PHS Geography
Streams and Floods
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Hydrologic cycle
The hydrologic cycle is a summary of the
circulation of Earth’s water supply
Processes involved in the hydrologic cycle
• Precipitation
• Evaporation
• Infiltration
• Runoff
• Transpiration
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The Hydrologic Cycle
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Channel flow and sheet flow
Longitudinal profile of a stream
Head area erosion predominant
Mouth area, deposition mainly
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Downcutting
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Drainage Pattern
Dendritic
Rectangular
Radial
Trellis
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Dendritic Drainage
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Factors affecting erosion and deposition
Velocity
Gradient – rise over run
Channel shape
Channel roughness
Discharge – amount of water
flow per unit time, as discharge
increases, load increases.
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Velocity
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Velocity
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Channel shape and roughness
Semicircular
Flow fast
Wide channel
Flow slow
Rough bottom
Flow slow
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Channel shape and roughness
Narrow channel
Flow fast
Wide channel
Flow slow
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Stream Valleys
Most common landform on Earth’s surface
Two general types of stream valleys
• Narrow valleys
– V-shaped
– Downcutting toward base level
– Features often include rapids and waterfalls
• Wide valleys
– Stream is near base level
– Downward erosion is less dominant
– Stream energy is directed from side to side
forming a floodplain
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Stream Valleys
Features of wide valleys often include:
• Floodplains
– Erosional: river erodes laterally
– Depositional: fluctuation in conditions, base
level
• Meanders: sweeping bends in river channel
– Cut bank: active zone of erosion
– Point bar: zone of deposition
– Cutoffs: shortened channel segment
– Oxbow lakes: abandoned bend
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Stream Valleys
Features of narrow valleys often
include: Rapids & Waterfalls
Both occur where stream profile
drops rapidly,
• Rapids: resistive bed acts as temporary base
level upstream, downcutting downstream
• Waterfalls: stream makes vertical drop
– Resistive rock underlain by erosive rock
– Water plunges and erodes the underlying rock
– Niagara Falls
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Changes from Upstream to
Downstream
• Profile
– Cross-sectional view of a stream
– Viewed from the head (headwaters or source) to the
mouth of a stream
– Profile is a smooth curve
– Gradient decreases downstream
• Factors that increase downstream
– Velocity
– Discharge
– Channel size
• Factors that decrease downstream
– Gradient
– Channel roughness
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Stream erosion
Lifting loosely consolidated particles
• Abrasion: particles scrape, rub, bump
together and wear down
• Dissolution: dissolve soluble rock by
chemical reaction
Stronger currents lift particles more
effectively
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Active Stream Erosion
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Base level and graded streams
• Base level: lowest point to which a stream
can erode
• 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
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Adjustment of Base Level
to Changing Conditions
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Local Base Level
(waterfall)
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Sediment Transport by Streams
Rolling
Suspension
dissolution
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Sediment Deposition by Streams
• Caused by a decrease in velocity
– Competence is reduced
– Sediment begins to drop out
• Stream sediments
– Generally well sorted
– Stream sediments are known as alluvium
• Channel deposits
– Bars
– Braided streams
– Deltas
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Sediment Deposition by Streams
• Floodplain deposits
– Natural levees: form parallel to the stream
channel by successive floods over many years
– Back swamps: marsh
• Alluvial fans
– Develop where a high-gradient stream leaves a
narrow valley
– Slopes outward in a broad arc
• Deltas
– Forms when a stream inters an ocean or lake
– Consists of three types of beds (Foreset, Topset,
Bottomset)
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Deposition
Bar
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Braided stream
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Braided River
Resurrection River, AK
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Erosion and Deposition Along
a Meandering Stream
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Meander Loop on the
Colorado River
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Flood plains
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Stream Valley in Arid Region
Owens Valley, CA
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Creation of oxbow lake
Creation of
oxbow lake
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Formation of a Delta
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Delta
southern, AK
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Nile and Mississippi Deltas
Triangle shape
Bird-Foot
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Alluvial Fan
Death Valley, CA
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Floods
Floods are the most common and
most destructive geologic hazard
Causes of flooding
• Naturally occurring
• Human-induced factors
Types of floods
• Regional floods
• Flash floods
• Ice-jam floods
• Dam failure
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Flooding
Salt River, AZ
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Floods
What is a 100-year flood?
It is not a flood that occurs every 100
years
Flood of a given size that has the
probability of 1 in 100 of occurring in
that year
Better term: 1-in-100 chance flood
Urban planning based on FEMA 100-yr
flood maps
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Flood Control
Engineering efforts
• Artificial levees
• Flood-control Dams
• Channelization
Nonstructural approach through
sound floodplain management
• Identify high risk areas
• Zoning regulations for development
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Flood-Control Dams
Store water for slow release
Lowers crest of flood, spread out over time
Often have other functions
• agricultural irrigation water
• hydroelectric power
• recreation
Reservoir covers previous land use: fertile
farmland, historic sites, scenic valleys
Sediment deposition behind dam
Impediment for fish migration
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Channelization
Altering a stream channel to speed flow of
water to prevent reaching flood height
• Clearing channel of debris
• Dredging to widen and deepen channel
• Lining channel with concrete
Artificial cutoff: straightening the channel
• shorter stream increases gradient and velocity
of water flow
• larger discharge associated with flooding
dispersed more quickly
Army Corp of Engineers
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Artificial Levees
Earthen mounds on river banks to
increase the volume capacity of river
Steeper slopes than natural levees
Trap sediment that otherwise would
have been deposited in floodplain
River bed build up often requires raising
the height of levee over time
Many artificial levees not built to
withstand extreme flooding
Levee failure numberous on Miss. (1993)
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Erosional Floodplain
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Incised Meanders
and Stream Terraces
• Incised meanders
– Meanders in steep, narrow valleys
– Caused by a drop in base level or uplift of the
region
• Stream Terraces
– Remnants of a former floodplain
– River has adjusted to a relative drop in base
level by downcutting
– Results in horizontal plane above current
floodplain
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Stream terraces
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Incised Meanders
Delores River, CO
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Incised Meanders
Colorado River, Canyonlands NP, UT
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