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How valid is the concept of grade in a river's long profile

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Introduction

How valid is the concept of grade in a river's long profile? A graded channel is a channel with an idealised smooth concave long profile: a steep youthful source declining into an elongated gentle elderly channel downstream toward its mouth. According to Collard 'As rivers evolve through time they appear to work towards the achievement of a smooth, concave profile ... from which irregularities are gradually removed.' G.K. Gilbert, an American geomorphologist first put forward the concept of grade believed that as a channel gradually wore away the land to give a flat peneplain resulting in the decline of the gradient downstream whilst upstream the channel continues to eat into the uplands consequently a graded-channel would eventually form. To this graded channel Gilbert devised the following classic definition; 'Where the load of a given degree of comminution is as great as the stream is capable of carrying, the entire energy of the descending water is consumed in the translation of the water and load and there is none applied to corrosion.' ...read more.

Middle

Theoretically a graded profile is not an unrealistic assumption of the general shape of most rivers however in reality countless factors play a part in creating irregularities in the smooth concave long-profile. Clowes & Comfort outlined and example of this in the creation of waterfalls and rapids resulting in irregular steep drops in the long profile of a river. 'Where a river flows from hard to soft rock, the softer is eroded relatively rapidly and the gradient is locally steepened to form a waterfall or section of rapids.' Where waterfalls and rapids occur an uneven distribution of energy is produced causing an eventual lowering of the gradient in the section of the river immediately followed by the feature. On a smaller scale that waterfalls and rapids, pool and riffle sequences along a meandering channel produce fluctuations in a streams long-profile whereby a sequence of deposited material, riffles, raise the gradient along the straight section of a meander whilst eroded sections at the apex of the bend, pools, lower the gradient resulting in a fluctuating local-profile. ...read more.

Conclusion

This process is called rejuvenation.' Rejuvenation has a larger effect on altering a rivers profile than a positive chance as new land is created which in time will be eroded by the river. According to Waugh, Britain experienced rejuvenation on most coast lines at the end of the Pleistocene glacial period; 'Britain was depressed by the weight of ice. Following deglaciation, the land slowly rose again (isostatic uplift). Thus rejuvenation took place on more than one occasion with the result that many rivers today show several partly graded profiles.' With sufficient time the profile of this particular river may become completely re-graded with the knickpoints, the extent of a newly graded profile often a waterfall, disappearing. Although hypothetically the idea of grade is certainly a logical and valid concept it does not take into account many of the factors affecting a rivers long-profile. As a river's circumstances change regularly it is unlikely a river will ever reach a perfectly graded state as equilibrium between erosion and deposition in a river is not an instant process and by the time it has happened a river's circumstances will have changed. ...read more.

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