The Afon Glaslyn, SnowdoniaCase Study of fluvial landforms and processes

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The Afon Glaslyn, Snowdonia

Case Study of fluvial landforms and processes

All rivers aim to reach equilibrium so they are balanced and when they are in equilibrium from source to mouth a smooth curve is created called the graded long profile. However, as you can see from the diagram many rivers such as The Afon Glaslyn, do not acheive equilibrium and reach their graded long profile:

Long Profile of Afon Glaslyn                                        River with graded long profile

Rivers become out of equilibrium when there is a change in base (sea) level. There can either be a positive change (rise) in the sea level or a negative change (fall) in sea level, resulting to a change in base level and a new graded long profile. Sea level changes happen because the Earth is in or out of glacial periods. Therefore sea level is changed either eustatically- a change in sea level due to a change in the amount of water locked up in ice sheets, or isostatically- when the the change in sea level is due to a change in the level of land caused by growing or retreating ice sheets. The change in base level affects the long profile of a river and as the river tries to reach its new graded profile in order to reach equilibrium there are various processes and landforms found along the river's course. By looking at The Afon Glaslyn we can see how and why these processes occur and landforms formed.

The Upper Reaches 

0-4km   

  • Processes

The main process in this section is erosion, in the form of: attrition, corrasion, hydraulic action and corrosion. Attrition is when the boulders collide with each other, breaking up into smaller pieces. Corrasion is when the ,material carried in suspension rubs against bank of river. Hydraulic action is the rivers force dislodging particles and finally corrosion is when acids dissolve rock. Erosion occurs because at this point the river is above its graded profile (base level decreased) and the lithologhy (made up of hard rock), this means that the river has to erode downwards to reach its graded profile and be in equilibrium. As well as erosion transportation also occurs - the load is made up of large boulders sand and clay.Therefore it is mainly traction and saltation taking place- rolling stones across bed and sand size+particles bouncing along bed. As the sediment is large has not been eroded yet there is little suspension (clay sized particles  carried with flow) but no solution (when minerals dissolve in water). Chemical and physical weathering is also taking place with the rain making the frost shatter to give large boulders and sands and the acids in vegetation being leached downwards into the rock to give clays and some sands.  

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  • Landforms

The landforms in this section are waterfalls and rapids and potholes. Waterfalls form due the decrease in base level which results in vertical erosian (to reach equilibrium) and also due to the lithology and the horizontal bands of alternate rock. Many waterfalls form when rivers meet a band of band of softer rock after flowing on hard rock. The underlying softer rock is worn away more quickly, and the harder rock is undercut and overhangs. In time the hard rock becomes unsupported and collapses. Afterwhich some of the rock will be swirled ...

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