Transcript Rivers - Cal State LA - Instructional Web Server

Weathering
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Leads to sediment source
Rocks must be broken down into smaller components
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Sediment generated from rocks termed clastic sediment
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Clastic refers to fragments of rocks
Sediment generated transported to oceans
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Coarser sediment forms beaches or other depostional
features
Deltas
Or transported to deep ocean
Fine grained sediment transported to shelf, slope, deep ocean
Weathering
•Weathering: the disintegration, or breakdown of
rock material
2 Types of Weathering
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Mechanical Weathering:
no change in chemical
composition--just
disintegration into smaller
pieces
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Chemical Weathering:
breakdown as a result of
chemical reactions
Mechanical Weathering
Physical breakup
• pressure release
• water: freeze - thaw cycles
• crystallization of salt in cracks
• thermal expansion and contraction
All this increases the total surface area
exposed to weathering processes.
Mechanical Weathering
Exfoliation:
Rock breaks apart in layers that are
parallel to the earth's surface; as
rock is uncovered, it expands (due to the
lower confining pressure) resulting in
exfoliation.
Weathering
Mechanical
Weathering
Sheet Joints
(Exfoliation)
Stone Mountain, GA
Half Dome,
Yosemite, CA
Exfoliated Domes, Yosemite
Stone Mountain, Georgia, showing the product of
exfoliation due to unloading
Stone Mountain, GA
Frost Wedging: rock breakdown caused
by expansion of ice in cracks and joints
Shattered rocks are common
in cold and alpine
environments where
repeated freeze-thaw cycles
gradually pry rocks apart.
Weathering
Thermal expansion due to the extreme range of temperatures can
shatter rocks in desert environments.
Repeated swelling and shrinking of minerals with different expansion
rates will also shatter rocks.
Role of Physical Weathering
1)
Reduces rock material to smaller
fragments that are easier to
transport
2)
Increases the exposed surface area
of rock, making it more vulnerable to
further physical and chemical
weathering
Chemical Weathering
Definition: transformation/decomposition
of one mineral into another
Mineral breakdown
• carbonate dissolves
• primary minerals --> secondary
(mostly clays)
minerals
Net loss of elements retained in the soil.
Dissolution
H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3-
water + carbon dioxide + calcite
dissolve into calcium ion
and bicarbonate ion
Biological activity in soils
generates substantial CO2
Bicarbonate is the dominant
ion in surface runoff.
Erosion
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Sediments produced by weathering are then
removed by erosion
Erosion - the incorporation and transportation
of weathering products by a mobile agent such
as wind, water, ice.
Rivers
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Introduction
• Rivers and streams are
dynamic systems that
continually adjust to natural
and human-caused changes
• Running water is the most
important geologic agent
modifying Earth’s land
surface and is a source of
fresh water for industry,
agriculture, and domestic
use
• Management of erosion and
flooding requires
considerable effort and cost
http://www.co.monterey.ca.us/OES/floods_files/Salinas_River_Floods.jpg
Salinas River, CA
Hydrologic Cycle
• Water is cycled
from oceans, through
atmosphere, to continents, and
back to the oceans
• Powered by solar
radiation and occurs
because water changes
readily from liquid to
a gas on surface
Evaporation, condensation,
precipitation, and runoff
characterize the movement of
water, though some is stored
in lakes, groundwater, and ice
Moving Water
Sheet flow
Sheet Flow
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water moves in a
continuous sheet of
shallow watermoving
over the surface
www.duluthstreams.org/.../ impact_impervious.html
Channel Flow
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water is confined to
long trough-like
depressions
LA River Channel
http://upload.wikimedia.org/wikipedia/commons/0/07/Los_Angeles_River_Glendale.jpg
A Drainage Basin
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A drainage basin is extent
of land where water from
rain and melting snow or
ice drains downhill into a
body of water, such as a
river, lake, reservoir,
estuary, wetland, sea or
ocean. The drainage basin
includes both the streams
and rivers that convey the
water as well as the land
surfaces from which water
drains into those channels
Drainage BasinsSouthern California
http://www.urbanedpartnership.org/target/units/river/drainage_map.gif
OC Public Works
Important River Parameters
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Gradient, Velocity, and
Discharge
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Gradient is the average
slope of the channel bed
Velocity is the distance
water travels in a given
amount of time
Discharge is the volume of
water passing a point in a
given amount of time
Ability to transport
sediment depends on
velocity which also
depends on gradient
River Sediment Transport
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Sediment transported as:
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Bedload
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Suspended loads such as clays
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Saltation, traction, rolling of
grains—e.g. sand, pebbles,
boulders
Most comes from slopes
Most comes from soils
Solution load from dissolved
minerals
Deposition by Running Water
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Braided Streams
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network of connecting
dividing channels,
separated by sand and
gravel bars
develop when sediment
supply exceeds transport
capacity of running water
Deposition by
Running Water
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Meandering Streams
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defined by a single channel
with broadly looping curves
cutbanks are found on the
outside of meanders, point
bars on the inside
unequal flow velocities in
channels accounts for
deposition and erosion in
predictable patterns
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Incised Meanders
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Develop where an older
meandering pattern is
cut into underlying
bedrock as tectonics
uplift the region
Channel Width and Depth
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Width mostly controlled by silt & clay in banks
More cl & slt = wider channel
 The wider the channel the shallower the depth.
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The Significance of Base Level
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Base level is the lowest
level to which any stream
can erode
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River cannot cut below its
mouth
Sea level is taken to be the
ultimate base level, but the
rising of the sea or
subsidence of land over
geologic time make this
concept a relative one
Local base levels may
control erosion and
deposition
Graded Streams
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Graded streams develop over
time as a balance between
gradient, discharge,
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flow velocity, channel shape,
and sediment load is reached
• The concept is an ideal, but
gives us a model to understand
responses to changes in these
parameters
Think about dams on our LA,
San Gabriel and Santa Ana
Rivers
Predictions of Adjustment to Base
Level Changes
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Malibu, Bolsa Chica, Bolinas
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Describe how these creeks would change and the impacts that these
changes will have on the sediment supply to beaches and other regions of
the coast if :
Sea Level Dropped due to global cooling
Tectonic uplift of coastal area
Sea Level drops and tectonic uplift of coast occurred together
Sea Level rises due to global warming
Tectonic down dropping of coast
A dam is placed on the mid-point of the creeks
A resistant outcrop occurs in the mid-point of the creek
Excavation of the creek bottom in the mid-point of the creek occurs
because of gold mining
Deltas
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Flow velocity
decreases as water
flows into lakes or
oceans
deposition occurs
and may lead to the
origin of a delta,
which can prograde
as sediment is
continually supplied
by the stream
 topset, foreset, and
bottomset beds
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Sacramento River Delta
NASA
Ganges River
Nile River
Mississippi Delta
Santa Clara River
Nasa
Malibu Creek
www.santamonicabay.org/.../
tabid/77/Default.aspx
LA River
scmi.us/.../ ci-core/la-river
San Gabriel River
Nasa
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Meandering Streams
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Oxbow lakes form
when meanders
become so sinuous that
the thin neck of land
between them is cutoff
during floods
How Do Valleys Form and
Evolve?
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Valleys usually have
rivers running their
length, with tributaries
draining highlands on
either side
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downcutting occurs
when a stream has
excess energy to
deepen its course
Flooding
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Rating Floodplain
Risk
Streamflow
Measurement
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Most common
practices are dams and
levees
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both require large
capital investments and
constant maintenance
they are constructed to
control finite amounts
of water and sediment;
if that is exceeded the
water will end up in the
floodplain anyway
Estuaries
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Where the Rivers meet the Sea
Semi- enclosed body of water where there is a
mix of river and seawater, and mix of fluvial and
marine processes
Mouths of many rivers flooded at last sea level
high (last interglacial)
Boundary between fluvial and marine processes
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Influenced by tides
Tectonic
San Francisco Bay
Fjord
Chesapeake Bay: Drowned
Shoreline
ESTUARY
The Netherlands
Barrier Island
Wetlands – lands covered with water
all or part of a year
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Wetlands purify water, act as flood control, & help stabilize
shoreline. Are biologically diverse
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Wetland Conditions
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Hydric (saturated) soils – formed under conditions of saturation,
flooding, or ponding long enough during growing season to produce
anaerobic conditions in upper
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Hydrophytic plants – adapted to thrive in wetlands despite the stresses of
an anaerobic and flooded environment
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Hydrologic regime – dynamic or dominant presence of water
Wetland Classification Chart
Major Categories
General Location
Wetland types
Marine (undiluted salt
water)
Open coast
Shrub wetland, salt
marsh, mangrove swamp
Estuarine (salt/freshwater
mix)
Estuaries (deltas,
lagoons)
Brackish marsh, shrub
wetland, salt marsh,
mangrove swamp
Riverine (associated w/
rivers and streams)
River channels and
floodplains
Bottomlands, freshwater
marsh, delta marsh
Lacustrine (associated w/
lakes)
Lakes and deltas
Freshwater marsh, shrub
and forest wetlands
Coastal Wetlands:
Inland Wetlands:
Palustrine (shallow ponds, Ponds, peatlands,
misc. freshwater wetlands) uplands, ground
water seeps
Ephemeral ponds, tundra
peatland, ground water
spring oasis, bogs
Physical/Hydrological Functions of Wetlands
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Flood Control
 Correlation between wetland loss and downstream flooding
 can capture, store, and slowly release water over a period of time
Coastal Protection
 Serve as storm buffers
Ground Water Recharge
 Water has more time to percolate through the soil
Sediment Traps
 Wetland plants help to remove sediment from flowing water
Atmospheric Equilibrium
 Can act as ‘sinks’ for excess carbon and sulfur
 Can return N back to the atmosphere (denitrification)
Chemical Functions of Wetlands
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Pollution Interception
Nutrient uptake by plants
 Settle in anaerobic soil and become reduced
 Processed by bacterial action
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Toxic Residue Processing
Buried and neutralized in soils, taken up by
plants, reduced through ion exchange
 Large-scale / long-term additions can exceed a
wetland’s capacity
 Some chemicals can become more dangerous in
wetlands (Mercury)
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Value of Estuaries
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Greatly influenced by river discharge and tidal mixing.
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Environmental conditions fluctuate widely.
Biomass is high because estuaries have high nutrient levels.
Species diversity is low because fewer species can cope with
the wide fluctuations in temperature, salinity etc.
“nursery grounds”
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young stages develop in the estuary
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(abundant food; low predation
may move offshore to adult habitats
Riparian System
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What is a Riparian Area?
Riparian zones or areas have been defined in several ways, but they are
essentially the narrow strips of land that border creeks, rivers or other bodies of
water. Because of their proximity to water, plant species and topography of
riparian zones differ considerably from those of adjacent uplands. Although
riparian areas may occupy only a small percentage of the area of a watershed,
they represent an extremely important component of the overall landscape.
This is especially true for arid-land watersheds, such as those in Eastern
Oregon.
Functions of a Healthy Riparian System:
1. Sediment Filtering
2. Bank Stablilization
3. Water Storage and Release
4. Aquifer Recharge
5. Wildlife Habitat
Riparian System
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A healthy system would have
some or all these characteristics:
1.) High water table & increased
storage capacity,
2.) High forage production,
3.) Good shade-Cool water,
4.) Good fish habitat-Good water
quality,
5.) High wildlife habitat diversity,
6.) Vegetation & roots present to
protect & stabilize banks,
7.) Higher late summer stream
flows.
•Denton Creek