Transcript Running Water

Water Cycle - Running Water
Northwest Geology
Chapter 15
http://www.kidzone.ws/water/
Streams
• Streams are the major geological agents
operating on the surface of the land. The chief
• focus of streams are
-How water flows in currents
-How currents carry sediment
-How streams break up and erode solid rock
-How streams carve valleys and assume a
variety of forms as they channel water
downstream
Stream Water Flow
Begins as sheetflow
Infiltration capacity is controlled by
• Intensity and duration of rainfall
• Prior wetted condition of the soil
• Soil texture
• Slope of the land
• Nature of the vegetative cover
Sheetflow develops into tiny channels
called rills
Two types of flow determined primarily
by velocity
– Laminar flow
– Turbulent flow
Factors that determine velocity
– Gradient, or slope
– Channel characteristics including shape, size,
and roughness
Streamflow
• Laminar – water particles flow in
straight paths giving flat, gentle surface
• Turbulent – water moves in an erratic
fashion characterized by swirling eddies
and whirlpools (white water)
Laminar
vs.
Turbulent
Streamflow
Factors that determine velocity
– Discharge – the volume of water moving past
a given point in a certain amount of time
- Changes from upstream to downstream
- Profile
* Cross-sectional view of a stream
*Viewed from the head (headwaters or source)
to the mouth of a stream
• Changes from upstream to downstream
-Factors that decrease downstream
• Gradient
• Channel roughness
-Base level and graded streams
(Base level is the lowest point to which
a stream can erode
• Two general types of base level
– Ultimate (sea level)
– Local or temporary
• Changing conditions causes readjustment
of stream activities
• – Raising base level causes deposition
• – Lowering base level causes erosion
• • Factors that increase downstream
-Velocity
-Discharge
-Channel size
• Stream erosion
-Lifting loosely consolidated particles by
abrasion
-Dissolution
(Stronger currents lift particles more
effectively)
Streamflow Discharge
• Discharge is the volume of water that flows past a
given point in a given time.
• The volume is calculated by multiplying the area
water in the stream channel (depth x width) by the
velocity (distance/time) of the stream flow.
• The dimensions of the stream channel are
expressed in feet or meters and velocity is
measured in feet or meters per second.
Consequently the units of discharge are cubic
meters (or feet) per second, i.e. discharge is the
volume (cubic meters or cubic feet) of water that
passes a given point in one second.
Discharge (m3/sec) =
channel width (m) x channel depth (m)
x velocity (m/sec)
Discharge can be calculated by
multiplying the area of the stream channel
in cross section (width x depth) by the
distance traveled in a given time (second).
Stream Transport
• Stream “load”
– Dissolved load – dissolved and carried by the
water
– Suspended load – suspended in and moved
by the water
– Bed load – moved along the bottom (bed)
– Flotation load – light material floating on
surface
Stream Capacity
• Maximum load a stream can support
(total sediment load from previous slide)
Sediment Deposition
• Caused by a decrease in velocity
-Competence is reduced
• Sediment begins to drop out
• Stream sediments generally well sorted
• Stream sediments are known as alluvium
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Deposition of sediment by a stream
• Channel deposits
• Bars
• Braided streams
• Deltas
• Floodplain deposits
• Natural levees – form parallel to the stream
channel by successive floods over many years
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Deposition of sediment by a stream
• Floodplain deposits
• Back swamps
• Yazoo tributaries
• Alluvial fans
• Develop where a high-gradient stream leaves
a narrow valley
• Slopes outward in a broad arc
Stream Valleys
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Most common landforms
Downcutting toward base level
Often include falls and rapids
Two general types
– Narrow and steep
– V-shaped
Drainage Networks
• The pattern of the interconnected
networks of streams in an area
• Common drainage patterns
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Dendritic
Radial
Rectangular
trellis
http://id.waterdata.usgs.gov/nwis/
current?type=flow
Streamflow Summary
• Discharge is the volume of water that flows past a
given point in a given time.
• The USGS has over 7,000 stream gaging
stations. Data from over half of which are available
on-line
• Stream gages measure the depth of water in a stream
channel and the velocity of flow
• The recurrence interval is the average time in years
between floods of the same size
• A hydrograph illustrates discharge over time
Stream velocity may be measured
quite simply. The technique is to
drop a floating object in the stream,
time its travel along a meter stick
several times, and average the
readings
Stream velocity
varies within a
stream
channel. Maximum
velocity occurs on
the outside of
channel bends;
minimum velocity
occurs on the inside
of bends.
Erosion occurs on the outer banks of streams (cutbanks) where velocity is greatest and deposition
occurs on inner banks (forming point bars) where
velocity is least. The stream channel slowly migrates
across the valley floor in the direction of the erosion to
form long, looping bends termed meanders.
Fluvial features
Oxbow lake
Factors of stream velocity that
control erosion
• Gradient
• Channel shape, size roughness
• discharge
Summary of Stream Velocity
• Stream velocity varies around curves in stream channels
• Velocity is greatest on the outside of curves, least on the
inside
• Erosion occurs in areas of higher velocity forming cutbanks on the outside of stream curves
• Deposition in regions of low stream velocity forms point
bars on the inside of curves
• Erosion and deposition cause stream channels to migrate
laterally
• Velocity decreases as the length of the wetted perimeter
(channel banks and bed) increases
• A floodplain is a broad, flat plain adjacent to a stream
channel