Rivers Flashcards
V-shaped valley, interlocking spurs
The valley is narrow with a narrow, shallow river channel, steep sides/Channel has a steep gradient
Water is mainly slow flowing as most of the river’s energy is used to overcome the friction of the riverbed & obstructions
The load is mainly large, angular and rough
Interlocking spurs: In the upper valley a river is in the mountains. Water takes the easiest path downhill so twists & turns around the high land (spurs) forming interlocking spurs.
The river cuts down into the valley to form a narrow, steep-sided V-shaped valley. If there are areas of hard rock which are harder to erode, the river will bend around it. This creates interlocking spurs.
Waterfalls
They occur because the river flows over hard rock which erodes slowly.
Beneath is a softer rock which is eroded faster to form a “step”.
The force of the water erodes the bottom of the
waterfall to form a plunge pool.
The hard rock gets undercut as the soft rock erodes so that it eventually collapses.
Meanders and Oxbow lakes
As the river erodes laterally, to the right side then the left side, it forms large bends, and then
horseshoe-like loops called meanders.
The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream.
The force of the water erodes and undercuts the river bank on the outside of the bend where
water flow has the most energy due to decreased friction. This creates a steep-sided riverbank on the outer bend called a river cliff.
On the inside of the bend, where the river flow is slower, material is deposited, as there is more friction. This forms a gently sloping inside bend, known as a slip-off slope.
Over time the horseshoe becomes tighter until the ends become very close together.
As the river breaks through, e.g. during a flood when the river has a higher discharge and more energy, and the ends join, the loop is cut off from the main channel.
The cut-off loop is called an oxbow lake.
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 where river mouths become choked with sediment, causing the main river channel to
split into hundreds of smaller channels or distributaries.
Levees
When a river floods, the coarsest material is deposited first, on the edges of the river, forming a natural embankment called a levée.
Deposition happens due to an increase in friction when the river comes into contact with the bed meaning that energy is lost.
Flood plains
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 building up into a large flood plain.
The opportunities presented by a river or rivers, the associated hazards and their management
The Mississippi River, USA
Key facts
Key facts: * Mississippi is 3800km long
* Flows through ten states
* Has over 100 tributaries
* Has a drainage basin covering 1/3 of the USA
1993 River Floods
Causes of river flooding Mississippi
Urbanisation reduces permeability in the river
basin and increases the flood risk. Before urbanisation, there would have been more flood plains available for water to occupy during times of flood.
Like many floods this flood was caused by large amounts of precipitation over a long period, in June and July of 1993 much of the area experienced over 150% of normal rainfall and some areas
On top of this, the ground was already saturated not only by the previous rainfalls but during the previous year 1992 the weather conditions were considerably cooler which meant less evaporation and so there was less rainfall absorbed by the soils to start with.
Impacts of 1993 flooding:
- 43 deaths
- 50,000 people evacuated
- 26,000km of land flooded
- $2.46 billion crop losses
- River traffic stopped for several months
- $12 billion in damages
- Contents of and the buildings themselves destroyed
- Threat of disease from sewage
- Insurance claims high
- Stagnant water attracted mosquitoes and rats
Management:
Flood management on the Mississippi
Hard engineering:
* 6 huge dams and 105 reservoir
* Strengthening the levees with concrete mattresses 25mx8m
* Making the course shorter and straighter by cutting through the neck of meanders to get the water passed towns more quickly to the sea
* Diversionary spillways – overflow channels
Soft engineering:
* Afforestation to delay runoff
* Less construction on the floodplain e.g. St Louis.
What are the benefits of living next to the Mississippi River?
- Settlements-in the lower course of the river the land is flat and easy to build on.
- Tourism
- Industry – The land in the lowlands of the river course is flat and easy to build on
- Transportation - Waterways are useful and important for transportation. Often this is for cargo boats. The total cargo of 2004 was around 31.5 million short tons consisting of 72% imported cargo and 28% exported cargo.
- Commercial fishing (and shrimping in the delta) are important to the people of the Mississippi drainage basin economically and for a food supply.
- Farming - The flood plain in particular is rich in nutrients and the soils are used for
agriculture. - The Mississippi is home to many Native Americans who live up in the Upper Mississippi
Valley
What are the problems of living next to the Mississippi River?
- Water Pollution is problematic. Poor farm management results in an inch of topsoil being lost every 12 years and this will flow into the river.
- Farmers spray herbicide and insecticide chemicals on the soil and this too will flow into
the river causing algal blooms (eutrophication). - Poor quality drinking water - Many cities along rivers take their drinking water from the river. The chemicals inside the water cannot be removed by present purification methods.
- Fish in streams have been observed with cancer, fin rot, deformed bones and skin diseases.
- Fish-eating birds such as the Bald Eagle and Peregrine Falcon, were, until recently, on the verge of becoming extinct due to problems in the food chain.
- Hurricanes strike by the delta of the Mississippi which can cause flooding, storm surges and problems for the people of places like St. Louis.
- Flooding –
