Chapter 17
Streams and Floods: The
Geology of Running Water
Copyright © 2022 W. W. Norton & Company, Inc.
Review – Mass Wasting
§ We can distinguish
between different
types of mass wasting
based on several
factors:
1) Composition and
character of the
moving materials.
2) Rate of movement.
3) Environment where
the movement
occurs.
Slowest to fastest
Review – Mass Wasting
1)
2)
3)
4)
5)
Creep and Solifluction
Rock Glaciers
Slumps
Mudflows
Debris Flows
6)
7)
8)
9)
Lahars
Avalanches
Rockfalls
Rockslides
Submarine Mass Wasting
Slowest to fastest
Review – Mass Wasting
Why Mass Wasting Occurs
§ Weakened substrate
§ Development of relief (slope)
§ Triggering Event (gravity >>>> resisting force)
How Can We Protect Against Mass Wasting
§ Identifying regions at risk
• Landslide potential mapping
• Surveying for surface indicators
§ Mitigation
Chapter 17
Streams and Floods: The
Geology of Running Water
Copyright © 2022 W. W. Norton & Company, Inc.
Streams and Floods
Chapter Learning Objectives:
1) Explain how streams and drainage networks form and evolve.
•
•
•
•
Erosion and deposition by streams.
Characterize the changes that take place along the length of a stream from its
headwaters to its mouth.
Describe the evolution of meanders in a stream.
Explain the concept of a floodplain.
2) Distinguish between different types of floods and the conditions that lead to each.
• Describe the various engineering methods used to protect areas from flooding.
3) Characterize environmental issues associated with streams.
Streams and Floods – Anatomy of a Stream
§ Stream: body of running
water that flows in a
channel (an elongate
depression or trough).
§ Large streams = Rivers
§ Medium streams = Creeks
A’
A
Stream in Colorado
https://education.nationalgeographic.org/resource/stream/
Streams and Floods – Anatomy of a Stream
§ Stream: body of running water that flows in a channel (an
elongate depression or trough).
§ Banks = Channel Edges
§ Streambed/Bed = Bottom of Channel
Cross
Section of
a Stream
A
A’
https://geo.libretexts.org/Bookshelves/Geography_%28Physical%29/The_Environment_of_the_Earth%27s_Surface_%28Southard%29/05%3A_Rivers/5.05%3A_The_Morphology_of_Rivers
Streams and Floods – Anatomy of a Stream
§ Water in streams flows its
source (Headwaters) to
its mouth.
§ Downstream = direction
from Headwaters to
Mouth. High to low
elevation.
§ Upstream = direction
from Mouth to
Headwaters. (Salmon like
this direction!)
https://gotbooks.miracosta.edu/geology/images/stream_profile.jpg
Streams Are Important Geologic Agents
§ Streams drain water from the
landscape and carry it into lakes
or to the sea.
§ Important part of the
hydrologic cycle!
§ How do streams form?
How Do Streams Form? (1)
§ The water in streams can come from any of the reservoirs in the hydrosphere.
• Precipitation
• Glacier melt
• Groundwater springs
§ Some water can
be temporarily
stored relatively
non-flowing
accumulations in
lakes and
wetlands.
How Do Streams Form? (2)
§ Streams form through Erosion.
§ Sheetwash: A film of water less than a
few mm thick on the ground surface.
§ Sheetwash will flow towards
depressions and slowly erode them as
it flows creating rills (very small
shallow streams).
§ As more water flows towards the
depression, erosion increases and the
channel deepens, and downcuts.
§ Can also initiate at the mouth of a
spring (where groundwater meets the
surface).
How Do Streams Form? (2)
§ Erosion efficiency depends on:
• Velocity
• Resistance of substrate to erosion
How Do Streams Form? (3)
§ Headwater erosion:
lengthening the stream
through erosion at the
head of the channel.
§ Headwater erosion occurs
when water flow at the
head of a channel has
sufficient erosive power.
Streams to Drainage Networks
§ As erosion deepens the main
stream channel, the
surrounding land surfaces
start to slope towards it.
§ As a result, smaller tributaries
form following the sloped
topography to join the larger
trunk stream.
§ The array of linked channels
forms a drainage network.
Drainage Networks
§ Drainage networks are
formed when numerous
smaller streams flow into one
large stream.
§ The land drained by the
network of streams is a
Watershed.
Drainage Networks
§ Drainage networks
provide conduits for the
removal of runoff.
§ Runoff: The water the
flows on the surface of
the Earth to drain the
land
• Streamflow &
Sheetflow.
Drainage Networks
§ Different drainage
networks will have
different configurations/
patterns of tributaries
and trunk streams. The
pattern that develops
depends on:
1) Landscape
2) Substrate
Drainage Networks Configurations
1) Dendritic
2) Radial
3) Rectangular
4) Trellis
5) Parallel
Drainage Networks: Dendritic
§ Dendritic = branching (branches
connecting to a trunk).
§ Forms when rivers flow over a
relatively uniform substrate with
a gentle slope.
Drainage Networks: Radial
§ Radial = flow outward from
the mountain peak, like
spokes on a wheel.
§ Develops from a point uplift
(a mesa, volcano, etc.)
Drainage Networks: Rectangular
§ Channels form along the pre-existing fractures, and streams join one
another at right angles, creating a rectangular network.
Drainage Networks: Trellis
§ Trellis drainage develops in
deformed strata with
alternating resistant and
weak rocks.
§ Only main channel is strong
enough to erode resistant
strata. Smaller tributaries
restricted to weaker rocks.
Drainage Networks: Parallel
§ Several streams form parallel
to each other on the sides of
steep escarpments of weak
substrate.
§ If the surface of the substrate
hosts no vegetation and many
channels form, the resulting
landscape can be called
badlands topography.
Drainage Networks
§ Drainage networks are
formed when numerous
smaller streams flow into one
large stream.
§ Many different patterns.
§ The land drained by the
network of streams is a
Watershed.
How are
watersheds
defined?
Watersheds and Divides
§ Watersheds are separated
by drainage divides.
§ Drainage Basins = term
often used interchangeably
with watershed.
§ Drainage Divide: relatively
high ridge that separates
watersheds.
• Defined on a variety of
different scales (local,
regional, continental).
Drainage Divides – Examples
§ Drainage divides and
watersheds are defined on a
variety of scales.
§ Tiny tributaries—tiny
watersheds
§ Large continental rivers
have large watersheds.
§ A continental divide
separates flow to different
oceans.
• North American Corderilla
Watersheds in Detail
Watershed: an area of land with a topographic divide that collects rain &
snow, and discharges most of this water to stream, river or other water
body
• Synonymous with catchment & drainage basin
• Total area that drains to a common outlet
Outlet (lowest
point)
topographic
divide or
watershed
boundary
Watersheds in Detail
Ontario Watershed Mapping:
https://www.lioapplications.lrc.gov.on.ca/SourceWaterProtection/index.html?viewer=SourceWaterProtection.SWPViewer&locale=en-CA
Watersheds in Detail
• High points divide which way water will flow
• Anything within those points drain to a common outlet
The Amazon Watershed
§ The Amazon watershed is one of
the largest in the world.
Precipitation falling in the
highlighted region makes its way
through tributaries to the Amazon.
Stream Types – Volume & Flow
§ Streams can be classified in many different ways. As we often rely on
streams for water resources it is helpful to describe them based on
water volume, and duration of flow.
§ Water Volume
• Gaining Stream = Volume of
water increases in the
downstream direction
• Losing Stream = Volume of
water decreases in the
downstream direction
Stream Types – Volume & Flow
§ Streams can be classified in many different ways. As we often rely on
streams for water resources it is helpful to describe them based on
water volume, and duration of flow.
§ Duration of Flow
• Permanent streams
• Ephemeral streams
– All ephemeral streams are
losing streams, but not all
losing streams are ephemeral!
Permanent Streams
§ Permanent streams are defined by water flowing all year. These
streams are common where there is abundant rainfall, groundwater
discharge, and low rates of evaporation (tropical and temperate
climates).
Permanent stream in Wyoming.
Ephemeral Streams
§ Ephemeral streams do not flow all year. They are common in places
with low annual rainfall, and high rates of evaporation (arid
environments).
The Importance of the Water Table
§ The location of the water table is a major determining factor on whether a
stream is permanent or ephemeral.
§ Water Table: Boundary that separates substrate in which pores between
particles are filled with groundwater, and where pores from substrate are
filled with air.
Arid Environment = low water table
Low Rainfall
High
Evaporation
Pores
filled with
air
Pores
filled with
water
Temperate Environment = high water table
High Rainfall
Low
Evaporation
Pores
filled with
air
Pores
filled with
water
The Importance of the Water Table
Arid Environment = low water table
Temperate Environment = high water table
Water table is below stream bed = no
groundwater can “feed” the stream
Water table is above stream bed =
groundwater can “feed” the stream
Stream bed
elevation