S-shaped curves, forming loops called meanders.

MEANDER FORMATION

• Meanders form due to the greater volume of water carried by the river in lowland areas which results in lateral (sideways) erosion being more dominant than vertical erosion, causing the channel to cut into its banks forming meanders.

MEANDER FORMATION (contd)• Water flows fastest on the outer bend of the

river where the channel is deeper and there is less friction. This is due to water being flung towards the outer bend as it flows around the meander, this causes greater erosion which deepens the channel, in turn the reduction in friction and increase in energy results in greater erosion. This lateral erosion results in undercutting of the river bank and the formation of a steep sided river cliff.

2. In contrast, on the inner bend water is slow flowing, due to it being a low energy zone, deposition occurs resulting in a shallower channel. This increased friction further reduces the velocity (thus further reducing energy), encouraging further deposition. Over time a small beach of material builds up on the inner bend; this is called a slip-off slope.

OX-BOW LAKE FORMATION• Over time meanders gradually change shape and migrate

across the floodplain. As they do so meander bends becomes pronounced due to further lateral erosion and eventually an ox-bow lake may form.

Visualising Meander formation:

This excellent animation (from Cleo Net) looks at the process of meander formation and how meander develop overtime becoming more sinuous, resulting in the narrowing of the meander neck and the formation of an ox-bow lake.

• Another excellent animation - this one focuses particularly on the development of an ox-bow lake as a meander continues to grow.

RIVER TRANSPORTATIONMaterial may be transported by a river in four main ways: solution, suspension, saltation and traction (see diagram). The type of transport taking place depends on (i) the size of the sediment and (ii) the amount of energy that is available to undertake the transport. In the upper course of the river there is more traction and saltation going on due to the large size of the bedload, as a river enters its middle and lower course there is alot of finer material eroded from further upstream which will be carred in suspension. Here is a great little movie showing the process of saltation.Check out the following animation showing the processes of river transport

RIVER DEPOSITION• DEPOSITION

Deposition is where material carried by the river is dropped. This will occur when there is no longer sufficient energy to transport material. Deposition of material may result in the formation of distinctive features such as slip off slopes (on the inner bends of meanders); levees (raised banks) and of course the floodplain itself. Remember - it is the largest material that will be dropped first as it requires the most energy to be transported. eroded from further upstream which will be carried in suspension.

This animation looks at sediment deposition as a river enters a lake - look at what size material is deposited first.

LOWER COURSE• At this point in the river’s journey

the flow is slower, the valley at its widest, and deposition is the dominant process operating.

• Where the river enters the ocean river sediments are deposited and deltas are formed.– Deltas are formed by the

deposition of any sediment remaining in the stream when it reaches its mouth, where the greater depth of the ocean slows the river and its ability to transport. Where such heavy deposition occurs the river divides into many streams.

LOWER COURSE• Estuaries occur where a

coastal area has subsided or ocean level has risen, causing the lower part of the river to be drowned, and therefore exposed to tidal influences.

• The Swan River originally had an estuarine delta (now dredged to provide for river transport with Fremantle harbour). Today it’s mouth remains an estuary and tidal influences are experienced as far upstream as Burswood.

• Now Check your understanding...

Try out this Hydrological Cycle / Drainage Basins Walk the Plank Game

and check your understanding of Drainage Basin Features by matching up these key terms and definitions within the 30 second time limit!