Transcript Streams and Floods

Chapter 11: Streams and Floods
1.
Humans and Rivers
2.
The Hydrologic Cycle
3.
Drainage Networks and Patterns
4.
Factors Affecting Stream Flow
5.
The Work of Streams
6.
Floods
7.
Flood Control
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Learning Objectives
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Students will explain concepts related to streams and
floods.
Students will synthesize all the components and
processes of the hydrologic cycle.
Students will describe and distinguish among various
stream drainage patterns.
Students will identify the factors that influence stream
velocity.
Students will explain the erosion, transport, and
deposition of streams.
Students will list factors that contribute to flooding.
Students will identify examples of flood protection and
adjustment.
Humans and Rivers
What is going on here, and what does it have to do with streams and floods?
The Good Earth/Chapter 11: Streams and Floods
Humans and Rivers
A stream is any flow of water
through a channel (defined by its
banks), from the smallest creek to
the largest river.
The Nile River Basin:
The Nile is 6,825 km long and
drops 1,800 meters in elevation
from its source to its mouth.
Flows from Burundi to the
Mediterranean Sea, covering 35
degrees of latitude.
Egypt became one of the earliest
sites of civilization because of the
Nile.
Humans began diverting water
from the Nile for irrigation over
5,000 years ago.
The Pyramids were built during a
time of ample water in the Nile.
The Good Earth/Chapter 11: Streams and Floods
Humans and Rivers
The history of the Nile illustrates how organisms in the vicinity of a
stream can be placed under considerable stress when significant
changes occur in stream systems.
We tend to take streams/rivers for granted, but they perform many
positive functions in today's society:
- Bring water to irrigate crops
- Provide drinking water
- Supply coolant for power plants
- Transportation (barges)
- Provide ecosystems for wildlife
- Recreation sites
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Self Reflection
Survey
Answer the following questions as a means of
uncovering what you already know bout streams and
floods.
1. What river is nearest to where you live? Where does
the river begin and where does it end?
2. How do people interact with streams? How do
streams fit within the context of biology, economics,
culture, politics, history, recreation, and aesthetics?
The Good Earth/Chapter 11: Streams and Floods
Go back to the Table of Contents
Go to the next section: The Hydrologic Cycle
The Good Earth/Chapter 11: Streams and Floods
The Hydrologic Cycle
The hydrologic cycle. Water evaporates from oceans, rises through
atmosphere, condenses to form clouds, is then released as precipitation
that may flow over land in streams, sink underground, or be absorbed by
plants.
The Good Earth/Chapter 11: Streams and Floods
The Hydrologic Cycle
Water moves in and around the earth system, changing
from one physical state (liquid, solid, vapor) to another, in a
process called the hydrologic cycle.
Water in streams doesn’t just come from rainfall!
The majority of precipitation reenters the atmosphere as
water vapor through evaporation.
Water vapor can enter the air when it released from plants
through transpiration.
The Good Earth/Chapter 11: Streams and Floods
The Hydrologic Cycle
The vast majority of Earth’s water is in the oceans (97%) with only about
3% on land.
The Good Earth/Chapter 11: Streams and Floods
The Hydrologic Cycle
Most of the water on land is stored in ice or groundwater.
The Good Earth/Chapter 11: Streams and Floods
The Hydrologic Cycle
Land receives more moisture by precipitation than it supplies by evaporation.
The balance comes from evaporation from the oceans. This excess water is
returned to oceans via surface streams.
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.1
Sort the following twelve terms into six pairs of terms that
most closely relate to one another.
groundwater
plants
transpiration
stream
ice
infiltration
rainfall
precipitation
water vapor
gas
melt water
run off
The Good Earth/Chapter 11: Streams and Floods
The Hydrologic Cycle
Where do streams come from?
Many (such as the Colorado river) come from small streams on the
slopes of mountains. Their source water can be snowmelt from the
mountains or rainwater running off the slopes.
The source of the mighty Mississippi is a small lake in Minnesota!
The Good Earth/Chapter 11: Streams and Floods
The Hydrologic Cycle
Rivers often come together to form another river. In the image
above, the Allegheny (left) and the Monongahela (right) join to
form the Ohio river in Pittsburg, PA.
The Good Earth/Chapter 11: Streams and Floods
The Hydrologic Cycle
The majority of precipitation on land runs off to the oceans in streams.
On average, the length of time that a given volume of water remains in
streams is about 14 days.
Only a small amount of precipitation on land sinks into the ground, as
infiltration rates are slow compared to run off rates.
However, the volume of water stored in groundwater is 70 times greater
than the volume of water in streams and lakes.
Ice stores about 3 times the volume of water found in groundwater.
Many rivers have their sources in springs that bring groundwater to the
surface (e.g. the Nile).
Many rivers have sources in glacial melt water (e.g. Brahmaputra,
Ganges).
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.2
Use the hydrologic cycle to suggest a
hypothesis about why rivers in South
America carry approximately twice as much
fresh water as do rivers in North America.
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.2
Draw a concept map that identifies the links
among the components of the earth system
and the hydrologic cycle.
The Good Earth/Chapter 11: Streams and Floods
The Hydrologic Cycle
Regardless of where or how they start, streams:
- flow down slope.
- often join other streams to form a network.
- empty into another body of water (another
stream, a lake, an ocean, a reservoir, a
wetland). This lowest point to which a stream
flows is called the base level.
The Good Earth/Chapter 11: Streams and Floods
Go back to the Table of Contents
Go to the next section: Drainage Networks
and Patterns
The Good Earth/Chapter 11: Streams and Floods
Drainage Networks and Patterns
What controls the amount
of water in a stream
channel?
The size of the area it
drains
The average precipitation
over the area
The Nile and the Mississippi have
the larges drainage basins, each
covering over 1 million sq mi.
Drainage basin = the area drained by a stream and its
smaller streams (tributaries). Above, the Mississippi
drainage basin is composed of several smaller drainage
basins (colored areas). It takes up half the land area of the
U.S.
The Good Earth/Chapter 11: Streams and Floods
Drainage Networks and Patterns
Drainage divides are
found along the high
ground separating
drainage basins.
Left – drainage divide and
drainage basins, Ohio.
Streams north of the
divide flow into the Great
Lakes.
Streams south of the
divide flow to the Ohio
River, which joins the
Mississippi and flows to
the Gulf of Mexico.
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.5
Why is the volume of water in the Mississippi River about 10
times greater than the volume of water in the Nile River?
a) The Mississippi River drainage basin is ten times bigger
than the Nile basin.
b) The Mississippi River drainage basin receives more rain.
c) The Mississippi River is a longer stream.
d) There is less vegetation to absorb rainfall in the
Mississippi River drainage basin.
The Good Earth/Chapter 11: Streams and Floods
Drainage Networks and Patterns
Why do stream patterns
look like the branches
of a tree?
Streams will follow the
path of least resistance,
forming valleys where
rock is most readily
eroded or following the
steepest slope.
Dendritic drainage
patterns are
characteristic of areas
where the geology is
relatively uniform.
The tips of the v’s
where streams come
together point down
stream.
The Good Earth/Chapter 11: Streams and Floods
Drainage Networks and Patterns
Trellis patterns – streams
intersect at right angle
forming rectangular
drainage patterns.
Found in areas of
alternating layers of weak
and resistant rocks.
Main streams flow parallel
to ridges, cutting across at
water gaps.
Smaller streams flow
down slope perpendicular
to the course of the main
stream.
The Good Earth/Chapter 11: Streams and Floods
Drainage Networks and Patterns
Rectangular patterns – occur in
regions where the streams are
controlled by joints, fractures in
the underlying bedrock. Streams
exploit these fractures and flow in
them.
Radial patterns – typically found
on the flanks of volcanoes, where
streams flow downhill from the
summit.
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.8
What type of
drainage patterns
can you observe in
this map of part of
the Appalachian
Mountains and
adjacent areas?
Explain any
differences in
patterns that you
observe in different
parts of the map.
The Good Earth/Chapter 11: Streams and Floods
Go back to the Table of Contents
Go to the next section: Factors Affecting
Stream Flow
The Good Earth/Chapter 11: Streams and Floods
Factors Affecting Stream Flow
Gradient = the slope of a stream. It is the change in elevation
of the stream over a horizontal distance.
Where is a stream’s gradient
highest, near the headwaters or
near the mouth?
The Good Earth/Chapter 11: Streams and Floods
Factors Affecting Stream Flow
How might the features you see in
this picture compare to what you
would observe in a low-gradient
river?
Rocky banks and channel
As you continue down stream a
channel becomes less rock,
changing to gravel, then sand,
and eventually fine-grained silt
and mud = channel roughness.
Which would you expect to flow
faster, the high gradient part of a
stream or the low gradient part?
Why?
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.9
Explain why stream velocity would
change along the same section of a
stream at different times of the year.
The Good Earth/Chapter 11: Streams and Floods
Factors Affecting Stream Flow
Cross-sectional area and
hydraulic radius affect rates
of flow.
The wetted perimeter = the
length of the surface of the
channel banks and bed in
contact with the water.
A large cross-sectional area
compared to wetted
perimeter = large hydraulic
radius, and therefore a
relatively high stream
velocity.
Higher hydraulic radius =
higher velocity
The Good Earth/Chapter 11: Streams and Floods
Factors Affecting Stream Flow
Downstream, channel roughness is generally reduced and hydraulic
radius increases.
Therefore, stream velocity increases downstream.
Stream discharge = volume of water that passes a given point in one
second.
Discharge (m3/s) = width (m) x depth (m) x velocity (m/s)
Fun fact: 50 million
gallons of water
discharge into the
Atlantic ocean from
the Amazon river
every second!
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.11
Some scientists predict that global warming will result
in a corresponding increase in evaporation from
the oceans. How would this affect the discharge
of the Amazon River?
a) Discharge would increase
b) Discharge would decrease
c) Discharge would stay the same
The Good Earth/Chapter 11: Streams and Floods
Go back to the Table of Contents
Go to the next section: The Work of Streams
The Good Earth/Chapter 11: Streams and Floods
The Work of Streams
All streams erode particles
from channel beds and
banks and carry them
downstream.
a. About 5 months after the
eruption of Mt. St. Helens
this channel of the upper
Muddy river had formed in
the soft erupted materials.
b. Note the increase in width
and depth of the channel
12 months later.
The Good Earth/Chapter 11: Streams and Floods
The Work of Streams
Erosion produces stream load, which is the combination of bed load,
suspended load, and dissolved load.
How is stream color affected by load?
Suspended load increases when discharge increases (higher velocity = more
energy to move particles).
The Good Earth/Chapter 11: Streams and Floods
The Work of Streams
The Amazon River has 10 times the discharge of the Mississippi but it
carries only 3 times the load. Why might this be? What factors account for
this difference?
Source materials are different – soils with different characteristics.
Different weathering histories of the two regions.
The area of the Amazon is much more densely vegetated – vegetation protects the
underlying soil from erosion.
Yellow River, China – carries more sediment relative to its discharge than
any other major river on Earth!
A
braided
channel,
Alaska
The Good Earth/Chapter 11: Streams and Floods
The Work of Streams
As a stream
slows down it
drops some of
its load. Heavier
particles drop
out first.
When a stream
has a heavy
load, sediment is
often deposited
in the stream
channel itself to
form bars or
islands.
These deposits
split the channel
into smaller
channels
forming a
braided channel.
The Good Earth/Chapter 11: Streams and Floods
The Work of Streams
As a meander
becomes larger
the channel
migrates across
the flood plain in
the direction of
erosion.
Meanders = broad curves in a stream. Velocity of the stream varies as it
moves through these large bends.
The Good Earth/Chapter 11: Streams and Floods
The Work of Streams
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.15
Examine the image of part of the lower Mississippi River. This picture was taken from the
space shuttle. The long axis of the image is about 67 miles long. This region is characterized
by rich farmland (purple) where a variety of crops are grown. The gray regions bordering the
river are undeveloped forested areas. The river is the black band that curves across the
image. The river flows from north to south.
1. Interpret the image
and discuss the
geologic history of
this section of the
river.
2. Identify where
erosion and
deposition are
occurring.
3. Use the blank map to
draw an earlier
course of the
channel.
The Good Earth/Chapter 11: Streams and Floods
The Work of Streams
Rivers dump much of
their sediment, forming
a delta, where they
enter the relatively
quiet waters of an
ocean or lake.
Velocity rapidly
declines as the river
hits the ocean causing
the drop of sediment.
At right – Mississippi
River delta. Constant
dredging is required in
order for oceangoing
ships to navigate the
main river channel.
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.16
Create a concept map that links
together the components of erosion,
transportation, and deposition in a
stream channel.
The Good Earth/Chapter 11: Streams and Floods
Go back to the Table of Contents
Go to the next section: Floods
The Good Earth/Chapter 11: Streams and Floods
Floods
Flood = a temporary overflow of a river onto adjacent
lands not normally covered by water.
The Good Earth/Chapter 11: Streams and Floods
Floods
Floods usually occur when the amount of water on the land
surface exceeds the volume of water that can be transported
in stream channels and absorbed into the surrounding soil.
Flooding is caused/influenced by:
- Magnitude, timing, and type of precipitation
- Human modifications of the physical landscape
- Capability of the ground to absorb water
- Evaporation rates
- Physical characteristics of the stream system
The Good Earth/Chapter 11: Streams and Floods
Floods
a. Graph of monthly rainfall for Cedar Rapids, Iowa. b. Distribution and magnitude
of precipitation (cm) associated with a single storm over southern Iowa July 4 and 5.
The most common cause of flooding is excess precipitation.
1993 Mississippi River flood – caused by both long term and short term
precipitation.
Jan – July several midwestern states received over 150% of their normal
precipitation.
Heavy storms added large amounts of rainfall all at once
The Good Earth/Chapter 11: Streams and Floods
Floods
This was spatially the largest flood in U.S. history!
It covered 44,000 square km or 17,000 square miles.
The Good Earth/Chapter 11: Streams and Floods
Floods
In arid southwestern states, floods occur because channels often have
no surface water in them most of the year and brief, intense storms
cause flash flood.
In areas with lots of snow and ice, floods occur when temperature rises
sufficiently to cause rapid snow melt.
When streams of snow melt flow from warmer to colder regions ice
jams can form blocking flow and making the flooding worse.
Ice jam flooding along the Red
River, North Dakota. Ice
blocked the flow, backing up
the river and forming a
temporary lake.
The Good Earth/Chapter 11: Streams and Floods
Floods
How do humans influence flooding?
- Paving covers or alters natural surfaces that otherwise
might absorb water.
- Storm sewers that divert water from surfaces dump it into
natural streams causing them to flood.
- Housing developments or agricultural fields replace
natural wetlands, which act as water storage reservoirs.
- The collapse of constructed dams.
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.17
List five factors that influence flooding.
Use one sentence to briefly describe the
role of each factor in flooding.
The Good Earth/Chapter 11: Streams and Floods
Floods
We can assess the potential of a stream to flood:
-Stream gauges measure the stream stage, depth of water in a channel
relative to a starting point for measurement
-Data from most stream gauges become available via satellite for scientists
-USGS has over 7000 stream gauges across the U.S.
The Good Earth/Chapter 11: Streams and Floods
Floods
Stage and discharge
relationships for the Cuyahoga
River, Akron, Ohio, for the
stream gauge in figure 11.26.
Hydrograph for the Cuyahoga
River, Akron, Ohio. Graph
shows stage measurements for
the gauging station in Figure
11.26. Red arrows show
measurements for figure b and
c.
The Good Earth/Chapter 11: Streams and Floods
Floods
U.S. stream discharge information is available at http://water.usgs.gov
Historical flood information can be used to determine how frequently flooding has
occurred in the past and to estimate the likelihood of future floods.
Historical discharge data is arranged in order from largest flood discharge to
smallest, assigned a rank based on size, and graphed.
Recurrence interval = the average time in years between floods of the same size.
Scientists can use
graphs of maximum
stream discharge
versus recurrence
interval to estimate the
discharge of 100-year
floods.
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.20
An analysis of flood data in metropolitan areas
over the last century suggests that floods, caused
by similar volumes of precipitation are actually
larger and more devastating today than in the past,
despite advances in flood monitoring. Provide
some potential explanations for this apparent
paradox.
The Good Earth/Chapter 11: Streams and Floods
Go back to the Table of Contents
Go to the next section: Flood Control
The Good Earth/Chapter 11: Streams and Floods
Flood Control
Effects of the
Mississippi River flood.
a. The city of Des
Moines, Iowa was
inundated by the flood
waters. b. A corn crop in
Missouri was destroyed
by the flooding.
Why should we care
about flood control?
The Good Earth/Chapter 11: Streams and Floods
Flood Control
• Cities located on flood plains suffer the most
economic losses
• Floodwaters can carry contaminated water
(sewage, agricultural chemicals)
• Roads get submerged and have to be closed
and repaired after the flood
• Farmland submerged – production falls
• Floods can deposit thick layers of sediment
where it should not be
• Can halt barge traffic for months
• People die and are uprooted and evacuated
The Good Earth/Chapter 11: Streams and Floods
Flood Control
In 2000 700 people were killed by the floods in the Limpopo River drainage
basin in Mozambique. a) The river during a normal year and b) during the
floods. The width of the river swelled to more than 80 miles in some
locations.
The Good Earth/Chapter 11: Streams and Floods
Flood Control
We can try to stop floods (prevention) or adjust our lifestyle to deal
with them effectively (adjustment).
Levees. (a) Artificial levees are
raised embankments along a stream
channel constructed to protect
neighboring lands from rising
floodwaters. (b) A levee in West
Virginia protects homes from
potential flooding of the Potomac
River.
Levees and floodwalls protect cities
and fields on the floodplain.
Is this prevention or adjustment?
The Good Earth/Chapter 11: Streams and Floods
Flood Control
A floodway is a diversion channel
that will transport floodwaters away
from inhabited areas.
Approximately 500,000 people live
at the confluence of the Assiniboine
and Red Rivers in Winnipeg,
Manitoba, Canada.
Is this an example of prevention or
adjustment?
The Good Earth/Chapter 11: Streams and Floods
Flood Control
Dams are used for flood
control under the
premise that floodwaters
can be stored in
reservoirs to be
released slowly when
the threat of flooding
has receded. Prevention
or adjustment?
Potential problem?
Storage facility must be
big enough to
accommodate the
excess.
Wilson Lake dam,
central Kansas.
The Good Earth/Chapter 11: Streams and Floods
Flood Control
Examples of adjustment tactics:
-Relocate settlement to higher ground
-Sales tax increases to fund projects that
modify land use patterns in floodplains
-Restoration of wetlands along river banks
-Avoid development in areas prone to flooding
-FEMA created to provide financial assistance
to those affected by natural disasters,
including floods
The Good Earth/Chapter 11: Streams and Floods
Streams and Floods Checkpoint 11.22
Place a check mark in the appropriate column for each action listed at left
indicating whether it is a prevention or adjustment tactic.
Characteristic
Flood control by . . .
prevention
adjustment
Levee is constructed
Newspaper publishes flood evacuation route
New housing developments are elevated on pilings above ground
Dredging removes sediment from streams
Flood zone maps are available in local library
Buildings are relocated outside of flood zone
Dam is constructed upstream from the community
Zoning regulations are enacted to prevent new construction in floodplain
The Good Earth/Chapter 11: Streams and Floods
The End
Go back to the Table of Contents
The Good Earth/Chapter 11: Streams and Floods