Fluvial Geomorphology
Lab 7
Concepts
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Basic concepts and definitions
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Watersheds/Drainage Basins
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Drainage divides
Drainage patterns
Classifying stream order
Categories of rivers
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Types of work performed by streams
Alluvium
Base level
Straight channel, braided stream, meandering stream
Stream information
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Calculating discharge
Flood recurrence
Real-world example: Midwest Floods of 1993
Real-world example: Water Wars affecting Georgia
Basic Concepts & Definitions
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Fluvial = stream-related processes
Geomorphology = analysis of how landforms
evolve
Fluvial Geomorphology is important in
understanding how water shapes our earth
Basic Concepts & Definitions
Types of work performed by streams
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3 types of work:
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Erosion of landscape
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Streams carry mixture of water and solids
Alluvium = clay, silt, and sand that is transported & deposited by
running water
Transportation of materials
Deposition of materials
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Concept of “Base Level”
 Lowest (elevation) point to
which a stream can flow and
cut down to
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Ultimate base level = sea level
Watersheds/Drainage Basins
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Streams organized into areas called watersheds or
drainage basins.
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All the water in the area flows into one stream and exits in one spot.
Can be broken down into sub-basins (see map of Georgia).
Watersheds/Drainage Basins
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Watersheds are separated from others due to
drainage divides
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High ground which separates streams that are flowing
in adjacent basins.
Streams cannot cross the divide.
Watersheds/Drainage Basins
Drainage Patterns
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Within each watershed, streams develop into distinct
patterns due to the underlying geology.
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7 common patterns:
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Dendritic
Rectangular
Trellis
Radial
Parallel
Deranged or Chaotic
Centripetal
Watersheds/Drainage Basins
Drainage Patterns
Centripetal
Watersheds/Drainage Basins
Classifying stream order
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Classify patterns of streams in a basin
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“First-order” streams are smallest
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Any basin will have more first-order streams than any other category.
Think of these as headwater streams.
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Where 2 similar-ordered streams come together, they increase in order.
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1&1=2
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2&2=3
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3&3=4
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2 streams of same order must be joined to increase in order.
Otherwise, keep the higher number for the next stream.
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1&2=2
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1&3=3
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2&4=4
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5 & 12 = 12
Watersheds/Drainage Basins
Classifying stream order
Categories of Streams
Straight Channel
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Uncommon, usually only occurs in canyons or when
humans force rivers to straighten out
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Examples: portions of the Columbia River (states of Washington
and Oregon) and the Colorado River (southwest U.S.)
Categories of Streams
Braided Stream
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Forms when the stream cannot hold its sediment
load and dumps it in the middle of the channel.
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Channel bars are collected piles of sediment
Channel bars force the river to flow around them, which
separates the river into “braids”
Categories of Streams
Braided Stream example: Platte River
Notice the channel bars? Some of them are even covered in vegetation, and have wildlife
living on them! These are considered established channel bars and do not easily reshape.
Categories of Streams
Meandering Stream
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Sinuous channel
Form through deposition and lateral erosion
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Deposition = inside bank
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(slower velocity water allows alluvium to collect here)
Erosion = outside bank of the meander
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(higher velocity water erodes the bank)
Categories of Streams
Meandering Stream: common features
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Point Bar
Cut Bank
Cutoff
Oxbow Lake  Backswamp  Meander Scar
Floodplain and its features
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Levee
Yazoo stream
Categories of Streams
Meandering Stream: common features
Why streams meander
The fastest water in the
channel moves from side
to side.
Check out the blue line: this
is the path of the fastest
water, which is why erosion
takes place there.
Cut Bank
Erosion taking place on the outside of
the meander.
Point Bar
Look at the deposition on the right
side of the photo.
Categories of Streams
Meandering Stream: common features (oxbow lake formation)
Categories of Streams
Meandering Stream examples: Chattahoochee, Mississippi, Tennessee
Stream Information
Calculating discharge
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Velocity = how quickly water is moving through the stream
Discharge = volume of water transported by a stream
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Area (A) = Width (w) x Depth (d)
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The greater the discharge, the more ability it has to carry sediment
In times of flooding, discharge is higher than during periods of
infrequent/low precipitation.
Need to know velocity & area of the
stream’s channel.
If w = 5 feet, d = 4 feet, then A = 20 ft2
Discharge (Q) = Velocity (v) x Area (A)
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If v = 100 ft/s and A = 20 ft2,
then Q = 2000 ft3/s
Stream Information
Calculating discharge: effects of urbanization
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Less infiltration
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water can’t soak into pavement…
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More runoff
More flooding
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Specific causes:
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Deforestation
Construction and paving
Channelization (restricting the channel by building
man-made levees)
Stream Information
Floodplains
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Floodplains form when the river leaves its channel during
times of high flow.
Natural levees are produced from flooding
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Flood waters overflow the banks, river loses velocity as it spreads
out. It dumps its sediment load, which forms the levees.
Yazoo streams may form on the other side of levees – cannot join the
main channel because of the levee.
Stream Information
Floodplains: Flood recurrence
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Recurrence = average time period when
an event will be equaled or exceeded
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Likelihood of occurrence each year
Used to designate different flood zones
(1 ÷ Frequency) x 100
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“10 year flood” = (1 ÷ 10) x 100 = 10%
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10% chance that a flood will occur each year
by that stream
If a 10 year flood happens in 2007, it still has
a 10% chance to occur in 2008, 2009, 2010…
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Does not mean that another flood cannot
happen for another 10 years!
10 year
100 year
500 year
Stream Information
Example: Midwest Floods of 1993
Most devastating flood in recent US history ($20B in damages), 14 weeks of flooding.
Towns relocated rather than rebuilt!
Estimate that this was beyond a 100 or 500 year flood… perhaps a 1,000 year flood!
Can you calculate the likelihood of that type of flood?
Stream Information
Example: Water Wars of the ACT and ACF
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Tri-state water fight
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ACF = Apalachicola,
Chattahoochee and Flint
ACT = Alabama, Coosa, and
Tallapoosa
Fights due to conflicting water
needs
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Georgia: needs for urban areas
and farming
Alabama: fears GA is hogging
water
Florida: oyster fisheries in
Apalachicola Bay is suffering
since water is being held back in
GA and AL
Read more about the water wars:
www.metroatlantachamber.com/macoc/initiatives/img/tri-statefactsheet.pdf