Post by Senior on Oct 30, 2016 1:44:24 GMT 3
Parts of a river:
Drainage basin – the area drained by a river and all its tributaries.
Watershed – the stretch of land that forms the boundary around a drainage basin and
separates it from another drainage basin.
Course of a River:
1) Upper course
This is where the river begins.
River channel is small, narrow, rough and usually shallow.
There may be large angular boulders in the channel.
Many smaller streams and channels join up to form larger streams.
Gradient is usually quite steep.
2) Middle course
River channel is wider and deeper.
The channel starts to meander.
Many tributaries join the river.
The gradient becomes less steep.
3) Lower course
River channel is widest and deepest.
River has the largest volume and load.
The gradient is usually very gentle.
Meanders are very common.
A delta may form at the river mouth.
Energy of a river.
- look in your text book. The tables summarizes the relationship between the energy
possessed by a river and the volume and speed of flow.
How water gets into rivers
The most direct path is direct precipitation.
Rain water falls directly into lakes or rivers.
Part of the water that reaches the ground soaks into the soil.
Some of this water flows sideways down slope towards the river.
Some of the water moves further down into the underlying rock layers.
This collects in the rock as underground water.
Some of this underground water may flow sideways
and enter the river as base flow.
When the water is not absorbed into the ground fast enough,
the water flows over the ground down slope into the river.
River transport
1) Traction
Occurs when rock fragments are too heavy to be lifted by the water.
These fragments roll and slide along the river bed.
2) Saltation
Occurs when rock fragments are to heavy to be suspended in the water all the time.
These fragments bounce along the river bed.
3) Suspension
Occurs when fine particles can be kept in the water column by the flow of the water.
4) Solution
This takes place when minerals in the rocks dissolve in the water.
The minerals are carried downstream in solution.
Landforms
NOTE: If a question on waterfall formation comes out, give answer as 1a. or if you have time
both 1a and 1b.
1a. Waterfalls (differential erosion)
A river may flow across rocks of differing hardness (fig.a).
The less resistant rock gets eroded faster.
forming a slight depression at the boundary between the hard and soft rocks.
This causes turbulence and increases erosion (fig. b).
Over time this results in a sudden change in gradient.
causing the water to plunge forming a waterfall.
The water plunges and hits the bottom of the waterfall.
eroding the river bed to form a plunge pool (fig. c).
1b. Waterfalls (faulting)
A river may flow over a fault line (Fig a)
During faulting, rocks are uplifted (Fig b).
If a river flows across an uplifted area,
the gradient of the bed drops suddenly
forming a waterfall.
2. Floodplains and levees.
Description:
Floodplains are wide low-lying plains found on both sides of a river.
It is covered with alluvium deposited by the river.
Levees are natural embankments found along the banks of the river.
Made up of coarser materials than the flood plain.
Formation:
During heavy rain, the river may overflow its banks.
Out of the channel, the water encounters more friction.
This results in deposition.
The larger, heavier fragments are deposited first.
The smaller, lighter fragments are carried further from the bank (Fig b).
Repeated flooding causes alluvium to build up (Fig c).
The coarser fragments near the banks form raised embankments.
The finer fragments form the flood plains (Fig d).
(Figures on next page)
3. Deltas.
Description:
Deltas are broad platforms of alluvium that may be built up at the mouth of rivers.
Formation:
When a river enters a lake or the sea it loses speed.
This results in a decrease in the energy of the flow of water.
Deposition of sediments takes place.
Sand is deposited at the river mouth.
Silt and mud deposited further out to sea.
Over time layers of sediment build up.
Eventually rise above the water surface to form a delta.
Conditions for formation:
1. There is active erosion along the river
so that the river carries a lot of sediment.
2. Tidal currents along the coast must not be strong.
Sediments may be washed away from the mouth.
3. There is a gentle offshore gradient with shallow water.
Sediments may be dispersed out to sea instead of being deposited.
4. Meanders and ox-bow lake.
Description:
Meanders are loops in the course of the river.
Formation:
Erosion occurs on the concave bank of the meander because of faster flow.
Deposition occurs on the convex bank because of slower flow.
Continuous erosion and deposition forms a very pronounced meander.
The two neighboring concave banks get closer together (Fig 1).
The narrow neck of land is finally cut through.
A new straighter river channel and
an abandoned meander loop (cut-off) are formed (Fig 2).
Deposition eventually seals off the cut-off forming an ox-bow lake (Fig 3).
River Management Strategies (know at least three very well).
Introduction: There are a few ways in which people have tried to manage rivers, mainly to
prevent flooding. These methods are collectively referred to as channelisation. Types of
channelisation are re-alignment, re-sectioning, bank protection and planting vegetation.
Point: Re-alignment is the straightening of the river channel.
Example: The Mississippi River in the USA has been shortened by 240km.
Elaboration: This method removes meanders
and reduces the length of the river channel,
increasing the speed of the river.
Success: This allows water to flow away from the area quickly and
wash away sediments which have accumulated on the river bed.
This deepens the channel allowing it to hold more water reducing floods.
Point: Re-sectioning is the widening and deepening of the river channel,
and the smoothing of the banks and bed using cement.
Example: In Singapore, the Singapore River and Kallang River has been widened and
deepened and most of the river has been encased in concrete.
Elaboration: Re-sectioning increases the capacity of the channel
and decreases the friction between the water and the banks and bed.
Success: This allows the channel to hold more water.
More water from the surrounding area can also now enter the channel.
Smoothing the banks and bed allows water to flow away quickly.
Decreasing the chances of flooding.
Limitation of re-alignment and re-sectioning:
Increasing the speed of the river may result in flooding downstream.
The channel downstream may not be able to handle the increased flow.
Increased speed also caused more erosion.
This sediment is deposited downstream reducing the river’s volume
and increasing the chances of flooding.
The sediments that enter the sea may block sunlight reaching the seabed.
This can affect the growth of sea grasses and corals
which depend on sunlight for photosynthesis.
The cost of building and maintaining these structures are also very expensive.
Point: Planting vegetation can protect river banks against erosion.
Example: Countries along the Mekong River have initiated a program to protect mangroves
along the river.
Elaboration: The roots of trees and other plants hold the soil firmly together.
Vegetation slows down surface runoff, preventing large amounts of water from entering the
river.
Success: This reduces the amount of eroded sediments entering the river.
The river does not become shallower reducing chances of flooding.
Limitation: Trees could collapse into the river and also cause the banks to collapse.
Woody debris may slow down the flow of the water encouraging flooding.
Woody debris may also deflect water towards the bank causing bank erosion.
Point: Bank protection utilizes gabions or revetments to reduce bank erosion.
Example: Revetments have been built along the Mississippi River, especially when it flows
through urban areas.
Elaboration: Gabions and revetments divert the flow of water towards the centre of the
channel.
This protects the banks from being eroded, reducing the build up of sediments.
Success:
The decreased bank erosion reduces the amount of sediment in the river, keeping it deeper.
Limitations: The protection of one part of the bank leads to erosion at unprotected areas.
The cost of building and maintaining these structures are also very expensive.
They may also be aesthetically unappealing.
Conclusion: Although these measures have decreased the chances of flooding in some
parts of the river, other parts of the river may experience greater incidences of flooding.