presents several examples of landform evolution, including the long-term cycle of erosion and
evolution of incised stream channels.
In the context of a rivers age, the process of channel rejuvenation refers to an increase in
erosional activities in mature or old channels caused by lowering base level elevation, tectonic
activities or other causes. Rejuvenated mature or old channels then exhibit some properties of
youthful channels such as channel incision and erosion processes.
5.2.2 Floodplain and Delta Formations
Over time, the highlands of an area are worn down. The streams erode their banks. The
material that is eroded is utilized downstream to build banks and bars to further enhance the
meandering process. Streams move laterally pushing the highlands back. Low flat valley land
and floodplains are formed. As the streams transport sediment to areas of flatter slopes, and
in particular to bodies of water where the velocity and turbulence are too small to sustain the
transport of the material, the material is deposited forming deltas. As deltas build outward the
up-river portion of the channel is elevated through deposition and becomes part of the
floodplain. Also, the stream channel is lengthened and the slope is further reduced. The
upstream river bed is filled in and average flood elevations are increased. As they work across
the river valley, these processes cause the total floodplain to raise in elevation. Hence, even
old streams are far from static. Old rivers meander, are affected by changes in sea level, are
influenced by movements of the earth's crust, are changed by delta formations or glaciation,
and are subject to modifications due to climatological changes and as a consequence of
human development.
5.2.3 Alluvial Fans
Alluvial fans are very dynamic landforms that can create significant hazards to highways as a
result of floods, debris flows, deposition, channel incision, and avulsion (Schumm and Lagasse
1998). They occur whenever there is a change from a steep to a flat gradient. As the bed
material and water reaches the flatter section of the stream, the coarser bed materials can no
longer be transported because of the sudden reduction in both slope and velocity.
Consequently, a cone or fan builds out as the material is dropped. There is considerable
similarity between a delta and an alluvial fan. Both result from reductions in slope and velocity
and both tend to reduce upstream slopes. Alluvial fans, like deltas, are characterized by
unstable channel geometries and rapid lateral movement. An action very similar to the delta
develops where a steep tributary enters a main channel. The steep channel tends to drop part
of its sediment load in the main channel building out into the main stream. In some instances,
the main stream can be forced to make drastic changes at the time of major floods by the
stream's tributaries.
Fans can be of two types, dry or mudflow fans formed by ephemeral streamflow, and wet or
fluvial fans formed by perennial stream flow. Two different conditions of fan morphology are
observed on modern dry fans. The first situation occurs when deposition is near the mountain
front and the fan surface is undissected. The second situation occurs when sediment material
is moved through a fan-head trench and deposition occurs at the toe of the fan. Good
relationships exist between fan area and drainage basin area (Schumm 1977). These
relationships among fan slope, area, and drainage basin characteristics are not surprising. The
presence of fan-head trenches, however, is sometimes attributed to tectonic activity or climate
change.
5.2
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