Running water
• 97.2 % oceans
• 2.2 % ice
• 0.6 % liquid fresh water most of which is
ground water
• Water is always on the move in the water
cycle
Water Cycle
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Infiltration = soak in
run off - surface flow
evaporation = back into the atmosphere
transpiration = back into the atmosphere
by plant
• evapotranspiration - combined
Running water
• Run off begins as sheet flow.
• How much goes into the ground depends on the
infiltration capacity of the soil
• Factors of infiltration
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intensity and duration of rain fall
soil wetness(saturation)
soil texture
slope
vegetation
Running water
• Rills - tiny channels
• Gullies - larger channels
• stream - any running body of water
– sometime called rivers if really large
Stream characteristics
• Gradient or slope is the vertical drop of the
stream over a fixed horizontal distance,
– The steeper the slope the more energy
available for stream flow.
• Cross-sectional shape determines the
amount of drag.
– The less drag the faster the water can flow
– Size and roughness also influence drag
Stream characteristics
• Discharge is the amount of water flowing
past a certain point in a given amount of
time
– Discharge (meter 3/ second)=
– channel width(meters) X depth (meters) X
velocity (meters/second)
Changes downstream
• Longitudinal profile - a look at the stream
from head to mouth
– gradient decreases downstream
– discharge increases downstream
– width, depth and velocity increase
downstream
• When gradient is high, discharge is small
Base level
• Running water in a stream will erode
downward over its course until it reaches a
balance point.
• This balance point is called the base level.
• Base level is the lowest elevation to which
a stream can erode.
• Two types of base level
Base level
• Ultimate base level - sea level
• Temporary ( local) base level - anything
which has the capacity to limit downward
erosion.
– Lakes, resistant bedrock, or a larger stream
• Kalamazoo River
– Ultimate base level the Atlantic ocean
– Local base level Lake Michigan
Base level
• Base level can be changed.
– Raised base level results is velocity dropping
and deposition begins up stream.
– Lowered base level results in velocity
increasing and erosion increasing.
• How do you raise base level? Build a dam.
• How do you lower base level? Uplift the
land or lower sea level.
Base level
• Sooner or later a stream will reach base
level and be neither eroding or depositing
just transporting.
• A graded stream is at base level
Stream Erosion
• Waters in streams are turbulent. They
whirl and eddy.
• The material in streams (sand, gravel, silt,
and clay sized particles, plus dissolved
minerals) abrades the streambed and
become rounder and smoother with
transport. This is called the load.
– Potholes are gouged by gravel caught in an
eddy.
Stream Transport
• Dissolved load - minerals dissolved by
ground water and added to the stream.
• Suspended load - usually the largest load
of a stream. Fine sand, silt and clay sized
particles.
– How much suspended sediment depends on
velocity of the stream and settling rate of the
sediment.
Stream Transport
• Bed Load - the material on the stream bed
too large to be carried by suspension.
– This is the stuff that does the grinding to
produce downward erosion.
• This material moves by rolling, sliding and
saltation.
– Saltation is jumping or skipping down stream
Stream Transport
• The ability to carry material is measured in
capacity and competence.
• Capacity is the maximum load a stream can
carry.
• Competence - the maximum sized particle
a stream can carry.
– The greater the stream’s discharge the greater
the capacity and competence.
Stream deposition
• When velocity slows the stream can no
longer carry its load.
• Particles will be deposited by size with the
largest settling out first.
• This sorting explains why particles of the
same size are found together.
• Alluvium is the term for all stream
deposited sediments
Stream deposition - channel
• Channel deposits - the materials deposited
mid-stream usually sand or gravel in bars.
• Point Bars form in the inside bend of a
stream meander.
• Braided streams have bars mid channel.
– Merger of two streams with different velocity,
– after glacial melting, or
– after heavy down pour.
Stream deposition- floodplain
• Floodplain - the area adjacent to a stream
that is covered with water during a flood.
• Natural levees flank the stream and are an
accumulation of gravel deposited during a
flood when the water lost it competence
and capacity upon leaving the channel.
Stream deposition- floodplain
• Some levees are so high that tributary
streams can’t enter the main stream.
• These streams (Yazoo tributaries ) flow
along the flood plain of the larger stream in
swampy (back swamp) poorly drained
areas.
Stream deposition- Fans and
Deltas
• When a stream loses its competency
abruptly it will drop its load.
• Alluvial fans will form if the deposition
occurs on land. These are fairly steep.
• Delta will form if the deposition occurs
under water.These are fairly shallow.
Stream deposition- Fan
• Alluvial fans form when a high gradient
stream in a narrow mountain valley enters
a broad flat basin.
• The change is gradient is drastic resulting
is immediate deposition.
• Coarse material is deposited at the top of
the fan while finer material is carried lower.
Stream deposition- delta
• Deltas form when a stream enters a lake or
the ocean.
Stream deposition- delta
• The main stream will become sediment
choked, so it will seek a higher gradient
route to base level.
• It may do so by splitting into branches
called distributaries.
Stream deposition- delta
• Mississippi River Delta - a bird-foot delta.
– This delta extends far out into Gulf of Mexico .
– Many Smaller sub deltas have grown since the
ice age.
• Threat to the Mississippi includes a
potential capture and diversion of water
further upstream.
• Could spell disaster for New Orleans.
Stream Valleys
• Playfair’s law - all streams have a main
trunk, fed by branches and have carved the
valley in which they exist.
• Two types of stream valleys
– narrow V shaped valleys
– wide valleys with flat floors
Stream Valleys - V shape
• Characterized by
– downcutting
– narrow canyons with steep wall
– rapids and waterfalls
• Valley walls undergo weathering and mass
wasting at the top to help with the profile.
Stream Valleys - Wide valleys
• More common in a graded stream
• Erosion shifts from down-cutting to lateral.
• Streams flow in floodplain is sweeping
bends called meanders.
• Erosion takes place on the outside of the
bend at the cut bank.
• Deposition takes place on the in side of the
bend at the point bar
• Causes the meander to migrate laterally
and downstream
Stream Valleys - Wide valleys
• Sometimes one meander will cut off
another to create a shorter channel
• The abandoned meander is called an
oxbow lake.
• Eventually the lake fills with sediment to
create a cut off scar
• Incised meanders- form when there is a
rapid change in base level.
Drainage networks
• All streams drain to an ocean.
• The drainage basin is all the land
contributes water to the stream.
• A divide is the imaginary line that
separates two drainage basins.
• Individual drainage basins have one of four
patterns depending on the material on
which they flow.
Drainage Patterns
• Dendritic - the most common - tree like
branches emptying into larger trunks
• Radial - like spokes on a bike tire draining
the side of an isolated cone or dome.
• Rectangular - flowing over jointed rocks
making sharp right angle turns
• trellis - also rectangular but looking more
like a trellis with lots of branches.
Stream Piracy - Ahrg!!
• Streams erode headward, downward and
laterally.
• As Niagara Falls erodes it is doing so up
stream (headward erosion)
• When the headward erosion of one stream
breaches the divide it can divert water
from a slower stream. This is stream
piracy.
Floods and Flood Control
• What causes floods? Too much water from
rain or melt water.
• Controls include building artificial levees,
dams,channelization (dredging) and better
flood plain management (non-structural
controls).