Freshwater Resources

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Freshwater Resources Natural Systems, Human
Impacts
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Figure 4.6a
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Freshwater depletion Aral Sea

• The Aral Sea, in central Asia, was the fourth
  largest freshwater body on Earth, but it could
  disappear completely
• Overirrigation for cotton

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Freshwater depletion Aral Sea
Satellite photo Deep water today
Shallow water today Dry former lake bed
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Freshwater depletion Aral Sea

• The Aral Seas depletion has been devastating to
  the local people and their economies.
• Hundreds of ships lie stranded in the sand,
  because water fell so fast.

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Water pollution Point and non-point sources
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The Hydrologic Cycle

• The continuous movement of H2O from one
  reservoir to another

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The Distribution of Water on Earth
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Distribution of the Earths water, by relative
volume
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The Hydrologic Cycle
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Hydrologic Cycle
Sun
Sublimation
Transpiration
Evaporation
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Hydrologic Cycle
Sun
Net movement of clouds toward land
Extra water on land
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Hydrologic Cycle
Sun
Surface Water Runoff
Infiltration
Groundwater Flow Discharge
Water Returns to Ocean
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Available freshwater resources

• Nations vary by more than a factor of 100 in
  water per capita.

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Annual Precipitation
Potential Evapotranspiration
Runoff
All 3 factors together ? Local water supply
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Rain-Shadow Effect
Moisture-depleted air-mass sinks and warms,
reducing relative humidity
Moist air-mass rises and cools, causing
precipitation
resulting in a rainy windward slope.
Moisture-laden winds come onshore.
forming a rain shadow on the leeward side of
the mountains.
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Stream Terminology Processes
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The Dynamic Equilibrium of a Stream System is
Controlled by

• Topography (including slope)
• Climate
• Streamflow
• Resistance of underlying bedrock

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Drainage Basin(Catchment, Watershed)

• An area of land that funnels all water that
  falls on it into a network of streams
• The boundaries of the drainage area are called
  drainage divides

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Drainage Divide Marks the edge of two adjacent
drainage basins
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Tacoma Watersheds
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http//www.sightline.org/maps/maps/cascadia_cs05m

• Cascadia bioregion, defined by the watersheds of
  the rivers that flow into the Pacific Ocean
  through North America's temperate rainforest zone.

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Tacoma
The Green River Watershed and groundwater wells
are both used to supply Tacoma's water.
http//www.ci.tacoma.wa.us/water/images/WaterSrvAr
ea209_07.jpg
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http//www.savingwater.org/education_regional_wate
r_system.htm
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Colorado River Drainage Basin
Continental Divide
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http//www.epa.gov/watertrain/protection/r1.html
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Typical Erosional Drainage Patterns
Dendritic
Trellis
Rectangular
Radial
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Discharge water cross section x velocity
(width x depth) (distance
/ time)
Discharge is three-dimensional (volume) per unit
time
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Low Discharge (30 meters3 / second)
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High Discharge (180 meters3 / second)
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Average Discharges of Major Rivers of the World
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Sediment Transport
Bed load Coarse particles moving along the
bottom of river channel Suspended load Accounts
for about 90 of its total load Dissolved load
Carried in chemical solutions
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Increased Flow Velocity leads to
Increased suspended sediment Increased
bedload transport Increased saltation
Velocity largely depends on stream gradient,
discharge, channel shape, and turbulence
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Measures of a Streams Ability to Transport
Sediment Include

• Competence A measure of the maximum size
  particle a stream can transport under a given set
  of flow conditions
• Capacity A measure of the total volume of
  sediment a stream can transport under a given set
  of flow conditions

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Hjulstroms Curve(consider rocks and sediment
in/near rivers)
(Streams Speed)
(Sediments Grain Size)
Clay (argile), Silt (silt), Sand (sable), Gravel
(gravier)
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Hjulstroms Curve
Sediments SMALLER than 0.6 mm in diameter are
TRANSPORTED by streams flowing 20 cm/sec.
Sediments LARGER than 0.6 mm in diameter are
DEPOSITED by streams flowing 20 cm/sec.
20 cm/sec flow
(Streams Speed)
(Sediments Grain Size)
Clay (argile), Silt (silt), Sand (sable), Gravel
(gravier)
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Hjulstroms Curve
ERODED (if stream gt 80 cm/sec)
TRANSPORTED ( if stream 25-80 cm/sec)
(Streams Speed)
DEPOSITED (if stream lt 25 cm/sec)
(Sediments Grain Size)
Clay (argile), Silt (silt), Sand (sable), Gravel
(gravier)
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Clay particles have a high surface-to-volume
ratio, as well as residual charges on their
outer layers ...thus they tend to be cohesive
and stick together making clay relatively
harder to erode than coarser silt and
fine-grained sand!
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Channels, Valleys, and Floodplains
Longitudinal profile
Cross-sectional profile
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Waterfall formed by headward erosion
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The Two Main Types of Channel Patterns on
Floodplains Are

• Meandering Streams
• Have a single channel with a sinuous pattern
• Are the most common pattern on floodplains
• Braided Streams
• Have an interlacing network of channels
• Are relatively uncommon

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Meandering Streams
Cut Bank
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A Braided River in Alaska
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Variables that Encourage Channel Braiding Include

• Highly variable water discharge
• Large sediment load (volume)
• Easily eroded bank material

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Potholes form by pebbles and gravel grinding
inside eddies
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Uplift can ? Incised meanders
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Formation of an Antecedent Stream System
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Base Level
The elevation at which a stream ends, by
entering a large standing body of water, such as
a lake or ocean
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Changing base level caused by building a dam, and
resultant change in longitudinal profiles
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The stream deposits sediment in the upper part of
the reservoir
The sediment-depleted stream begins to erode
downstream of the dam
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Figure 4.17
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Deltas formed at the mouth of a large river
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Silt carried by Atchafalaya River discharge
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Rivers NaturallyChange Position
(End of Ice Age)
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Rivers NaturallyChange Position
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Changing Location of the Mississippi Delta Over
the Last 6000 Years
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Natural vs. Artificial Channels
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Natural Channels

• Shaded ? Hospitable
• Sorted gravels ? Habitats
• Variety of water depths, speeds, and gradients ?
  Diversified habitats species
• Deep enough ? Supports life during dry season
• Big rocks, undercut banks ? Resting areas

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Artificial Channels

• Not shaded ? Water T too high
• Unsorted gravels ? Few habitats
• Uniform water depths, speeds, and gradients ? Few
  habitats species
• Shallow ? Cannot support life during dry season
• Uniform topography ? No resting areas

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Los Angeles River (CA)

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Kissimmee River (FL)

• 1960s - Channelized, straightened, and shortened
  the river
• Water quality suffered,
• Flood hazard increased

1991 - Started to un-channelize it (most
ambitious restoration project attempted so far)
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• http//www.ipcc.ch

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• http//mbayaq.org/cr/seafoodwatch.asp