Landscape Development
Slopes Dominate
Natural Landscapes
• Created by the interplay of tectonic and
igneous activity and gradation
– Deformation and uplift
– Volcanic activity
• Agents of gradation
– Mass wasting
– Streams
– Wind
– Glaciers
Slope Formation
• Slopes are temporary, in the context of
geologic time
– Change may result from renewal of the activity
which created them, or by other actions
• Change may be rapid, but, overall, is typically
quite slow
– Earthquakes, eruptions, floods, landslides, glaciation
– Creep: the very slow downward adjustment of hillsides
• Geologists attempt to understand landforms in light of
their formation and continued adjustments
The hydrologic cycle
Figure 16.3
Slope Formation
• Rainsplash, sheetflow, rIlls, runoff and streams
– Water falls on the land and quickly enters (forms)
small channels
• These join together in a drainage network to remove
the water
• Channels lengthen by headward erosion, intercept
other channels and grow in size and flow volume
• An efficient drainage system is developed
Longitudinal profile
of a stream
Figure 16.7
Running water
• Changes from upstream to downstream
• Profile
– Profile is a smooth curve
– Gradient decreases downstream
• Factors that increase downstream
– Velocity
– Discharge
– Channel size
Running water
• Changes from upstream to downstream
• Factors that decrease downstream
– Gradient
– Channel roughness
• Base level and graded streams
• Base level is the lowest point to which a
stream can erode
Running water
• Base level and graded streams
• Two general types of base level
– Ultimate (sea level)
– Local or temporary
• Changing conditions causes readjustment of
stream activities
– Raising base level causes deposition
– Lowering base level causes erosion
A waterfall is an example
of a local base level
Adjustment of base level
to changing conditions
Figure 16.9
Drainage basin
Figure 16.30
The drainage basin of the
Mississippi River
Watershed of Chesapeake Bay
Drainage patterns
Figure 16.32
Running water
• Stream valleys
• The most common landforms on Earth’s
surface
• Two general types of stream valleys
– Narrow valleys
» V-shaped
» Downcutting toward base level
» Features often include rapids and waterfalls
Erosion in Fast-moving Streams
Figure 7-7 A Steep-Gradient Stream
Gradients are steep in headwaters, and
streams flow fast, cut steep-sided channels,
erode vigorously, and carry
coarse sediment.
Figure 7-8 Rivers Erode the Geosphere
Arizona’s Grand Canyon is an example of
how effective river erosion can be. In this
case the Colorado River downcut rapidly
In response to uplift on the Colorado Plateau.
Sideways Erosion—Cutbanks
Figure 7-9 Erosion along River Cutbanks
Can Be Hazardous
• Rivers obviously erode downward
but they also erode sideways or
laterally, especially if the gradient
is lower.
• On the outside bend of a stream a
channel may be eroded; this is
called the cutbank (the stream
cuts into the bank here).
• Opposite the cutbank is the
pointbar—a focus of deposition
rather than erosion as streams
migrate laterally.
Running water
• Stream valleys
• Two general types of stream valleys
– Wide valleys
» Stream is near base level
» Downward erosion is less dominant
» Stream energy is directed from side to side
forming a floodplain
Running water
• Stream valleys
• Features of wide valleys often include
– Floodplains
» Erosional floodplains
» Depositional floodplains
– Meanders
» Cut bank and point bar
» Cutoffs and oxbow lakes
Erosion and deposition along
a meandering stream
Figure 16.14
Link: Map of Licking River
Erosion and Deposition—Low Gradient
Streams Near Base Level
• Broad loops and curves are common along the lower
reaches of rivers, near their base levels.
• These bends, or meanders (Fig. 7-10a), change as a result
of erosion and deposition working hand in hand.
Figure 7-10 Meanders and
Oxbow Lakes
(a) Meanders erode banks
where river’s gradient is low.
(b) If a river cuts across
narrow neck of a meander
loop, sedimentation may seal
off the loop, transforming it
into an oxbow lake.
Sediment Transport—Loads
• Bedload, is dragged, rolled,
skipped, or bounced over the
river bottom—the latter is
referred to as the process of
saltation
• Finer particles carried along in
the river water itself are the
suspended load.
• Dissolved minerals and some
Figure 7-11 Mechanisms of Sediment
organic materials leached from Transport
soil/rock
Gravel-rich bedload is dominant along
steeper river gradients, whereas suspended
make up the dissolved
and dissolved loads are common at lower
load.
gradients.
Stream Carrying Capacity
• At a specific velocity and
discharge, a stream has a
maximum amount of bed and
suspended load it can carry =
capacity.
• Capacity ↑ with Velocity and
Discharge ↑ , therefore more
erosion and sediment
transport occurs during
flooding.
• Material transported in large
rivers consists mainly of
suspended load, such as silt.
Figure 7-12 The World’s Muddiest River
China’s Huang Ho (Yellow River) carries a
tremendous suspended load of silt—up
to 70% of the river’s volume. Its
watershed has much loess, a silt-rich
windblown sedimentary deposit.
Running water
• Incised meanders and stream terraces
• Incised meanders
– Meanders in steep, narrow valleys
– Caused by a drop in base level or uplift of the
region
• Terraces
– Remnants of a former floodplain
– River has adjusted to a relative drop in base level
by downcutting
A meander loop on the
Colorado River
Running water
• Headward erosion and stream piracy
• A stream can lengthen its course by either
– Building a delta
– Headward erosion
• Headward erosion may result in stream
piracy – the diversion of the drainage of one
stream into another