2. Outline
• Water and the Hydrologic Cycle
• Water on the Ground
• Water Under the Ground
• Water and Society
3. Water and the Hydrologic Cycle
• Water plays a central role in moderating
temperature and controlling climate
• The erosional and depositional effects of
streams, waves, and glaciers, coupled
with tectonic activity have produced the
diversity of Earth’s landscapes
• The unique chemical properties of water
make life possible
• The hydrologic cycle maintains a mass
balance on a global scale
5. Water and the Hydrologic Cycle
• The largest reservoir in the hydrologic
cycle is the ocean
– Contains more than 97.5% of Earth’s water
– Most of the water in the hydrologic cycle is
saline, and not usable by humans
• The largest reservoir of fresh water is
the polar ice sheets
– Contain 74% of the Earth’s fresh water
• The largest reservoir of unfrozen fresh
water is groundwater
7. Water and the Hydrologic Cycle
• Movement of water through the
hydrologic cycle is powered by the Sun
– Evaporation
– Condensation
– Precipitation
– Transpiration
– Surface runoff
– Infiltration
9. Outline
• Water and the Hydrologic Cycle
• Water on the Ground
• Water Under the Ground
• Water and Society
10. Water on the Ground
• During a heavy rainfall, water moves
downhill
– Initially as sheet flow (overland flow)
– Gets concentrated into stream flow
• Consists of storm flow and base flow
• Streams with no base flow are ephemeral
• Streams with base flow are perennial
11. Water on the Ground
• Streams are part of a complex natural
system that includes
– A channel
– A drainage basin
– A divide
• The stream’s load is the total sediment
and dissolved matter it is transporting
14. Water on the Ground
• Streams and drainage systems
– Play a fundamental role in both the
hydrologic cycle and the rock cycle
– Support complex ecosystems
– Are constantly evolving in response to
changing relief, climate and vegetation
15. Water on the Ground
• The size and shape of a stream
channel are controlled by several
factors
– Erodibility of rock
– Steepness of descent
– Volume of water
• The vertical distance that a stream
channel descends along its course is its
gradient, overall this decreases
downstream, though not smoothly
16. Water on the Ground
• Stream behavior is controlled by 5
basic factors
1. Average channel width and depth
2. Channel gradient
3. Average water velocity
4. Discharge
5. Sediment load
• All streams experience a continuous
interplay among these factors
17. Water on the Ground
• Following a stream from its source to its
mouth, orderly adjustments occur
– Width and depth increase
– Gradient decreases
– Flow velocity and discharge increase
– Turbulence decreases
19. Water on the Ground
• Meandering channels
– Straight channels are rare
– Low gradient streams typically assume a
sinuous shape, each bend is a meander
– The shape reflects the way the stream
minimizes resistance to flow
– Velocity is lowest along inside meanders
– Velocity is highest along outside meanders
20. Water on the Ground
• Meandering channels
– Sediment accumulates on the inner side of a
meander, forming a point bar
– Collapse of the stream bank occurs on the
outside of a meander, forming a cut bank
– In this way meanders tend to migrate
– Sometimes a stream bypasses a channel
loop, cutting it off and forming an oxbow lake
23. Water on the Ground
• Braided channels
– A stream that is unable to transport the
entire available sediment load tends to
deposit the coarsest and densest sediment
to form a bar, which locally divides and
concentrates the flow
– A stream with many interlacing channels
and bars is called braided
– Tends to have variable discharge and
easily erodible banks
25. Water on the Ground
• The size of clasts a stream can
transport is mainly related to velocity
• However, the size of clasts decreases
downstream from the rocky headwaters
• A stream’s load consists of three parts
– Bed load
– Suspended load
– Dissolved load
26. Water on the Ground
• Bed load
– 5-50% of total sediment load
– Move by rolling, sliding, or saltation
• Suspended load
– Particles of silt and clay provide the muddy
character of many streams
• Dissolved load
– Comprised primarily of 7 ions
• Bicarbonate, calcium, sulfate, chloride, sodium,
magnesium, and potassium
30. Water on the Ground
• Streams form three major depositional
landforms
– Floodplain: deposition of fine sediment
beyond natural levees during a flood
– Alluvial fan: a fan-shaped body of alluvium
at the base of an upland area
– Delta: triangular shaped deposit formed
when a stream enters the standing water of
a sea or lake
33. Water on the Ground
• All continents are divided into large
regions from which major rivers flow to
one of the world’s major oceans
– The line separating any two of these is
called a continental divide
• Continental divides often coincide with
crests of mountains, the result of uplift,
we know that there is a close
relationship between plate tectonics
and the locations of stream basins
35. Water on the Ground
• Water can remain stored in any of several
surface water reservoirs
– Ice caps are the greatest of these
– Lakes: mainly in high latitudes
• formed by glaciation, volcanism, tectonism,
streamflow, natural dams, cave collapse, ice dam
collapse, permafrost thaw, and coastal processes
– Wetlands: permanently or intermittently moist
• Include swamps, marshes and bogs
• Highly biologically productive
37. Water on the Ground
• Flooding
– Occurs when a stream’s discharge
becomes so great that it exceeds the
capacity of the channel
– Major floods occur infrequently, but can be
devastating or catastrophic
– During a flood, the peak discharge comes
well after the rains that produced it
• After rainfall, surface runoff moves into stream
channels, quickly increasing discharge
41. Water on the Ground
• Because floods can be so dangerous,
prediction has become essential
• Frequency of past floods can be
plotted, calculating the average time
interval between two floods of equal
magnitude
– This is called the recurrence interval
• In addition, real-time monitoring during
storms in combination with information
about the river basin’s geometry helps
43. Water on the Ground
• Flood prevention and channelization
– River channels are often modified for the
purpose of flood control
• This is called channelization
– Channelization protects our well-being, for
a time, and at a price
• Interferes with ecosystems
• Can aggravate pollution
• Does not always protect against flooding, and
in fact increases the chances of it
• Can lead to subsidence
• Renders historic hydrologic data invalid
44. Outline
• Water and the Hydrologic Cycle
• Water on the Ground
• Water Under the Ground
• Water and Society
45. Water Under the Ground
• Less than 1% of the liquid water in the
hydrosphere lies beneath the ground
– Groundwater
• It comprises a volume 35 times larger
than the volume of all the freshwater
lakes and streams, and is nearly a third
as large as all the glaciers and sea ice
• More than 50% of it is within 750 m depth
46. Water Under the Ground
• The elements dissolved in groundwater
consist of chlorides, sulfates, and
bicarbonates of calcium, magnesium,
sodium and potassium
– Which dissolve from common rock-forming
minerals
– Consequently the composition of ground-
water varies from place to place depending
on the surrounding rocks
47. Water Under the Ground
• From the ground surface to beneath the
water table, the regolith
– Is filled with air: aerated zone
– Is filled with water: saturated zone
• The upper surface of this is the water table
• The water table represents the upper
limit of all readily usable groundwater
– It follows the shape of the ground surface,
higher under hills, and lower under valleys
49. Water Under the Ground
• Groundwater flows between pore spaces
by percolation
– This flow depends on the porosity and
permeability of the rock it moves through
– Porosity: the percentage of the total volume
of rock that consists of open pore spaces
– Permeability: a measure of how easily a
solid allows fluids to pass through it
51. Water Under the Ground
• Groundwater flows from high water table
areas to low water table areas in response
to gravity
• Replenishment occurs when rainfall and
snowmelt enter the ground in areas of
recharge
• Water moves through the system to areas
of discharge, where it meets the surface,
streams, lakes, ponds or wetlands
53. Water Under the Ground
• An aquifer is a body of rock or regolith
sufficiently porous and permeable to store and
conduct significant quantities of groundwater
– If it has a water table, it is unconfined
– If the rate of withdrawal exceeds the rate of local
groundwater flow, a cone of depression may form
– A confined aquifer is bounded above and below by
impermeable rock (aquiclude)
– If it has high hydrostatic pressure, it is an artesian
aquifer, freely flowing
55. Water Under the Ground
• A spring is a flow of groundwater
emerging naturally at the ground surface
56. Water Under the Ground
• Slow moving groundwater has the
capacity to dissolve a lot of material
– Limestone and marble are very susceptible
• A cave will form when circulating
groundwater dissolves an underground
void with only no opening to the surface
• The passage is enlarged in the most
favorable flow route by the water that
fills the opening
57. Water Under the Ground
• Spectacular cave
formations are
deposited by
precipitation of
materials from the
groundwater
58. Water Under the Ground
• In contrast to a cave, a sinkhole is a
large dissolution cavity open to the sky
• In regions of exceptionally soluble rock,
sinkholes and caves are so numerous
that they combine to form a distinct
topography of smalls basins, ridges,
and pinnacles called karst
– This is best developed in moist, tropical
regions underlain by limestone
60. Outline
• Water and the Hydrologic Cycle
• Water on the Ground
• Water Under the Ground
• Water and Society
61. Water and Society
• A reliable water supply is critical
– For human survival and health
– For industry and agriculture
– For environmental services
• Water is under threat almost everywhere in
the world in terms of quality and quantity
• Laws and policies are confusing and
complicated, and groundwater is difficult to
monitor
62. Water and Society
• Crop irrigation demands 75% of water
• Industry demands 20%
• Domestic use demands 5%
• However, proportions vary greatly from
one region to another
• Population growth is partly responsible
for increasing demand, as are
improvements in living standards
63. Water and Society
• 29 countries worldwide suffer from water
shortages (450 million people)
• Interbasin transfer of water from one
drainage basin to another to meet high
water demands raises political issues,
and can have environmental impacts
• Excessive groundwater withdrawal can
lead to lowering of the water table, drying
of springs, compaction and subsidence
65. Water and Society
• About 1.2 billion people, mainly In
developing countries, do not have
access to clean drinking water
• In North America, water is drawn from
relatively clean sources, but is still
monitored and treated with chlorination
to kill microorganisms
66. Water and Society
• The accessibility of surface water
makes them useful resources, but
highly susceptible to contamination
• Contaminants come from
– Urban, suburban and agricultural runoff
– Industrial and landfill effluents
– Mining, logging and petroleum discharge
– Airborne contaminants
– Thermal pollution
67. Water and Society
• A common form of surface water
contamination results from excess plant
nutrients from fertilizers and detergents
• Triggers algae growth, and aquatic weeds get
out of control: algal bloom
• When they die, their breakdown causes
oxygen depletion, killing other organisms in
the water: eutrophication
• If accelerated by the addition of anthropogenic
pollutants, it is called cultural eutrophication
69. Water and Society
• Groundwater contamination is caused by
many of the same pollutants that affect
surface water
• However, it can be much more difficult to
detect, control, and clean up
– Passive remediation involves relying on natural
environmental processes to clean up the site
– Active remediation involves intervention by
injecting oxygen or other chemicals to speed
the breakdown of contaminants
74. Outline
• Earth’s Cover of Snow and Ice
• Glaciers
• Glaciation
• Sea Ice
• Ice in the Earth System
75. Earth’s Cover of Snow and Ice
• The part of Earth’s surface that remains
perennially frozen is the cryosphere
– Sea ice
– Glaciers: 10% of Earth’s land surface
– Frozen ground: 20% of Earth’s land
76. Earth’s Cover of Snow and Ice
• In the Northern hemisphere almost 1/4
of the land is covered by snow and
frozen ground during the winter
• The accumulation of snow fall that is
greater than seasonal melt is the main
contributor to glaciers and ice caps
• This highly reflective surface bounces
sunlight back into space, reducing
surface air temperature
78. Earth’s Cover of Snow and Ice
• Snow melt is a major source of water for
rivers and moisture for soils
• During a typical Northern hemisphere year:
– Sept-Oct: snow appears in Alaska/Siberia
– Nov: expands south and thickens
– Dec: snow reaches Russia/Europe/USA
– Dec-Mar: snowpack thickens, southern edge
retreats
– Late Spring: snowpack recedes rapidly
– June: confined to high mountains and Arctic
79. Earth’s Cover of Snow and Ice
• The lower limit of perennial snow is the snowline
– Its shape is controlled by variations in thickness of
winter snowpack and local topography
– Altitude typically changes from year to year
• Dependent on winter snow accumulation and summer melting
• Above this, part of winter’s snow has survived
summer
• In polar regions annual snowfall is very low
because the air is too cold to hold moisture
– Polar deserts
81. Outline
• Earth’s Cover of Snow and Ice
• Glaciers
• Glaciation
• Sea Ice
• Ice in the Earth System
82. Glaciers
• When snow and ice become so thick
that the pull of gravity causes the frozen
mass to move, this is a glacier
– Cirque glacier: the smallest
– Valley glacier: extends down from a cirque
– Ice cap: cover mountain highlands or low
lying land at high latitudes, flow radially out
– Fjord glacier: glacier in a fjord
– Piedmont glacier: spreads out from a valley
glacier
84. Glaciers
• Earth’s high mountain ranges contain
glacier systems tens of kilometers long
• Continent-sized ice sheets overwhelm
nearly all the land within their margins
– Greenland and Antarctica include 95% of
Earth’s glaciers and reach 3000 m thick
• Ice shelves hundreds of meters thick
occupy Antarctica’s embayments
86. Glaciers
• Glaciers form from snow that has
accumulated and been compacted until
it is so dense that it is impenetrable to air
– Glacier ice is considered a rock
• Ice grains recrystallize at depth within
the glacier, reaching 1 cm near the base
88. Glaciers
• Glaciers form wherever snow and ice
can accumulate
– High latitudes
– High mountains at low latitudes
• Ice temperatures vary among glaciers
– Warm (temperate) glaciers: at pressure
melting point, can coexist with water
– Cold (polar) glaciers: below pressure
melting point
90. Glaciers
• A glacier’s advance or retreat is the
balance of the amount of snow and ice
added (accumulation) and lost (ablation)
– Upper zone is the accumulation area
– Below this is the ablation area
– Between these is the equilibrium line
– The front of the glacier is called the terminus
93. Glaciers
• The terminus of a glacier responds to
changes in mass balance
• The movement of glacier ice occurs by
1. Internal flow
• Ice at critical thickness deforms and is pulled
downslope by gravity
• Occurs within individual ice crystals under stress
• The surface is brittle, cracks under tension, forms
crevasses, <50 m deep
1. Basal sliding
• Meltwater at the base acts as lubricant
95. Glaciers
• The uppermost ice in the central part of a
glacier flows faster than the sides and the
base - similar to a river
• Flow velocities vary from cm to m per day
• Even as a terminus is retreating, down-
glacier flow of ice continues
• The response lag is the time for effects of
change in accumulation to be transferred
through the glacier to the terminus
97. Glaciers
• Coastal glacier retreat is characterized
by frontal calving
– Where the terminus is in deep water
– Front breaks off to form icebergs
• Fjord glacier termini may remain
grounded on a shoal, preventing calving
100. Glaciers
• Most glaciers move slowly, but some
experience episodes of unusual rapid
behavior called surges
– Ice in the accumulation area moves rapidly
down-glacier
– Produces a chaos of crevasses and broken
pinnacles in the ablation area
– May advance several km during a surge
– The cause is not fully understood
102. Outline
• Earth’s Cover of Snow and Ice
• Glaciers
• Glaciation
• Sea Ice
• Ice in the Earth System
103. Glaciation
• The Earth fluctuates between periods of
extended cooler and warmer
temperatures
– Glaciations: glaciers expand, and new
ones form
– Interglacials: ice sheets retreat, sea level
rises
• We are in an interglacial period
104. Glaciation
• As glaciers flow, they sculpt the land
– As plows, scraping up weathered rock
– As files, rasping firm rock
– As sleds, carrying away sediment
• A glacier can carry very large boulders
and very fine sediment, but due to its
viscosity, there is no segregation by
size or density
– Glacial deposits are neither sorted nor
stratified
105. Glaciation
• Glaciers pluck rock fragments from
leeward outcrops
• These rocks embedded in the base of
the ice scrape underlying bedrock
producing long parallel scratches called
striations, or deeper glacial grooves
• Fine fragments of silt called rock flour
polish the bedrock until it has a smooth
reflective surface
109. Glaciation
• The debris carried by the glacier
eventually gets deposited
– Till: unsorted glacial debris deposited by
glacial ice
– Outwash: debris reworked, transported and
deposited by meltwater
– Moraines: glacially bulldozed ridges of
sediment
– Esker: curved ridge of sand and gravel
– Kettle: closed basin
112. Glaciation
• Land beyond the limit of glaciers is
called periglacial and is mainly found in
circumpolar regions
– Characterized by permafrost
• The active layer of this thaws in summer,
becoming unstable, and refreezes in winter
• When it melts, ground collapses
– Patterned ground: ice-wedge polygons
– Pingoes: frost-heaved hills
– Gelifluction: mass wasting
116. Glaciation
• Trapped in the snow that accumulates
each year on a glacier is evidence of
local and global environmental conditions
• The oldest ice in most cirque and valley
glaciers is several hundred to several
thousand years old
• Large ice sheets contain ice that dates far
back into the ice ages
• This can be examined through ice cores
118. Outline
• Earth’s Cover of Snow and Ice
• Glaciers
• Glaciation
• Sea Ice
• Ice in the Earth System
119. Sea Ice
• Forms by the solidification of fresh
water at the ocean surface, not by
precipitation, salt crystals are excluded
– In glacial periods, the ocean becomes
saltier, during interglacials, sea ice melts
and it becomes less salty
• 2/3 of Earth’s persistent ice cover floats
as a thin veneer on polar oceans
• But it only comprises 1/1000 of Earth’s
total volume of ice
121. Sea Ice
• Once the ocean surface cools to the
freezing point of sea water, slight
additional cooling leads to ice formation
– First small platelets or needles called frazil
– Then a soupy mixture at the surface
– Without waves, crystals freeze together to
form a 1-10 cm thick blanket of ice
– With waves, crystals form 3 cm diameter
pancake-like ice masses
123. Sea Ice
• Sea ice is distributed differently
between the two hemispheres due to
contrasting geography
– Antarctica is covered by a vast, thick ice
sheet, and sea ice forms a ring around it
– The North Pole is within the deep Arctic
Ocean, mostly covered by sea ice
• Sea ice is in constant motion and is
constantly changing
127. Outline
• Earth’s Cover of Snow and Ice
• Glaciers
• Glaciation
• Sea Ice
• Ice in the Earth System
128. Ice in the Earth System
• Influence on ocean salinity and circulation
– Interactions among ice, water and
atmosphere influence ocean structure,
salinity and circulation
– Sea ice is very sensitive to temperature
change, and the exclusion of salt as it forms
leads to the production of dense, cold saline
water on the continental shelves
– This produces deep bottom ocean water
129. Ice in the Earth System
• Influence on atmospheric circulation
and climate
– Floating ice isolates the ocean surface
from the atmosphere, cutting off heat
exchange
– Ice has high albedo, reflecting incoming
solar radiation rather than absorbing it
– This results in a steep temperature
gradient between the equator and poles,
driving atmospheric circulation
130. Ice in the Earth System
• Ice cover and environmental change
– If the climate became colder and ice over
expanded, the result would be a positive
feedback due to raised albedo
– If the climate warms and ice cover shrinks
or disappears, a similar but opposite effect
of positive feedback warming would occur
as the Earth’s overall albedo decreases
• While melting of sea ice would contribute little
to ocean levels, however melting of land ice
would contribute significantly to water volume
• Both would drastically affect ocean salinity
