Year 13 IB Geography

Fluvial Landforms and Formation - A collaborative Effort

● Understand the relationship between landforms and the processes that have

created them, especially the interplay between erosion and deposition.● Understand the evolution of fluvial or water-formed landscapes as the outcome of

dynamic processes such as slope erosion, stream erosion and deposition.● Understand the formation of waterfalls and rapids, meanders and associated

landforms, flood plains, braided channels, levees and deltas. Key terms and definitions from above. Cara Landforms : Any feature of the Earth's surface having a distinct shape and origin. Landforms include major features (such as continents, ocean basins, plains, and mountain ranges) and minor features (such as hills, valleys, and slopes). Erosion: The removal of part of the land surface by wind, water, gravity, or ice. These agents can only transport matter if the material has first been broken up by weathering. Deposition: The dropping of material which has been picked up and transported by wind, water or ice. Slope erosion: All slopes are affected by gravity and, consequently, by one or more of the several mass movement processes by which weathered material is transported downhill. Stream Erosion: Streams are one of the most effective surface agents that erode rock and sediment. A stream constantly abrades and weathers the individual rock and soil particles carried by its water.

How does erosion and stream flow affect the shape of the channel? SophieThere are four forms of erosion, corrosion, attrition, hydraulic action and solution. These processes consist of materials carried by the river that contribute to the wearing of the river bank and bed.

Corrasion is when the river picks up material that wears the bank and bed away by abrasion as it rubs along the surface. This process is the major method in which the river erodes horizontally and vertically.Attrition occurs when the bedload is moved downstream and the impact between colliding materials such as boulders, cause the rock to break into smaller pieces.

Eventually these sharp rocks become more rounded and smooth in shape.Hydraulic action takes place when the force of the turbulent river current hits the bank and pushes water into the cracks. This causes the air in the cracks to compress and pressure is increased and as times passes the bank is weakened enough and therefore collapses.Solution/ corrosion occurs continuously and is independent of river discharge or velocity. It is related to the chemical composition of the water for example the concentration of carbonic acid and humic acid.

Therefore the shape of the channel is affected by the type of erosion for example corrassion and hydraulic action can erode one side more then the other causing it to be asymmetrical. Attrition and corrosion cause the bed to be eroded away making the channel deeper. Solution, hydraulic action and corrosion can erode the channel horizontally making the channel wider etc. There are two forms of stream flow, Laminar flow and turbulent flow.

Laminar flow is very uncommon, when it does it flows over the sediment on the bed on the river without disturbing it. On the other hand turbulent is the dominant method, it consists of both vertical and horizontal eddies (which can produce minor whirlpools on the surface).The turbulence varies with velocity of the river, which depend on the energy available after the friction has been overcome.

The shape of the channel is affected by the power of the stream flow, the more powerful the current the more effective the erosion will be. Waterfalls - Conal Waterfalls are usually formed when a river is relatively young. At these times the channel tends to be deep and narrow. When the river flows over resistant bedrock and then flows down over for example a small hillock, erosion occurs slowly at the top of the hillock where the erosion rates differ from that of the hillocks slope, this means that hard bedrock must be present at the top of the hillock. Therefore, when water flows over the hillock it eats away at the softer rock and earth on the slope of the hillock, this gradually steepens the slope until it becomes vertical. At this point the water begins to eat into the softer rock under the harder rock, this causes an overhang to form and thus the waterfall is formed.As the river increases its velocity nearing the edge of the waterfall it grabs material from the riverbed. Whirlpools created in the turbulence as well as sand and stones carried by the river increase the erosion capacity.This then makes the waterfall carve deeper into the riverbed and to recede upstream. Sometimes, over a period of time, the waterfall will fall back to form a canyon or gorge downstream at the same time as it recedes upstream. As it recedes upstream it will then carve deeper into the ridge above it, slowly eating (undercutting) away at the soft rock and earth underneath the overhang. The water that falls down from the waterfall eats away at the hard rock.

Rapids Dan

they areformed by obstructions that the river faces on its travel downstream. such as rocks, sharp turns etc. anything that causes friction to the flow of water will cause a rapid.

Meanders and Associated

Landforms - AmyMeandersMeanders are a bend in a watercourse in rivers and streams. They are formed when the moving water in a river erodes the outer banks and widens the valley.In meandering streams, erosion and deposition occur at the banks. Point bars form where slow moving water deposits fine material. Cut banks

occur where faster moving water erodes away bank materials.Think of the water flowing in a channel like cars moving down a highway. At the outside of a bend the water flows faster than the water on the inside of the bend.As it moves downstream the past of the deepest flowing water moves to the opposite bank. Therefore erosion still occurs on the outside of the bend and deposition occurs on the inside. This is the River Strule near Omagh. The narrow strip of land between the two stretches of the river is called the meander neck. In time the river will erode through the neck and the loop will be cut off leaving an oxbow lake. The river will flow straighter and shorten its course. This could happen gradually over a number of years or suddenly in a flood when the river has masses of energy. The water is always being thrown to the outside of the bend and it is here that erosion is at its strongest. Oxbow LakesAn oxbow lake is a U-shaped body of water formed when a wide meander from the main stem of a river is cut off to create a lake, by the force of erosion. It will be slowly created as soil erodes and re-deposits, changing the river’s original course.On the inside of the loop, the river travels more slowly leading to deposition of silt, while water on the outside edges tends to flow faster, eroding the outside banks.

Over time the loop of the meander widens until the neck vanishes altogether.

Eventually the bend becomes isolated from the river’s path and a horseshoe shaped oxbow lake is formed.

Once the water stops flowing in the former river bed, sediment begins to build up in the lake. Eventually the lake will become a wetland, then a meadow; then trees will take root. In a process known as ‘succession’ what was once a river will once day become a forest.

This oxbow lake is in Rio Madre de Dios which runs through the Peruvian Region which then becomes the Beni River in Bolivia and then turns northward into Brazil.

Flood Plains - Borja

When rivers reach the highest level

(bankfull stage), water starts to overflow and the land beside the river starts to flood. This adjacent land is known as the floodplain.

The floodplains are normally reached by flooded due to the excess of rainfall which raises the river level, by this there is a visible increase in the wetted perimeter (the total length of the bed and the bank sides in contact with the water in the channel) and the hydraulic radius (the ratio between the area of the cross-section of a river and the length of its wetted perimeter. This results in an increase of friction causing a decrease in velocity, causing the sediment that was previously in suspension to be deposited on the river bed. The fertility of the land is improved by the silt which is deposited by each flood; this layer of silt causes it to rise in height. The floodplains can decrease in area, due to lateral erosion on meanders. The edge of the flood plain is limited by a slope often called the bluff line. Braided Channels- Katie.A river channel in which have been deposited bars and islands around which the river flows. It has been shown that, for a given discharge, braided channels slope more steeply than meandering channels. Braiding occurs when the discharge fluctuates frequently, when the river cannot carry its full load, where the river is wide and shallow, where banks are easily eroded, and where there is a copious bedload, as is common in periglacial environments.

Levées - TomNatural levees are effects of fluvial deposition

● The process of this is when a river overflows its banks, the increase in friction produced by the contact with the flood- plain causes material to be deposited.

● The coarsest materials dropped first to form a small, natural embankment alongside the channel.

● During subsequent periods of low discharge, further deposition will occur within the main channel causing the bed of the river to rise and the risk of flooding to increase.

Human changes● To try to contain the river, the embankments are sometimes artificially strengthened and

heightened (the levee protecting St Louis from the Mississippi is 15.8m higher than the floodplain which it is meant to protect).

● Some rivers, such as the Mississippi and Yangtze; flow above the level of their floodplains which means that if the levees collapse there can be serious damage to property.

● This has occurred twice now in the state of Mississippi, once in 1993 and then again in 2008, many levee’s broke in the process of heavy rains and high precipitation.

DeltasBen What forms them?

A delta is a landform that is formed at the mouth of a river located where the river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. A delta is usually composed of fine sediment which is deposited when a river loses energy and competence as it flows into these relatively flat areas of slow moving water expanding the width of the river. Over long periods of time, this deposition builds the characteristic geographic pattern of a river delta.

Deltas vary greatly in shape; however, geomorphologists have grouped them into three basic forms:

· Arcuate: having a rounded , convex outer margin, e.g. the Nile· Cuspate: where the material brought down by a river is spread out evenly on

either side of its channel, e.g. the Tiber· Bird’s foot: where the river has many distributaries bounded by sediment and

which extend out to sea like the claws of a bird’s foot, e.g. the Mississippi