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

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:






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
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
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
 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
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:



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.
Flood-hazard map