Transcript Unit 6 Presentation

Tearing Down Mountains II:
Groundwater and Rivers
GEOSC 10: Geology of the National Parks
Presented by Dr. Sridhar Anandakrishnan
The Pennsylvania State University
Water
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Falls from the sky
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Plants and trees drink & evaporate 2/3
Some runs off into streams
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Streams carry water & sediments
Some is soaked into the Earth to make
groundwater... which eventually goes to
streams
Groundwater makes caves and drinking water
Streams go to rivers go to ocean goes up into
air
GEOSC 10 - Geology of the National Parks
Hydrologic Cycle
GEOSC 10 - Geology of the National Parks
Water
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Unique to Earth
‣ Maybe some on Mars, Moons of Jupiter
‣ Life needs water
2/3 of Earth’s surface is water
97% of Earth’s water is in the oceans
Most of the rest in glaciers
Water is most important surface process
modifying the landscape
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Johnstown
GEOSC 10 - Geology of the National Parks
Streams Change Landscape
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Streams erode their bed
Move water (duh) and sediments, boulders, all
sizes of rocks in between
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Sediment is relatively finely ground up rock
When rivers deposit sediment they do it in flat layers
Floods do most of the work
GEOSC 10 - Geology of the National Parks
Human Uses
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Agriculture & industry
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Irrigation & transport
Commerce
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Travel & trade
Hydroelectric power
‣ Norway is rich because they don’t use oil (but have
lots)
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Hydrologic Cycle
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Water evaporates from the ocean...
It rains onto land
‣ It evaporates right away
‣ Plants take up and then release into atmosphere
‣ 2/3 of rainfall goes straight back up
‣ This is evapo-transpiration
Rest goes into streams and ground water
GEOSC 10 - Geology of the National Parks
Streams
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Driven by gravity
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Sea level is where water is heading (it can’t get
lower)
Recharged by direct rainfall
Also by groundwater springs
‣ That’s why rivers run even after long periods without
rain
GEOSC 10 - Geology of the National Parks
Erosion
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Streams carry sediments along with water
Mass wasting (last time) delivers sediments to
river
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That makes it higher and higher (at the place where
mass wasting is adding stuff)
That makes the stream steep
That makes the water flow faster
That carries away sediments!
GEOSC 10 - Geology of the National Parks
Moving Sediment
GEOSC 10 - Geology of the National Parks
Moving Sediment
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Suspended load
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Fine stuff floating up in water
Bed load
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Bigger rocks (gravel to boulders) that bounce along
Breaking up other rocks as they go, making smaller
rocks
GEOSC 10 - Geology of the National Parks
Types of Streams
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Meandering streams
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If the river has mostly fine sediments, it makes a
deep channel (it won’t get clogged)
If it has a curve, the curve gets more pronounced
- Water on the outside of the curve has to go faster, so it
erodes more, so it makes the curve deeper...
‣ Sometimes makes a big loop that gets cutoff: an
oxbow
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Braided Stream
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In areas with lots of gravel and boulders, the
main channel gets choked
New channels form all the time
Looks like a braid from above
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Dams
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A dam stops water temporarily but sediments
permanently
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Dams
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Sediments fill up the reservoir above the dam
The river below the dam is clean of sediments,
so that clean water can pick up more
sediments
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If no dam, the water would be “dirty” - lots of
sediments, so not so good at erosion
Gravel and sand bars below dams get washed
away
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Lots of critters live on those bars...
Also, the clean water makes life tough for
some fish (they can’t hide)
GEOSC 10 - Geology of the National Parks
Dams
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Eventually that reservoir above the dam will fill
up
GEOSC 10 - Geology of the National Parks
Katrina
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Levee protecting New Orleans broke
New Orleans is below sea level, so water
rushed in
GEOSC 10 - Geology of the National Parks
Floods
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Floods happen. Every year. Snowmelt,
rains...
When rivers flood they carry water and
sediment out into the fields beyond the main
channel
But we don’t like mud in our living room...
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Ask the government to build walls preventing
flooding, called levees
Levees built over the last 100 years
GEOSC 10 - Geology of the National Parks
Big Easy...
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Big Easy built on mud from long ago
floods
Water is slowly squishing out and the mud is
compacting
New Orleans sinks, but Mississippi (also
known as Big Muddy) stays the same...
Eventually Big Easy lower than Big Muddy
Stage set for disaster
GEOSC 10 - Geology of the National Parks
As if That Isn’t Bad Enough...
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Usually big rivers build natural levees
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Floods deposit more stuff near the river than far
away, making a low wall...
So for a while the river stays in one course...
But eventually, during a big flood it will breach
the levee and take a different course to the
ocean
All the riverbanks downstream of that breach
are left high and dry
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Groundwater
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Soil is like a sponge
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Spaces between the grains can be filled with air or
water
Below a line called the water table, the spaces
(pores) are filled with water. Above, they are mostly
air.
Rain seeps into the ground and slowly makes
its way deep into the Earth
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Because water has to trickle through small pores,
soil is like a natural filter - bacteria can’t get through
Water trickles slowly, so bacteria die over time
We can pump that water and drink it!
GEOSC 10 - Geology of the National Parks
Groundwater Pollution
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Soil is like a sponge
Pour a chemical into the soil (motor oil, drycleaning fluid), and it is very difficult to get it
out
Think of a sponge. Add soap - you need to
soak and rinse many many times to get all the
soap out...
GEOSC 10 - Geology of the National Parks
Caves
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In State College, lots of limestone
Groundwater is slightly acidic (critters in the
ground breathe in oxygen and breathe out
CO2)
Water + CO2 = carbonic acid - a weak acid that
dissolves limestone
Water seeping into cracks will enlarge those
cracks, making caves
Now water can go deep quickly!
GEOSC 10 - Geology of the National Parks
Sinkholes
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If a cave forms near the surface, the roof can
collapse, or it can fill with sediment
Putting a big building (or road) on a sinkhole
costs lots more...
Memorial Field (State High football field) is a
sinkhole
GEOSC 10 - Geology of the National Parks
Water Supply Pollution
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In areas with lots of limestone, we get caves
(know as karst regions, after the region in
Slovenia where this is common, and first
noted)
Water can go into caves (and sticks and
leaves and bacteria) - no filtering
Water can go quickly into the Earth - bacteria
don’t die
Water can be unsafe...
GEOSC 10 - Geology of the National Parks