River Basin Processes

River Basins

The Hydrological (Water) Cycle •97% of the world’s water is contained in the oceans. •2% of the world’s water is stored as ice and snow. •That leaves just 1% is either fresh water on land or water vapour

•As the amount of fresh water is limited, it has to be recycled over and over again. This is known as the hydrological cycle. •As no water is added or lost from the hydrological cycle it is called a closed system.

Hydrological Cycle

The Drainage Basin System Definition: A drainage basin is the land area or catchment area drained by a single river and its tributaries. An imaginary line called the watershed separates one drainage basin from another. The watershed usually follows a ridge of high land; any water falling on the other side flows into an adjacent drainage basin. • The drainage basin forms part of the hydrological cycle, but unlike the hydrological cycle it is an open system. This is because and open system has:

– INPUTS where water enters the system through precipitation (rain and snow) – OUTPUTS where water is lost to the system either by rivers carrying it to the sea or through evapotranspiration. (Evapotranspiration is the loss of moisture directly from rivers or lakes – evaporation, or from vegetation – transpiration.

The Drainage Basin System • • •

Within the system are stores and transfers (flows) STORES are places where water is held, eg: in lakes, on the surface, in soil and in rocks underground. TRANSFERS are processes by which water flows or moves through the system, eg: infiltration, surface runoff, throughflow, percolation.

Diagram of a typical drainage system. Source

River Basin

Tributary Confluence Watershed

River

Channel

Mouth of river

Sea or Lake

From its source, a river flows downhill. This is because of the Earth's gravity. Small trickles of water join up to form a stream. The amount of water in the stream increases steadily as more streams, called tributaries, join it. Eventually the stream becomes big enough to be a river. A drainage basin is all the land that supplies a river and its tributaries with water. If you could look down on a river from above, you would see that it branches. This is called a drainage pattern. The shape of the pattern depends on rocks, soil, climate and the changes made to the river. Radial drainage happens when streams flow down from a central high point, such as a mountain top. Other rivers, such as the Amazon, form a pattern like the branches of a tree. This is called dendritic drainage.

Rivers and their tributaries can form many other types of drainage patterns. Parallel drainage happens when streams flow in valleys that are parallel to one another. This might be.because movements in the Earth, millions of years ago, made the rocks 'fold' into parallel lines. Trellis drainage is common where massive layers of sedimentary rock have 'slipped'. Streams flow in channels that are parallel to each other, and tributaries join the streams at right angles.

•The force of a river depends on 2 things - the energy and the volume of the river. Energy = Speed + volume •The steeper it is the faster it flows. •Speed can be reduced by friction and turbulence, so a lot depends on the nature of the river bed. This is related to two things: •the material it is made of •the cross sectional shape of the river bed. •The most efficient shape is where the depth is equal to half the width W = 2D

A wide shallow channel means little erosion, but often plenty of deposition because of increased friction which slows down the flow.

A narrow, deep channel increases the speed of the water and so the amount of erosion increases and deposition decreases.

Corrasion, otherwise known as abrasion, is the grinding of rock fragments carried by the river against the bed and banks of the river. This action both widens and deepens the channel. The power of the grinding is especially powerful in floods when large fragments of rock are carried along the river bed eroding the bed and the banks. Solution, also known as corrosion, is the process by which river water reacts chemically with soluble minerals in the rocks and dissolves them. For example, when a river stream flows over an area of limestone (calcium carbonate), it erodes the limestone by reacting chemically with it and dissolves it.

Attrition is the knocking of rock fragments in the water against one another. The fragments are broken into smaller pieces and become smoother along the process.

Hydraulic action is the breaking down of rocks and dragging them away from the bed and banks by the force of the running water itself. When water from a fast moving stream enters cracks in a rock, the force breaks up the force into pieces.

•

Some rivers have greater volume than others. There are four factors that influence volume. Vegetation

Permeability of rocks

The presence of vegetation affec ts the ra te a t whic h water is intercepted and is able to infil tra te the ground. If an area is covered with vegeta tion, the vegeta tion will intercept the rain and allows some of i t to infiltra te into the ground. As a resul t, there w ill be less water as surface runoff into the river and so the volume of water into the river will be lowered. On the other hand, in an area with no vegeta tion, there will be no interception and li ttle infiltra tion into the ground. This will resul t in more wa ter as surface runoff into the ri ver and so the volume of the water is higher than tha t of a vegeta ted a rea.

The permeability of rocks i s determined by the size of the pores between the rock particles. If the rock has small pores, water cannot easily infiltra te into the rock and thi s means that the rock i s impermeable. On the other hand, if the roc k ha s large pores, water can easily infiltra te and thus the rock is permeable. When wa ter flows through an area of i mpermeable rock, little water infil trates the ground, as a resul t there is high surface runoff and leads to a high volume of flow of water.

Drainage basin size

Climate

A larger d rainage basin will have a greater volume of water. This is because there is a greater a mount of tributaries b ringing water to the river resul ting in a higher surface runoff.

The a mount of precipitation and temperature affec ts the volume of river. During dry season, a river channel may be al most empty therefore the volume is low. During the wet season, the volume will be high and water may overflow the c hannel and flood the surrounding land.

Drainage Ba sin refers to the area drained by a river and its tributaries.

Tempera ture also plays a pa rt in influencing the volume of rivers. When the tempera ture is high, there is more evapora tion and transpira tion which reduces the a mount of water in the river.

The three factors that influence the velocity of a river are the river gradient,channel roughness and channel shape. Gradient

Channel Roughness

Channel shape

A river flowing down a steep slope or gradient has higher velocity than one which flows down a gentler gradient. For example, the speed of flow in a river that plunges down a steep slope in the form of a waterfall is much higher than the speed of flow in a river that winds down a gentler slope.

In figure A, the channel is smooth while that in figure B is rough or uneven with boulders on the river bed as well as rocks that protrude out from the bank. A river that flows through such a river has to overcome such obstacles and therefore there will be more friction and the velocity of the river is reduced.

The shape of the channel or its cross section affects the wetted perimeter. The wetted perimeter refers to the extent to which water is in contact with its channel. The greater the wetted perimeter, the greater the friction between the water and the banks and the bed of the channel, and the slower the flow of river.

Diag A.

Diag B.

In figure A, the channel is smooth while that in figure B is rough or uneven with boulders on the river bed as well as rocks that protrude out from the bank. A river that flows through such a river has to overcome such obstacles and therefore there will be more friction and the velocity of the river is reduced.

• Wetted perimeter is calculating by adding the length and the breadth of the channel in contact with water.

• A river uses its energy to carry or transport eroded materials such as mud, sand, boulders and dissolved materials. These materials are called its load. River transport their load by four processes. These processes are:

Saltation | Traction | Solution | Suspension |

•

A river uses its energy to carry or transport eroded materials such as mud, sand, boulders and dissolved materials. These materials are called its load. River transport their load by four processes. These processes are traction, saltation, suspension and solution.

•Larger particles like pebbles and boulders roll and slide along the river bed.

•Dissolved materials containing minerals like calcium and sodium are carried in the water. Trying to look out for these type of load in a river? Too bad, they cannot be seen by a naked eye.

•Smaller particles such as clay, silt and fine sand are carried along without contact with the river bed.  Materials carried in suspension usually forms the greatest part of the total river bed.