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

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Jordan Beasley

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.’ Gilbert therefore believed that a graded channel is the inevitable product of a river using up excess energy and once a river reaches a graded condition it is in a state of equilibrium and unable to deepen their valleys or change the form and gradient of their long profiles directly.

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According to Briggs & Smithson a graded profile is necessary to erosion and transportation as ‘In the upper reaches of the stream, the discharge is low and the sediment coarse. A steep slope is therefore necessary to transport the material. Down-slope the discharge increases while the sediment is finer thus more energy is available, but the losses of energy through friction have not increased proportionately. The sediments can therefore be transported over gentler slopes.’ This explanation would tie in with the Hjulström diagram depicting the higher velocities required to erode and transport pebble and cobble sized particles, found upstream, ...

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