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Long Profile of a river How does the channel gradient change? Base Level • Permanent Base level: is the lowest point to which erosion by running water can occur. In the case of rivers the theoretical limit is the sea, although there are exceptions where a local base level may exist, such as with the great Lakes of the USA. • Temporary Local Base Level: Lowest level to which part of a river can erode downwards, e.g. where a river flows into a lake, where a tributary joins a main river or there is a resistant band of rock crossing a valley. Changes in base level result from: • 1. Climatic change: Glaciations and changes in rainfall. • 2. Tectonic change: Where land is uplifted after plate movement or volcanic activity. • Changes can be positive sea level rises in relation to the land, or negative sea level fall in relation to the land. GRADED PROFILE • River is an open system striving to achieve dynamic equilibrium – a balance between the rate of erosion and rate of deposition • Graded profile: gentle concave sloping profile. In reality the gradient is always adjusting to changes in discharge and load. Changes in Base level cause…changes in the graded profile • Positive Change results in a decrease in the gradient of a river as deposition becomes dominant. • Negative Change results in a steepening of the gradient as land emerges from the sea. This increases the river ‘s potential energy and thus the rate of downward erosion and the river undergoes a process called rejuvenation. Beginning at the lowest end, the river will try to regrade itself. Characteristic landforms are the result. • These are knickpoints, terraces and entrenched meanders. • Draw Figure 3.46 p. 76 Antalaya Turkey