Streams and Floods
Prepared by Betsy Conklin for
Dr. Isiorho
The Hydrologic Cycle
• the movement of water and water vapor
from the sea to the atmosphere, to the land,
and back to the sea and atmosphere
The Hydrologic Cycle (cont.)
Streams
• stream: a body of running water that is
confined in a channel and moves downhill
under the influence of gravity
• can imply size (rivers = large, streams =
smaller, creeks = smallest) but geologists use
the term when talking about any body of
running water
• headwaters of stream: upper part of stream
near its source in mountains
• mouth: place where a stream enters the sea,
lake, or larger stream
Streams (cont.)
• flood plain: a broad strip of land built by
sediment on either side of a stream
• stream channel: a long narrow depression
eroded by the stream into rock or sediment
• stream banks: the sides of the channel
• stream bed: the bottom of the channel
• sheetwash: a thin layer of unchanneled
water flowing downhill, common in deserts
due to lack of vegetation
Streams (cont.)
• sheet erosion: when a thin layer of surface
material, usually topsoil, is removed by a
flowing sheet of water
• rills: the tiny streams formed by overland
sheetwash that becomes concentrated in
small channels
• longitudinal profile: shows a typical stream
viewed from the side
• cross section: a representation of a portion
of a stream in a vertical plain
Drainage Basins
• drainage basin: the total area drained by a
stream and its tributaries
• can be outlined on a map by drawing a line
around the region drained by all the
tributaries to a river
• tributary: a small stream flowing into a
larger one
• divide: a ridge or strip of high ground
dividing one drainage basin from another
Drainage Patterns
• drainage pattern: the arrangement of a river
and its tributaries
• types:
– dendritic: resembles the branches of a tree or
veins in a leaf
Drainage Patterns (cont.)
– radial: when the streams diverge outward like
spokes of a wheel; formed on high conical
mountains
Drainage Patterns (cont.)
– rectangular: when tributaries have frequent 90o
bends and tend to join other streams at right
angles; develops on regularly fractured rock
Drainage Patterns (cont.)
– trellis: consists of parallel main streams with
short tributaries meeting them at right angles;
forms in a region where tiled layers of resistant
rock such as sandstone alternate with
nonresistant rock such as shale
Drainage Patterns (cont.)
Stream Velocity
• stream velocity: the distance water travels
in a stream per unit time
• moderately fast river = 5 km/hr (3 mph)
• stream reaches its maximum velocity near
the middle of the channel
• high velocity generally results in erosion
and transportation
• low velocity generally results in deposition
Stream Velocity (cont.)
• factors effecting stream velocity:
– shape of the channel
– roughness of the channel
– human interference (construction of a culvert or
bridge can partially block a channel, increasing
a stream’s velocity)
Stream Gradient
• stream gradient: downhill slope of a
stream’s bed or the water surface, if the
stream is very large
• gradient of 5 ft/mile means the river drops 5
ft. vertically for every mile that it travels
horizontally
• usually decreases downstream
Discharge
• discharge: the volume of water that flows
past a given point in a unit of time
• found by multiplying the cross-sectional
area of a stream by its velocity (or width X
depth X velocity)
• reported in cubic feet per second (cfs) in the
US, or in cubic meters per second (m3/sec)
Discharge (cont.)
• increases during a flood, may be up to 50 to
199 times normal flow
• stream erosion and transportation generally
increase as a result of a flood’s velocity and
discharge
Stream Erosion
• three types of stream erosion:
– hydraulic action: refers to the ability of flowing
water to pick up and move rock and sediment
– solution: usually slow but effective process of
weathering and erosion in which rocks are
dissolved by water
– abrasion: the grinding away of the stream
channel by the friction and impact of the
sediment load; can cause potholes (usually most
effective on a rocky stream bed)
Stream Transportation of Sediment
• bed load: the large or heavy sediment
particles that travel on the stream bed
• traction: movement of sediments by rolling,
sliding or dragging
• saltation: when sediments are carried
downstream in a series of short leaps or
bounces
Transportation of Sediment (cont.)
• suspended load: sediment that is light
enough to remain lifted indefinitely above
the bottom by water turbulence; the muddy
appearance of a stream during a flood or
after a heavy rain is due to a large
suspended load
• dissolved load: the portion of the total
sediment load in a stream that is carried in
solution
Bar
• bar: a ridge of sediment, usually sand or
gravel, deposited in the middle or along the
banks of a stream; formed by deposition
when a stream’s discharge or velocity
decreases; stream deposits heavier boulders
first and small particles last
Gravel bars along the banks and in the middle of a stream
Placer Deposits
• when the heavy sediment is concentrated in
the stream where the velocity of the water is
high enough to carry away lighter materials
but not the heavy sediment
• found in streams where the running water
has mechanically concentrated heavy
sediment
Braided Streams
• a stream that flows in a network of many
interconnected rivulets around numerous
bars
• a stream tends to become braided when it is
heavily loaded with sediment and has banks
that are easily eroded
A braided river
Meandering Streams and Point Bars
• meanders: a pronounced sinuous curve
along a stream’s course
• point bar: a stream bar deposited on the
inside of a curve in the stream, where the
water velocity is low
• meander cutoff: a new, shorter channel
across the narrow neck of a meander
A meandering stream
Meandering Streams/Point Bars (cont.)
• the simultaneous erosion on the outside of a
curve and deposition on the inside can
deepen a gentle curve into a hairpin-like
meander
1
Deposition
Erosion
Deposition
3
Erosion
2
Point
bars
Curve shifts outward
and downstream
Oxbow Lake
• oxbow lake: a crescent-shaped lake
occupying the abandoned channel of a
stream meander that is isolated from the
present channel by a meander cutoff and
sedimentation
Meander
neck
becomes
narrower
Neck cutoff
occurs
Oxbow lake
Flood Plains
• flood plain: a broad strip of land built up by
sedimentation on either side of a stream
channel
• natural levees: low ridges of flooddeposited sediment that form on either side
of a stream channel and thin away from the
channel
Flood Plain (cont.)
Flood Plains
1
2
Sediment deposited during floods
3
Natural leeves Backswamp
Deltas
• delta: a body of sediment deposited at the
mouth of a river when the river’s velocity
decreases
• distributaries: small, shifting channels that
carry water away from the main river
channel and distribute it over the surface of
the delta
• a delta can be represented by the Greek
symbol delta () due to the fact that some
deltas are broadly triangular
Deltas (cont.)
River
Land
Distributaries
Marshy delta surface
Topset beds
Lake
Foreset beds
Bottomset beds
• foreset beds: form the main body of the delta
• topset beds: nearly horizontal beds of varying grain
size formed by distributaries shifting across the delta
surface
• bottomset beds: deposits of the finest silt and clay that
are carried out into the lake by the river water flow or
by sediments sliding downhill on the lake floor
Deltas (cont.)
• wave-dominated delta: a delta formed by
the reworking of sand by wave action
Mediterranean Sea
Distributaries
Barrier
Islands
Nile River
(Egypt)
Deltas (cont.)
• tide-dominated delta: a delta formed by the
reworking of sand by strong tides
Bangladesh
Tidal sand bars
Deltas (cont.)
• stream-dominated delta: a delta with fingerlike distributaries formed by the dominance
of stream sedimentation (also called
birdfoot delta)
Alluvial Fans
• alluvial fan: a large, fan- or cone-shaped
pile of sediment that usually forms where a
stream’s velocity decreases as it emerges
from a narrow mountain canyon onto a flat
plain
Narrow mountain canyon
Braided
stream
Alluvial fan
Plain
Flooding
• recurrence interval: the average time
between floods of a given size
• flood erosion: caused by the high velocity
and large volume of water in a flood
• high water: covers streets and agricultural
fields and invades buildings, shorting out
electrical lines and backing up sewers
• flood deposits: usually silt and clay
Urban Flooding
• urbanization enhances flooding due to
paved areas and storm sewers which
increase the amount and rate of surface
runoff of water, making river levels higher
during storms
• bridges, docks, and buildings built on flood
plains can also constrict the flow of flood
waters, increasing the water heigh and
velocity and promoting erosion
Flash Flooding
• flash floods: local, sudden floods of large
volume and short duration, often triggered
by heavy thunderstorms
Controlling Floods
• upstream dams: trap water and release it
slowly after a storm
• artificial leeves: embankments built along
the banks of a river channel to contain
floodwaters within the channel
• protective walls: made of stone (riprap) or
concrete constructed along river banks to
slow erosion
• floodwalls: walls of concrete, may be used
to protect cities from flooding
Controlling Floods (cont.)
• bypasses: reduce the discharge in the main
channel by diverting water into designated
basins in the flood plain
Weir
Dam
Bypass
Leeve
Flood wall
Reservoir
Stream Valley Development
• valleys: the most common landforms on the
earth’s surface that are usually cut by
streams
• downcutting: the process of deepening a
valley by erosion of the stream bed
• base level: the limit of downcutting
Graded Stream
• graded stream: a stream that exhibits a
delicate balance between its transporting
capacity and the sediment load available to
it
• an increase in gradient causes an increase in
a stream’s velocity, allowing the stream to
erode and carry more sediment
• a change in sediment load can cause a
change in gradient as well
Lateral Erosion
• lateral erosion: the erosion and undercutting
of a stream’s banks and valley walls as the
stream swings from side to side across its
valley floor
1
2
3
Undercutting of
valley wall
Widening
flood plain
Headward Erosion & Stream Piracy
• headward erosion: the slow uphill growth of
a valley above its original source through
gullying, mass wasting, and sheet erosion
• stream piracy: the natural diversion of the
headwaters of one stream into the channel
of another
Stream Terraces
• stream terraces: steplike landforms found
above a stream and its flood plain
1
Flood plain
3
Terraces
2
Terraces
Terraces
New flood plain
Incised Meanders
• incised meanders: meanders that retain their
sinuous curves as they cut vertically
downward below the level at which they
originally formed resulting in a meandering
valley
Incised meanders of the Colorado River
Meandering river
Land surface
at base level
Base level
Incised meanders
Land surface has been
lifted above base level
Uplift
Base level
Superposed Streams
• superposed streams: a river let down onto a
buried geologic structure by erosion of
overlying layers
The stream initially cuts through
the horizontal sediment
Continued erosion removes horizontal
strata and stream cuts through
underlying rock forming
narrow valleys in resistant rock
Pictures
All pictures used in this power point presentation were taken from the
following:
Carlson, Diane H., David McGeary and Charles C. Plummer. Physical
Geology: Updated Eighth Edition. New York City, McGraw-Hill
Higher Education, 2001.