Geography- Rivers

• Condensation: The cooling of a gas so that it changes into a liquid, for instance as water vapour cools, it condenses to become water droplets, which, when heavy enough, fall as rain.
• Confluence: Where two rivers meet and join to form one larger river.
• Delta: A build up of sediment at the point where a river meets a sea or lake, due to the water velocity slowing and the river having less energy to carry the sediment.See later section for details.
• Drainage Basin:The area of land drained by a river and its tributaries.
• Estuary: The point at which a river begins to meet the sea. The river will be tidal, meaning that it will have both salt water and fresh water in it.
• Evaporation: Water that is warmed, usually by the sun, so that it changes into a gas (water vapour).
• Evapo-transpiration: The combination of evaporation and transpiration.
• Fluvial: relating to a river, from the Latin for water.
• Groundwater: see Percolation
• Hydrology: The study of water
• Infiltration: The downward movement of water that seeps into the soil or a porous rock.
• Mouth: The end of the river, where it meets the sea, or a lake.

• Overland Flow: When water flows over the surface of the ground. This occurs for a number of reasons: the soil may be saturated and therefore be unable to absorb any more water; the underlying rock may be impermeable or the ground may be frozen.
• Percolation: The movement of water through the soil or underlying porous rock. This water collects as groundwater.
• Precipitation: Waterfalling to Earth in any form: e.g. rain, sleet, hail, snow, and dew, all are encompassed by the term precipitation.
• Surface Run-off: see Overland Flow
• Throughflow: the movement of water with in the soil sideways, towards the river.
• Transpiration: The water loss from vegetation into the atmosphere.
• Tributaries: rivers running into the main one, that form part of the same drainage basin system.
• Velocity: The speed of the flow of the river.
• Watershed: The imaginary dividing line between neighbouring drainage basins.

The drainage basin of a river forms an integral part of the entire hydrological cycle (also known as the water cycle), which is shown in the diagram below.

The drainage basin acts as an open system,with a number of inputs, outputs, stores and transfers. 

The main input into the system is precipitation, mainly as rainfall, but also as things such as snow, sleet and hail.

This water is then transferred through the system by the processes of infiltration,percolation, overland flow and throughflow.

During the course of its movement between the sky and the river, water can also be stored in a number of places within the system. Vegetation may interceptthe falling precipitation and store it, water may be stored on the ground in lakes, it may be stored within the soil, or it may be stored as groundwater.

Finally the water will reach the river, which is the primary out put to the system. However water will also have already been lost due to the processes of evaporation, transpiration and evapo-transpiration.

As water flows downhill into rivers it can createa number of different drainage patterns. These are primarily influenced bythe underlying geology (rock type) of the area.

Centripetal

• The rivers flow inwards towards a point.
• Occurs due to the underlying rock forming a basin.
• Examples include the Sea of Galilee
• 

Dendritic

• The rivers form a tree shape, with the primary river forming the trunk.
• Occurs in areas where the rock type is uniform (it is all the same)
• Examples can be found commonly throughout the world, and include the Mississippi, in the United States.

Parellel

• The rivers run parallel to each other downhill.
• The underlying rock is uniform and the surface is flat.
• 

Radial

• The rivers flow outwards from a central point.
• The underlying rock has been uplifted to become a dome, or may be a cone of a volcano.
• Examples include the uplifted granite dome of Dartmoor, or the perfect volcanic cone of Mt. Taranaki in New Zealand.
• 

Trellis

• The river and its tributaries run parallel to each other, before turning at right-angles to meet up.
• The underlying rock is an alternating structure of resistant and less resistant rock.
• The main river, which flows in the direction that the underlying rock dips, is called the Consequent River. The tributaries flowing into it are called Subsequent Rivers.

Long Profiles -Over the long profile of the river, it is steeper near the source, where there is a lot more material for the river to cut through to reach its base level. As you go further down the river it has cut further down towards base level and is beginning to cut laterally (sideways) as well. The slope angle of the river decreases. By the time you reach the lower stages of the river it almost flows over a flat surface, as it meanders its way towards the sea.

Cross Profiles- The cross profile of the river channel changes throughout the course of the river. In the upper section the channel tends to be quite narrow, and comparatively deep. The bottom and sides are littered with many boulders and rocks, causing a great deal of friction for the water flowing past them. This slows the water down and means that this is where the river is flowing the slowest.

• In the mid course of the river has a wider channel, which is deeper than the one in the upper reaches, and the water flows faster, as it has less material to slow it down. The river may begin to meander in this section.
• The lower course sees the river flowing at its fastest until it slows down when it meets the sea. The channel is very wide, deep in places where the water is flowing quickest, and smooth sided.

River Profiles Overview

Upper Course Mid Course Lower Course Long Profile Steeply sloping towards the lower sections of the river. Shallow slopes towards the mouth of the river. Almost at sea level, very gently sloping towards its mouth. Cross Profile Steep sided v-shaped valley. Thin river channel, deep in places. V-shaped valley remains with a wider valley floor and the river begins to meander across it. The river channel begins to widen and become deeper. Wide, shallow valley, with large flood plains and meanders. The river channel is wide, deep and smooth sided.

Fluvial Erosion

The main processes of fluvial erosion occur throughout the course of the river. These are outlined below.

The river itself, however, will try to erode in different directions, depending on how far down the course you are. Very basically, rivers are trying to erode down to their base level. In most cases this is sea level, but it can also be the level of a lake that the river might be flowing into.

At the top of the river, near its source, the river has a huge amount of material to get through to reach base level, so it primarily cuts downwards, creating a steep-sided v-shaped valley.

In the mid-course of the river it continues to cut downwards but is also starting to cut sideways or laterally.

Once it has reached the lower course, and is nearing the sea, the river hasa lmost reached its base level, so most of its erosive energy is concentrated on cutting laterally, creating features such as meanders.

The main processes of fluvial erosion are:

• Abrasion- The erosion of the river bottom and the riverbank by material being carried by the river itself.
• Attriction/Corrasion- The rocks and pebbles being carried by the river crash against each other, wearing them down to become smaller, rounded pebbles.
• Corrosion- The chemical erosion of the rocks of the riverbank by the slightly acidic water. This occurs in streams running through rocks such as chalk and limestone.
• Hydraulic Action- The water forces air to be trapped and pressured into cracks in the rocks on the bank of the river. This constant pressure eventually causes the rocks to crack and break apart.

Once it has been eroded, material in the river is transported down the river.Whilst this is happening, erosion processes such as attrition and abrasion continue to occur. There are four main processes of fluvial transportation,depending on the size of the material being moved:

• Traction- The largest rocks in the river are slowly rolled along the bottom of the river by the force of the water.This primarily occurs in the upper reaches of the river.
• Saltation-Smaller rocks are bounced alongthe river bed. This occurs in the upper and middle sections of the river in general.
• Suspension- The water carries smaller particles of material. This process occurs throughout the course of the river, but increases the closes you are to the mouth of the river.
• Solution- Materialis dissolved within the water and carried along by it. Primarily this occurs in the middle and lower reaches of the river.

Fluvial deposition occurs where the river losses energy and therefore cannot continue to carry the material it is transporting. This could happen in an estuary when the river meets the sea and slows down, depositing its load, which may eventually lead to the formation of salt marshes or a delta. Material is also deposited further up the course of the river. For instance the slower moving water on the inside of a bend of a river will have less energy and therefore drop its load, helping to create a meander.

A major depositional feature of a river is the flood plain, in its lower reaches. This is made up of deposited sand and silt, which is known as alluvium. This is often very fertile and is the reason why many areas near rivers have large amounts of agricultural activity.

Upper Course Mid Course Lower Course Erosion & Deposition Primarily vertical erosion, through attrition, abrasion and hydraulic action. Large boulders deposited and eroded in situ. Continues to cut vertically, but it also begins to cut laterally as it gets closer to base level. Deposition occurs in the slower moving insides of meanders. Primarily cuts laterally as it has almost reached base level. The erosive energy of the river is almost totally concentrated on cutting sideways. Much deposition occurs. Transportation Traction and saltation. Saltation, suspension and solution. Mainly suspension and solution.

Erosional Features

Interlocking Spurs - As the river cuts its deep V-shaped valley in its upper course, it follows the path of the easies trock to erode. Thus it tends to wind its way along, leaving the more resistant areas of rock as interlocking spurs.

Meanders - Meanders occur in the mid course and lower course of the river, where it is beginning to cut laterally as it gets closer to base level. Meanders are basically bends in the river, where the faster water on the outside of the bend has cut into the bank, eroding it and creating a river cliff. At the same time the slow moving water on the inside of the bend deposits its load, building up a shallow slip-off slope. Meanders migrate downstream as they cut through the valley sides.This creates a line of parallel cliffs along the sides of the valley.

V-shaped valleys - In the upper course ofthe river, it cuts rapidly downwards, as the river puts almost all of its energy towards cutting down to base level. This causes the most distinctive river feature, the V-shaped valley. Rocks and other material are washed into the river from the steep valley sides during times of heavy rainfall, adding to the material being carried by the river.

Ox-Bow Lakes - In the lower course of theriver meanders can become so pronounced that they can form ox-bow lakes. Inthe lower course the rapid lateral erosion cuts into the neck of the meander, narrowing it considerably. Eventually the force of the river breaks through the neck, and as this is the easiest way for the water to go, the old meanderis left without any significant amount of water flowing through it. Quickly the river deposits material along the side of its new course, which completely block off the old meander, creating an ox-bow lake.

Waterfalls - Waterfalls are perhaps the most spectacular erosional feature of a river. They primarily occur in the upper course of the river. Often a waterfall will form where a band of harder rock lies over a softer one. As the river flows over the edge of the harder, more resistant rock, into its plunge pool, it erodes away the softer rock below, creating an overhang. Once the overhang is big enough the whole thing collapses due to gravity and its own weight. The whole process then occurs again. This means that over time waterfalls will move backwards up the valley, leaving a steep sided gorge in front of them.

Deltas - Deltas occur where a river that carries a large amount of sediment meets a lake or the sea. This meeting causes the river to lose energy and drop the sediment it is carrying. Deltas form two types, called arcuate and birds foot. An arcuate delta is one, which builds out into the sea, extending the coast line, as the Nile Deltadoes in Egypt. A bird's foot delta is an extension of this as "fingers"of material form further off the edge of the delta. The delta of the Mississippi river shows these characteristics.

Flood Plain - The Flood Plain is the area of alluvial deposits found beside the river in its lower course. As meanders move slowly down the course of the river they erode away the valley to create a wide valley floor, and they deposit layers of alluvial material on the slip off slopes. Over time this builds up into a large flood plain. A very good example is the Canterbury Plains in New Zealand, where many large rivers have contributed to a huge area of alluvial deposition that has become prime agricultural land.

Levee's - Levees are naturally formed banks along the sides of a river channel in its lower course, as it flows through the flood plain. They are formed by the river depositing material when it floods. During a flood the river deposits its heaviest, coarsest material closest to its normal course. Over years this deposition has built up the natural embankments, built of coarse material. Beyond them the flood plain has been built up of the finer material that was deposited further away from the normal course of the river.

Features Overview

Upper Course Mid Course Lower Course Features Interlocking spurs, waterfalls, V-shaped valley, gorges. Meanders, Slip-off slopes, ox-bow lakes. Deltas, flood plains, levees, meanders, ox-bow lakes. Velocity Relatively slow moving. Despite areas of fast flowing water, the large amount of material on the river channel bed means that friction will slow the water down. The water has increased in speed as the channel widens and becomes smoother. Some boulders cause friction to slow it down a little. The fastest section of the river, as the channel is widest, with very smooth sides, and the greatest volume of water.

The Uses of River Basins

For thousands of years, rivers have been the focal point of people's activities. Some of these are listed below:

• Rivers provide a source of fresh drinking water, a source of food (fishing) and a transport route, all of which were very important to the location of early settlements.
• Flood plains provide areas of rich, fertile alluvial soil. Hence areas like the Canterbury plains in New Zealand are intensively farmed.

• Rivers can act as a very effective power source. Initially waterwheels were used to power factories during the Industrial revolution. Latterly the development of hydroelectric power has meant a great increase in the building of dams to trap the water of a river and its drainage basin.

• Rivers have always been seen as a convenient way of waste removal. This has led to many rivers becoming very polluted and in some cases, dangerous.

• Estuaries commonly have been used for industry, which has been able to build its factories on the flat flood plain land. This location is ideal for many industries, such as oil refineries, as they then have easy access to the sea for transporting their goods. The land is flat, cheap and easy to reclaim. Usually a local labour source is not too far away. The ship building industry used also to be found in the estuaries of many of the great rivers around Britain, such as the Clyde and the Mersey. Now only a few remain.

As humans have increasingly used and abused river basins so management and planning of them has become increasingly important.

• Flooding is the most common thing to have to plan around. In many cities the flood plain has not really been built on. Oxford, Exeter and Salisbury are all good examples of where this is the case.
• To prevent the impact of flooding schemes have been introduced in many of these places. These methods can be very successful, or can cause greater problems further downstream. In Exeter, flood relief channels and raised riverbanks have been used to diminish the flood risk. The scheme in place is aimed at countering a "once in one hundred years" flood, and has been severely tested a couple of times.
• Building dams across rivers can also cause problems. Obviously there are the advantages of creating a large reservoir, which can be used for drinking water or as the source of water for a hydroelectric power scheme. The reservoir will often also be used for recreational purposes. However the building of a large dam can also cause problems by affecting the flow of water further down the river, by flooding areas of farmland and even towns or villages,and by affecting entire ecosystems.

• The Tucurui Dam in the Northern Brazilian rainforest did just this, flooding an area of 2875 square kilometres. It was built to provide power for local industries, but at quite a price. The lake it created displaced 40,000people, and is estimated to have destroyed hundreds of species of animals and plants, some of which may never have been actually known about.

• Rivers are used for the dumping of waste, such as sewage, agricultural waste, chemicals and oil. All will greatly harm the wildlife of the river as well as causing potential problems for humans when they drink the water. Schemes and by-laws have been introduced to try to prevent the pollution of our rivers.

A river regime is the difference in the discharge of the river throughout the year. The information can easily be shown on a graph.

During the year in Britain, it would be expected that the discharge of most rivers would be greater in winter months than in the summer. River regimes will also reflect the area that they are in.

For example: The regime of an Alpine stream, has a minimal discharge compared to that of a British stream during the winter. This is because most of the precipitation in the Alps during the winter falls as snow. During the spring months, when the snow melted, the discharge of the Alpine river shows an incredible increase, which often leads to flooding problems. In contrast a British river wil slowly reduces its discharge throughout the drier summer months, until the autumn.

Flood hydrographs

Flooding occurs due to a sudden increase in the amount of water travelling down a river, and can occur for a number of reasons:

• A rapid snow melt
• A prolonged period of heavy rain
• A sudden and intense fall of rain
• Human interventions, such as deforestation, which cause the water to run-off faster than previously.

• The discharge of a river is shown on a graph called a flood or storm hydrograph. It shows the rainfall amount and then the discharge of the river. Most of the rain falls onto the land rather than directly into the river. The water then will make its way into the river and you can use a hydrograph to see how quickly this occurs. By looking at the peak rainfall and comparing it with the peak discharge you can work out the lag-time (the time between the two peaks).

Different catchment areas will have different flood hydrographs. Some, with steep slopes and little vegetation, will rise very quickly, and can be described as a "flashy river". These are the most likely to flood.

Those drainage basins with shallower slopes, and greater vegetation cover will infiltrate the water more, and release it at a slower rate into the river. This means there is less chance of flooding occurring.

Flood hydrographs are very important in predicting how a certain river will behave in a time of intense rainfall. The information could then also be used to plan the most appropriate form of flood prevention scheme for that particular river.