24
Module 3, Lesson 2
What are Flood Plains and Deltas?*
Running water carries sediment. But i t does not carry sediment
forever. Sooner or later, the sediment must settle. Some streams
may carry sediment very far. At some time, however, the
streams will dejp^sjt^he sea^ment.
The sediment that streams deposit builds up two kinds of land
areas: flood plains and deltas.
Flood Plains
After a heavy rain, a stream carries more water than usual. This
raises the water level. Sometimes the level gets so high that the
water flows over its channel—the place it usually flows through.
Then there is flooding.
~
The flood water deposits fine sediment_on the banks, which is
land next to the stream. Some streams flood often. The sediment
builds up flat ajeas^called flood plains. Soil on flood plains is
very good for farming.
Deltas
Almost every stream empties its water into a larger body of
water, which empties into a larger river or an ocean. Theplace
where a stream empties its water is called the mouth.
A stream, flows slowly at the mouth. I t deposits its sediment at
the mouth. The sediment builds large landXreasTcalied deltas. A
delta is shaped like a triangle. I t gets its name from the Greek
letter "delta" (A).
Large pebbles are deposited when a fast stream slows as it levels
out. This is called an alluvial deposit. Fast-flowing water has
more power than slow-moving water to carry heavy objects.
From Science Workshop Series Earth Science: Geology by Seymour Rosen
© 1992 by Globe Fearon, an imprint of Pearson Learning, a division of
Pearson Education, Inc. Used by permission.
Grade 8 Science
25
Module 3, Lesson 2
Flood Plains and Deltas
Usually, stream water stays
in the channel. I t does not
flow onto its banks.
Figure A
In times of heavy rain,
however, the water may flow
over its channel.
Figure B
Flood water deposits fine
sediment on both banks of a
stream. The^sediment builds
flood plains.
Figure C
Module 3, Lesson 2
26
Grade 8 Science
Questions: Flood Plains and Deltas
1. Flood plain soil is very fertile. There are many good farms on
flood plains. But the farmers are taking a big chance. Why?
Pod< daq(4 U deemed
Wtwwis . (iwfj
2. Would you want to live on a flood plain?
3. Explain why or why not.
for n
A
y
t
How a Delta Grows
(
M
.
J
*
*
I
U
'
Imagine yourself looking
down from high in the sky.
"a" is land
"b" is a stream
*c" is a large body of water
The stream empties into
the water.
Figure
D
The stream deposits its
sediment at its mouth.
The sediment builds up.
All the sediment is under
water.
I
27
Module 3, Lesson 2
Grade 8 Science
The sediment builds up and
up. Much of it reaches above
the water level. Much is now
even with the land. The
sediment is now "new" land.
The stream has cut new
channels across this new
land.
How many new channels do
you see? 3 or 3
Figure E
The buildup continues.
More sediment is deposited.
The land grows outward i n
the shape of a delta. I t will
continue to grow.
Why will the delta keep
growing?
HAI dJa MMI bsip
Module 3, Lesson 2
23
Grade 3 Science
Questions: How a Delta Grows
F i l l i n the B l a n k s
Complete each statement using a term or terms from the list
below. Write your answers i n the spaces provided. Some words
may be used more than once.
deposited
deltas
erosion
slowly
flooding
mouth
flood plains
delta
farming
channel
1. The process by which parts of the earth's crust are carried
away is called JLttVU(M
.
*, ,
2. Material that is eroded eventually is duifioS'i $JC(
.
3. Soil on the flood plain is good for
4. The part of a stream that the water flows through is called
M
the
C!NW*A
.
5. Stream
sediment builds
areas caw
called
tream sediment
builds land
land areas
sbhocP f (CUMS
and
JU^W^
6. When water overflows its channel. "ffdoJi^ •
takes place.
7. Fine sediment laid down during floods builds a
fl6od
fldUti
-
"
' t
8. The plaeewhere a stream empties its water is called the
(MfA^fk
•
9. Thej water
v/ater aat the mouth of a stream usually flows
$
10. Sed^en^deposited at the mouth of a stream builds a
Matching
Match each term i n Column A w i t h its description in Column B.
Write the correct letter i n the space provided.
Column A
1. flood plain
2.
3.
4.
5.
delta
channel
triangle
mouth
Column B
^
a
&
&
qf
_
) shape of a delta
b)
c)
d)
e)
built by flood sediment
built at mouth of stream
stream's end
place a stream usually
flows through