Download Temporary Local Base Level

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
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