Transcript ch13

Classroom presentations
to accompany
Understanding Earth, 3rd edition
prepared by
Peter Copeland and William Dupré
University of Houston
Chapter 13
Streams: Transport to the Ocean
Streams:
Transport to the
Ocean
Gary D. McMichael/Photo Researecher
Rivers and streams
Stream : body of water flowing in a
channel
The floor of the channel is called the
bed.
When rainfall is very heavy or snow
melts rapidly, bodies of water
overflow their banks and water
covers the adjacent land called the
floodplain.
Rivers and streams
• Carry away runoff to lakes and seas
• Erode land (degradation)
• Transport and deposit sedimentary
debris
Stream behavior
• Mostly determined by velocity and
shape of channel.
• These factors combine to allow
either laminar or turbulent flow.
• Turbulent flow is much more
erosive.
• Stream velocities may vary from
0.25 to 7 m/s.
Laminar flow
• Smooth sheet-like flow at a low
velocity
• Usually confined to edges and top
of stream
Turbulent flow
• Irregular swirling flow
• Occurs at most rates of stream flow
• Keeps particles in suspension
Laminar flow
Fig. 13.1a
Turbulent flow
Fig. 13.1b
Laminar to turbulent transition
Laminar flow
ONERA
Turbulent flow
Fig. 13.1c
Streams move material
in three forms
• Dissolved load
• Suspended load
• Bed load (traction and saltation)
Sediment
Transport
Fig. 13.2
Saltation
Fig. 13.3
Grain Size and Flow Velocity
Fig. 13.1
Stream terms
competence: measure of the largest
particles a stream can transport
proportional to v2
capacity: maximum quantity of
sediment carried by stream
proportional to Q and v
Lower Velocities Form Ripples
ripple
Fig. 13.5a
Higher Velocities Form Dunes
ripples
dune
dune
Fig. 13.5b
Pebbles
Caught in
Eddies
Form
Potholes
Carr Clifton/Minden Pictures
Fig. 13.6
Waterfall Retreating Upriver
Donald Nausbaum
Fig. 13.7
Parts of a River System
Fig. 13.8
Two important stream types
1. Meandering Streams
• Gentle gradients, fine-grained
alluvium
• Minimizes resistance to flow and
dissipates energy as uniformly as
possible (equilibrium)
• Examples: point bars,oxbow lake,
migrating meanders
Two important stream types
2. Braided Streams
• Sediment supply greater than amount
stream can support.
•At any one moment the active channels
may account for only a small proportion
of the area of the channel system, but
essentially all is used over one season.
•Common in glacial, deserts, and
mountain regions.
Incised Meanders, Utah
Tom Bean
Fig. 13.9
Meandering River Over Time
Fig. 13.10
Lateral migration by
erosion at the outside
&
deposition
on the inside
of the river
Fig. 13.10a
Meandering River
Point Bar
Peter Kresan
Fig. 13.11
Braided River
Tom Bean
Fig. 13.12
Formation of Natural Levees
Fig. 13.1
Discharge
Total amount of water that passes a
given point in a stream per unit time
Q = wdv
Discharge
Discharge (m3/s) = width (m) 
depth (m)  average velocity
(m/s)
In the U.S., this is expressed as
cubic feet per second (cfs):
1 m3/s = 35.9 ft3/s
River at Low
Discharge
Fig. 13.14a
River at High
Discharge
Fig. 13.14b
Flooding
• Water in the stream is greater
than the volume of the channel.
• Interval between floods depends
on the climate of the region and
the size of the channel/
City Built on a Floodplain
Xie Jiahua/China Features/Sygma
Recurrence interval
Average time between the
occurrences of a given event
The recurrence interval of a flood
of
a given size at a given place
depends on:
• climate of the region
• width of the floodplain
• size of the channel
Annual Flood Frequency Curve
Fig. 13.1
Longitudinal Stream Profile of
the Platt and South Platt Rivers
Fig. 13.16
Base level
Elevation at which a stream
enters a large body of water
such
as a lake or ocean
Role of Base Level in Controlling
Longitudinal Profile of Rivers
Fig. 13.17
Effects of Building a Dam
Original Profile Graded to Regional
Base Level
Fig. 13.18a
Effects of Building a Dam
Dam Forms New Local Base Level
Fig. 13.18b
Effects of Building a Dam
Deposition Upstream
and Erosion Downstream
Fig. 13.18c
Graded stream
Stream in which neither erosion
nor
deposition is occurring, due to
an
equilibrium of slope, velocity,
and
discharge.
Geologic evidence of changes
in stream equilibrium
• Alluvial fans
• Terraces: erosional remnants
of former floodplains
Alluvial Fans
Michael Collier
Fig. 13.19
Formation of River Terraces
Fig. 13.20
Drainage divides
separate adjacent drainage basins
Fig. 13.21
Drainage basin
Area of land surrounded by
topographic divides in which all the
water is directed to a single point
Drainage Basin of the
Colorado River
Fig. 13.22
Typical Drainage Networks
Fig. 13.23
Antecedant Stream
Stream was present
before deformation
Deformation causes
gorge to form
Fig. 13.24&b
A Superimposed Stream
Deformation occurred
before stream was present
Downcutting causes
gorge to form
Fig. 13.25
Delaware Water Gap
A Superimposed Stream
Michael P. Godomski/Photo Researchers
Fig. 13.24c
Delta
Location of significant
sedimentation where a river
meet
the sea.
Mississippi Delta
Landsat 2 image annotated by Moore, 1979
Fig. 13.26
Typical Large Marine Delta
Fig. 13.27
Shifting Mississippi River Delta
Over the Past 6000 Years
Fig. 13.28