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Chapter 16: The Fluvial Processes

McKnight’s Physical Geography: A Landscape Appreciation,

Tenth Edition, Hess

The Fluvial Processes

• The Impact of Fluvial Processes on the Landscape

• Streams and Stream Systems• Stream Channels• Structural Relationships• The Shaping and Reshaping of Valleys• Floodplains• Theories of Landform Development

2© 2011 Pearson Education, Inc.

The Impact of Fluvial Processes on the Landscape

• Moving water is widespread

• Effective as an agent of erosion and deposition

• Influence on landscape dominant

• Smoothes irregularities

3© 2011 Pearson Education, Inc.Figure 16-1

Streams and Stream Systems

• Any channeled flow of water is a stream• Fluvial processes are those which involve running

water• Unchanneled downslope movement of water—

overland flow• Channeled movement of water—streamflow



• Valleys and interfluves– Valley—portion of terrain in

which a drainage system is established

– Interfluve—higher land above valley walls that separates adjacent valleys

• Drainage basins– Watersheds– Areas that drain to the valley– Drainage divide


Figure 16-2


• Stream orders– Small streams join larger

ones– Relationship called stream

orders– First order versus second

and third order– Smaller numbers in stream

order are more frequent– Larger numbers are longer in

length and watershed order


Figure 16-4


• Fluvial erosion and deposition– Splash erosion– Sheet erosion– Rill erosion– Gully erosion

• Erosion by streamflow– Once channeled, erosion

is greatly enhanced– Erosion increased by debris

the streamflow picks up– Chemical weathering—corrosion


Figure 16-6


• Perennial and intermittent streams– Perennial streams—permanent, in humid regions– Intermittent streams—seasonal– Ephemeral streams

• The role of floods– Amount of water in a stream, discharge– Most excavating of valleys accomplished during

floods– Stream gages used to measure flow characteristics– Flood recurrence intervals (i.e., the “100-year flood”)


Stream Channels

• Streamflow normally confined to channels, giving it a three-dimensional complexity

• Friction effect of flow along bottom and sides of channel

• Highly turbulent flow irregularities produce local currents which modify streamflow


Figure 16-11

Stream Channels

• Stream channel patterns, four categories– Straight channels: short and

indicative of strong geologic structure control

– Sinuous channels: irregular and gentle curvature that is sinuous in appearance

– Meandering channels: highly variable curvature

– Braided streams: multiple interwoven channels separated by loose bars or islands


Figure 16-14

Structural Relationships

• Stream development affected by many factors

• Most important factor is geologic-topographic structure

• Consequent versus subsequent streams

• Antecedent streams—those which existed before new uplift occurs

• Superimposed streams


Figure 16-16


• Stream drainage patterns– Dendritic pattern: treelike

pattern with random merging of streams

– Trellis pattern: forms in areas of hard and soft bedrock in close contact, shape modified by structure of bedrock


Figure 16-19


• Stream drainage patterns (cont.)– Radial pattern: streams

descend a concentric uplift– Centripetal pattern: streams

converge into a uniform basin– Annular pattern: forms in

areas of hard and soft domes or basins, flow follows soft bedrock and is confined by hard bedrock


The Shaping and Reshaping of Valleys

• Valley deepening– Downcutting—hydraulic power of

the moving water to lower the streambed

– Base level—lower limit to the amount of downcutting that can occur—ultimate base level is sea level

– Knickpoints—channel irregularities such as rapids and waterfalls

– Knickpoint migration


Figure 16-25


• Valley widening– Widening slows when

above the base level– As gradient decreases,

stream flow meanders and lateral erosion occurs

– Water moves fastest on outside of curves (cut bank)

– Slowest water on inside of curves accumulates alluvium (point bar)


Figure 16-28


• Valley lengthening– Headward erosion: fast

moving water from slope break between interfluve and valley wall causes material collapse

– Results in decrease of interfluve area and increase in valley area

– Causes a headward extension of the valley into the interfluve area


Figure 16-29


• Stream capture– Headward erosion causes

one stream to be diverted to the basin of another stream naturally

– Captor versus captured stream

– Beheaded stream– Elbow of capture


Figure 16-32


• Delta formation– Slowed flow when it

reaches an ocean or lake results in deposited sediment

– Debris builds up and forms a delta

– Distributaries• Deposition in valleys

– Aggradation


Figure 16-34

Floodplains

• Low-lying near flat alluvial valley floor that is periodically inundated with floodwaters

• Floodplain landforms– Bluffs– Cutoff meander– Oxbow lake– Meander scars– Natural levees– Backswamps– Yazoo streams


Figure 16-38

Floodplains

• Modifying rivers to control flooding– Humans live on floodplains

due to flat land, abundant water, and productive soils

– Manmade levees, dams, and overflow floodways created to avert disaster

– Example of the Mississippi River

– Human-induced changes to the deltas and floodplains


Figure 16-42

Stream Rejuvenation

• Lowering of sea level during ice ages or increase of elevation from tectonic uplift increases gradients

• Gradient increases restart or rejuvenate vertical incision by streams

• Stream terraces• Entrenched meanders


Figure 16-43

Theories of Landform Development

• Davis’s geomorphic cycle– Continuous sequence of uplift,

fluvial erosion, and denudation– Youth stage, streams become

established and drainage pattern develops

– Mature stage, streams approach equilibrium

– Old age stage, erosion reduces landscape to near base level

– Rejuvenation, new uplift restarts the cycle



• Penck’s Theory of Crustal Change and Slope Development– Criticisms of Davis’

geomorphic cycle– Stated the land slope has a

parallel retreat, same slope angle over time

– Many ideas have been substantiated by subsequent works


Figure 16-46


• Equilibrium theory– Variations in influence of crustal movement and resistance

of underlying rock– Slope forms adjust to geomorphic processes to achieve

energy balance– Shortcomings in areas that are tectonically stable– Prime theory used today


Figure 16-47

Summary

• Fluvial processes are those which involve moving water• Fluvial processes impact the landscape through erosion

and deposition• Valleys, interfluves, and drainage basins describe the

impacts of fluvial processes on landscape• Stream orders help describe the structure of stream

systems• Streams affect the landscape through numerous types of

erosion and deposition• Friction along stream channels and turbulence within the

channels affects their patterns


Summary

• There are four primary types of stream channels• Consequent and subsequent streams develop along

areas of new land formation• Antecedent and superimposed streams result from slow

uplift of land affecting the pattern• There are five patterns of stream drainage• Streams affect valleys by both widening them and

deepening them through downcutting• The base level is the lowest level at which a stream will

downcut


Summary

• Knickpoints are irregularities in the channel flow, such as waterfalls or rapids

• Valleys can be lengthened by headward erosion, stream capture, and delta formation

• Floodplains are flat land areas that have fertile soil but are prone to occasional flooding from nearby streams

• Numerous landforms related to floodplains exist• Stream rejuvenation occurs when uplift increases the slope

gradient of a stream• There are numerous theories of landform development and

their relationship to fluvial processes


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Technology

base level

fluvial processes

stream

landscape

water

impact

downcutting

figure 16