Chapter Fourteen
Streams & Floods
Earth’s Water
 Water in, on, and above Earth is ~ 1.36 billion km3 (326 million mile 3) and this amount is
fairly constant
 The continuous circulation of water through the ocean, land and the atmosphere is the process
called hydrologic cycle
 Distribution of Earth’s water
97.2 % in ocean; 2.15% ice/glacier; 0.65% Lakes, Streams, Groundwater, Atmosphere
 Types of water
Juvenile- initial Earth’s water
Meteoric- nearly all surface water originates in the atmosphere
Distribution of the Earth’s water: by volume
Hydrologic Cycle
A stream system network
Anatomy of a Stream
A stream is a surface water flow confined to a channel
 Floodplain- flat land immediately surrounding a stream which may be submerged if a river overflows
its bank
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Drainage Basin- areas of land that supply their water
Drainage Divide- topographic highland that separates two adjacent drainage basins
Tributaries- streams that contribute water to main (trunk) stream
Distributaries- main river splits into small channels that empties into a lake/sea
Graded Stream- state of temporary equilibrium
Base level- lowest point to which a river can erode
Discharge- the amount of water that flows through a given area (Q=V*A)
Streamflow & Discharge
 Gradient = Change in elevation per distance
– Ranges from 66 m/km (in mountains) to 0.1 m/km (on lowland plains)
– Turbulent Flos: Non streamline flow
– Stream Velociy:
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Velocity = Distance traveled in a given time
Ranges from >10 m/s to 0.27 m/s
Local velocity depends on: continental gradient, location of water within channel (slowest in straight segment at sides &
bottom; velocity greatest at the outside of the curve; fastest in straight segment in top center, in curved segment: top on
inside of curve, narrow places than in wide ones)
Velocity greater downstream than upstream because greater volume of water and stream bed is smoother
Stream discharge: Volume of water passing a given point for a given time
Discharge = (Width X Depth X Velocity) / 2 (for a hypothetical square or rectilinear stream, factor 2 is not there)
Streamflow and Discharge – contd.
 Discharge depends on:
– Size of drainage basin
– Amount of precipitation in basin
– Ranges from 200,000 to 5 m3/s (one day in Amazon for more than 5 yrs of New York need)
– GEOLOGICAL WORK OF STREAMS
 Stream Erosion
 Graded Streams- No net erosion, no net deposition (dynamic equilibrium)
 Aggregation- too much sediments-increased sediment load – steeper gradient – increases stream’s
velocity - Regrading
 Degradation- occurs when there is less sediments – steeper gradient - Regrading
 Abrasion: Scouring of Particles
 Hydraulic lifting – Erosion by water pressure
 Dissolution: Dissolution of soluble bedrock
Stream Transport
Streams erode their networks of tributary valleys and distinctive drainage patterns. A drainage pattern is a
reflection of the underlying rock material or structure.
 Drainage Types (Patterns)
 Dendritic: Branching drainage pattern – develop on relatively flat sedimentary rocks
 Radial: Streams typically drain from a Central high area like spoks of a wheel
 Rectangular: Looks like a grid of city streets
 Trellis: Develops where narrow valleys are separated by parallel ridges
 Stream Piracy
 Superposed/Antecedent streams
 Channel patterns- straight, braided or meandering, oxbow lake
Aggradation & Degradation of graded streams
Four types of drainage pattern
Four types of drainage patterns – contd.
Four types of drainage pattern – contd.
Four types of drainage patterns – contd.
Channel Patterns
 Straight Channels: In areas of active uplife
 Braided streams: Networks of converging and diverging stream channels separated by
sand & gravel bars
 Meandering Streams: Oxbow Lakes
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Sediment Deposition
Velocity of sediment transport is controlled by its discharge
Stream Capacity- the total volume of sediments it carries is controlled by its discharge
Competence- the maximum size of sediments a stream carries is controlled by its velocity
Sediments are transported in the following format
 Suspended load
 Bed load
 Dissolved load
Superposed streams
Superposed streams – contd.
Antecedent streams
Antecedent streams – contd.
Antecedent streams – contd.
Evolution of meandering streams
Pronounced stream meander bends
Waterfalls & Rapid
 Waterfall Formation
 Waterfall migration upstream
 Waterfall reduction to rapids
 Niagara Falls
 Stream Transport:
– Capacity = Maximum possible sediment load that a stream can transport; proportional to
stream discharge
– Competence = largest possible sediment that a stream can transport; proportional to the
square of a stream’s velocity.
Evolution of waterfalls and rapids
Evolution of waterfalls & rapids – contd.
The creation of stream terraces
Creation of stream terraces – contd.
Creation of stream terraces – contd.
Sediment Distribution
Floodplain features
Anatomy of a delta
Stream Deposition
Alluvium- sediment materials that settle from water
 Point bar- sediments deposited within the channel
 Flood plain Deposits
 Levees
 Backswamp- deposits that make a flood plain’s wetland
 Alluvial fans- formed where stream valley widens
 Delta- formed by the deposition of sediments in standing water where the main stream breaks
into smaller channels
Large sediment loads
The evolution of the Mississippi River delta plain
Hydrographs - Illustrations
Hydrographs - Charts
A flood frequency curve for a hypothetical stream
Controlling Floods
 Floods occur every 2 to 3 years
 Flooding caused when runoff is greater than stream channel can carry
 Flood Prediction:
– Seasonal flooding
– Predicting Extreme Floods
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Statistical probability
Stream hydrographs
10-yr and 100-yr floods
Difficulty of Predicting Floods
Inadequate Records underestimate 100-yr floods
Human development increases maximum discharge
Inadequacy of statistical probability
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Flood Prevention
 Artificial Strategies:
– Artificial Levees & Flood walls
– Flood-control dams
– Channelization: Benefits and problems
– Drawbacks to structural solutions: cost and false sense of security
– Nonstructural Strategies:
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Identifying high-risk areas
Zoning against floodplain development
Managing resources to minimize floodwaters
Artificial levees & flood wells
Artificial levees and flood walls – contd.
Artificial levees & flood walls – contd.
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Flood-hazard map
Summary
% of Ocean water in Earth’s water
Meteoric Water
In a straight section of a stream channel, fastest water
Stream’s discharge – calculation
Long-term discharge depends on what factors?
Erosive power & velocity – relationship
Causes for stream meander
Stream capacity & Stream competence
Difference between alluvial fan and delta
What is hydrograph
Flood prediction – how?