Rivers – The Essentials. Long Profile. A long profile can tell you how a river and its valley changes from source to mouth. Cross Profile. A cross profile is an outline of how a river or valley would look like if you cut a section of it vertically e.g. across a line from A to B.
A long profile can tell you how a river and its valley changes from source to mouth.
A cross profile is an outline of how a river or valley would look like if you cut a section of it vertically e.g. across a line from A to B.
Why do they change? Think about the effects of changes in gradient, processes of erosion, deposition, velocity etc.
This is the process by which the bed and banks are worn down by the river’s load. The river throws these particles against the bed and banks, sometimes at high velocity.
This process involves the force of water against the bed and banks.
This is the chemical action of river water. The acids in the water slowly dissolve the bed and the banks.
Material (the load) carried by the river bump into each other and so are smoothed and broken down into smaller particles.
Rivers pick up and carry material as they flow downstream. A river may transport material in four different ways:
Solution - minerals are dissolved in the water
Suspension - fine light material is carried along in the water
Saltation - small pebbles and stones are bounced along the river bed
Traction - large boulders and rocks are rolled along the river bed
Waterfalls are formed where a river flows over a band of hard rock next to a softer rock. The softer rock is eroded by hydraulic action and corrasion. The power of the water creates a deep plunge pool below the waterfall and undercuts the softer rock until the waterfall eventually collapses. This causes the waterfall to retreat upstream forming a gorge.
A meander is a bend or curve in the river.
The formation of meanders is due to both deposition and erosion.
The high velocity of the water erodes and undercuts the river bank on the outside of the bend where water flow has most energy (deeper, so less friction), forming a river cliff.
On the inside of the bend, where the river flow is slower (shallower, so more friction), material is deposited forming a slip-off slope (river beach)
Oxbow lake formation
Erosion on the outside of the bend of a meander and deposition on the inside make the meander wider and the neck gets narrower. Often during a flood, the river breaks through, creating a new straight course and deposition (silt) seals off the old meander to form an oxbow lake.
Over time, the water in the oxbow lake will evaporate (summer months) and disappear. Mud and silt will also get washed into the oxbow lake when it rains, so it will fill up and eventually vegetation will grow where water used to be. There will be a slight scar in the landscape left behind.
clays and silts
The formation of levees
When a river overflows, the velocity decreases (water flow on flood plain – shallower/more friction) and it begins to deposit its load, the largest material first (as it has more energy nearer the channel – deeper/less friction/higher velocity). Smaller materials like sands, clays and silts get deposited further away from the river channel and onto the flood plain as they are easier to transport (the river does not have the energy to transport large material – velocity is reduced in shallower water).
After repeated floods, high banks called levees are formed at the sides of the river.
A flood plain is the flat area of land (hillsides mark its boundary) where the river meanders across, either side of a river. During periods of high rainfall, a river will overflow its banks and deposit sediment (very small material e.g. silt) as the water evaporates and recedes. Over the years, this cycle will help build a layer of silt on the flood plain. This material is very fertile and excellent for growing crops.
Think of the drainage basin hydrological cycle (above) when answering a question about hydrographs.
Basin lag time
0 12 24 36 48 30 72
Hours from start of rain storm
What is a hydrograph? A hydrograph is a line graph drawn to show the discharge in a river after a period of rain.
Why are they used? They are used by river engineers to show how a particular river responds to a period of rainfall and can help with predicting floods and also preventing them.
a. impermeable – doesn’t let water infiltrate through it e.g. clay. Leads to fast rising/flashy (responds quickly)hydrograph
b. permeable – it lets water through e.g. sandy soil / porous rocks )
c. Porous – rocks with small holes in e.g. limestone/chalk/sandstone
HUMAN (man made)
Mozambique Floods 2000
Case Study of a flood in a LEDC
Case Study of a flood in a MEDC
Since the serious 1993 Mississippi floods (USA), some scientists began asking whether rivers should be allowed to flood naturally.
There are 3 main strategies in response to controlling rivers which flood:
1.HARD ENGINEERING (the use of technology to try to control rivers)
2.SOFT ENGINEERING (trying to work alongside/with the river
3.Do nothing (this is often necessary in poor countries)!
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21604 (waterfall animation)
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21394 (flood plain and levees)
http://www.geography.ndo.co.uk/animations3.htm (processes of erosion)
http://www.cleo.net.uk/resources/displayframe.php?src=309/consultants_resources%2F_files%2Fmeander4.swf (meander formation)
http://www.bbc.co.uk/schools/gcsebitesize/geography/riverswater/floodingrev4.shtml (Mozambique floods)
Geography at the movies (video clips) -http://www.geographyatthemovies.co.uk/ (Coasts)