River rejuvination Essay

Submitted By ajhudson
Words: 622
Pages: 3

The long profile of a river is not static entity and is prone to change due to changes in sea level. Indeed, a river is already in dynamic equilibrium because things may be balanced over long periods of time but over the short term more or less rainfall can increase or decrease the amount of energy available to the river to move material. Over very long periods of time but over the short term more or less rainfall can increase or decrease the amount of energy available to the river to move material. Over very long periods of time large changes in sea level can drastically alter the rivers potential energy and can increase the chances of vertical erosion. Sea levels change because of 2 principal causes - isostatic changes and eustatic changes, both of these changes can lead to rises or falls in sea level. Isostatic changes are local changes where the height of the land changes relative to the global sea level. An example of this can be found in glaciation. During glaciations the weight of ice pushing on the Earth can depress the crust, causing the land to fall relative to the level of the sea. When the ice goes in warmer periods the land rebounds upwards as Scotland and Northern Britain are today - rising relative to the level of the sea. This process is also seen in deltas where sediment loading depresses the Earth's surface. Eustatic sea level change is a global change in sea level, linked directly to the temperature of the Earth. In warmer periods there is less ice and the water is warmer, so expands and sea levels rise. In colder periods the ocean water is colder so contracts and sea levels fall. When sea levels fall we get raised beaches, abandoned cliffs, erosion surfaces created at the coast, and several adjustments made to river valleys. These changes to river valleys are attributable to the fact that the rivers graded profile is no longer in balance, and the gradient of the river at its mouth is now steeper because of the fall in sea level. In effect the water has further to fall on its journey from source to mouth. This means that there is more energy at the mouth and vertical erosion (which under normal conditions would occur at the river's source) will occur. This leads to the development of knick points, waterfalls, river terraces and incised…