Laboratory Manual for Introductory Geology 4e

FIGURE 13.8 A floodplain and its associated features.
Floodplain
Oxbow lake
Point bars
Bluff
Point bar
Natural levee
High-water level
Floodplain
deposits
Ancient floodplain
deposits
Stream bed
gravel
Meander
scars
Ancient channel
and point bar
flood. Sediment carried by the floodwater is deposited on the floodplain (an area of
flat land on either side of a stream that is covered with water during a flood---when
the stream overflows its banks, the sediment it is carrying is deposited within the
floodplain, enriching its fertility).
Many depositional and erosional features of floodplains can be easily recognized
on topographic maps or in photographs (FIG. 13.8). Natural levees are ridges of sediment
that outline the stream channel; they form when a stream overflows its banks
and deposits its coarsest sediment next to the channel. Several generations of natural
levees are visible in Figure 13.8, showing how the meanders changed position with
time. Point bars form where water on the inside of a meander loop slows down, causing
sediment to be deposited. At the same time, erosion occurs on the outside of the meander
loop because water there moves faster. As a result, meanders migrate over time,
moving outward (toward their convex side) and downstream. Sometimes a stream cuts
off a meander loop and straightens itself. The natural levees that formerly flanked the
meanders help outline the former position of the river, leaving meander scars. Meander
scars that have filled with water are called oxbow lakes.
13.4 Changes in Streams over Time
Streams erode vertically over time by leveling the longitudinal profile to the elevation
of the mouth. Streams also erode laterally, broadening their valleys by meandering (see
Fig. 13.7). Most streams erode both laterally and vertically at the same time, but the
balance between vertical and lateral erosion commonly changes as the stream evolves.
The headwaters of a high-gradient stream are much higher than its mouth,
and the stream’s energy is largely expended on vertical erosion, lowering channel
13.4 CHANGES IN STREAMS OVER TIME
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