Running water: Rivers and Streams

The Water Budget and Groundwater
Running water: Rivers and Streams
Total Water on Earth
1,360,000,000 km3
Oceans and Seas
1,331,746,800 km3 (97.9%)
Glaciers and Ice Sheets
24,000,000 km3 (1.8%)
Groundwater
4,000,000 km3 (0.3%)
Lakes and Reservoirs
155,000 km3
The Niagara River
Soil Moisture
83,000 km3
The 1997 Manitoba Flood
Vapor in the atmosphere
14,000 km3
Rivers
1,200 km3
Weathering, transport and deposition
The water budget and groundwater
The hydrologic cycle
Classification of Rivers
Braided Rivers
Meandering Rivers
Groundwater resides within the Earth.
The only water source for many areas (e.g., Walkerton)
Water filling void spaces in rocks and sediment.
Effluent Rivers: water
table rises to river bed
(groundwater adds to
the river)
Water table: the surface below which groundwater fills void spaces.
Zone of aeration: above the water table; voids filled with air.
Zone of saturation: below the water table; voids filled with water.
Effluent Rivers: water
table rises to river bed
(groundwater adds to
the river)
The Hydrologic Cycle
The constant exchange of water between all of the water reservoirs is
termed the Hydrologic cycle.
The cycle is balanced over time.
Influent Rivers: water
table is below the
river bed (river adds
to groundwater).
From Lutgens and Tarbuck, figure 6-9
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Rivers
The exchange between land and oceans is largely via rivers:
Of the 380,000 km3 of water in the cycle:
Land
Oceans
Precipitated:
25.3%
74.7%
Evaporated:
15.8%
84.2%
Net:
+9.5%
-9.5%
0.095 x 380,000 km3 = 36,100 km3 of excess water to the land.
Rivers deliver water and vast amounts of clastic sediment and
material in solution to the world’s oceans.
A river’s discharge is the volume of water moving through a river
over a given period of time.
A river’s sediment discharge is the volume of sediment moving with
the water of a river over a given period of time.
The ten largest rivers on Earth deliver 36% of all water that flows
into the oceans.
The Amazon River delivers 15% of the world total.
Total discharge into the oceans by rivers is 36,000 km3
6,300 km3/year
99.7% of excess water to the land is returned to the oceans by rivers.
or
200,000 m3/second
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River discharge depends on:
Climate
Relief
Drainage basin area
Geology
Orinoco 30% of Mississippi basin area; 190% of water discharge.
Tropical versus mid-latitude setting;
more rainfall.
Ganges-Brahmaputra: 24% of Amazon basin area;
20 billion tonnes of the products of weathering are carried annually
by rivers to the oceans:
15% of water discharge;
185% of sediment discharge.
Approx. 16 billion tonnes of clastic sediment
Approx. 4 billion tonnes of dissolved material
Clastic sediment is transported in rivers as:
Bed Load: large particles that move in contact with the bed.
The Ganges-Brahmaputra
drains the Himalayan
Mountains (high relief, high
rates of erosion)
Suspension Load: fine sand, silt and clay that “floats” along with the
water.
Over 90% of the total clastic sediment discharge into the oceans is as
suspended load.
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Flow in Rivers
20 billionTonnes/yr = 8 km3/yr
Discharge varies seasonally, daily and hourly.
= 800 km3/100yrs
Some rivers always have discharge.
= 8,000 km3/1000yrs
Some rivers are dry over much of the year
(ephemeral rivers).
= 8,000,000 km3/1,000,000yrs
etc., etc., etc.
So why aren’t the continents flattened out by now?
Plate Tectonics: volcanism, igneous intrusion, thrusting and folding
all build the continents.
Hydrograph: a graph showing the variation in a river’s
discharge with time.
Annual Hydrograph is a hydrograph showing variation in
dischage over the course of a year.
Single storm hydrographs show the discharge of a river for a single
rainfall event.
In a natural setting rainfall precedes the peak river discharge by
Lag Time:
time required for rain to soak into the ground until it is saturated
and then flows over the land surface into rivers.
Arroyos are steep-sided
valleys produced by
some ephemeral
stream.
Winter: accumulation of snow/ice cause lowest discharge).
Spring melt: maximum annual discharge.
Summer/Fall: rainstorms cause short duration discharge peaks.
Cities are vast, impermeable surfaces (water doesn’t soak into the
ground) with human facilities to rapidly direct runoff to rivers
(streets, storm drains, storm sewers).
Urbanization reduces lag time and increase the peak discharge.
Lag time can be hours to days,
depending on area, relief and
the nature of the drainage basin.
A natural surface behaves like a
sponge, soaking rainwater into
the ground; removing it from
the surface.
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Why does urbanization increase the risk of flooding?
Urban flooding
Paving streets and parking lots, storm drains.
Greater runoff = higher storm peak.
Flood Frequency Curves
Used to determine the risk of flooding by a given river.
Data plot as a straight line which defines the Flood Frequency Curve.
e.g. 20,000 ft3/s
50,000 ft3/s
every 1.2 years
every 5 years
Annual maximum discharge
versus time between each
occurrence of that discharge
for a given river.
100 year flood:discharge with a recurrence rate of 100
years.
Curve shows the the probability of a given discharge
occurring in a particular year.
20,000 ft3/s, 80% chance in a
year
Classification of Rivers
Braided Rivers
Multiple, interconnecting
channels.
Relatively steep slopes.
8 of 10 years 20,000 ft3/s will be
exceeded.
100,000 ft3/s, 1% chance in a
given year.
This is the 100 year flood
for the Skykomish River.
Coarse sediment in
transport, mostly
bedload.
Discharge highly variable over time (not uniform)
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Meandering Rivers
Braided Rivers
Channel system occupies the
entire floodplain.
Single, sinusoidal
(meandering) channel.
During high discharge the
entire floodplain is covered
forming a single channel.
Relatively gentle slopes.
Sediment is fine-grained,
mostly by suspension
load.
Longitudinal bars migrate downstream during floods.
Discharge is more uniform with time.
http://www.usra.edu/esse/ford/ESS205/fluvial/Rakaia1.jpg
Gravel longitudinal bars on the Athabaska River
Brahmaputra River, India
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Classification of Rivers
Braided Rivers
and
Meandering Rivers
During floods, longitudinal
bars migrate down stream.
Migration of a longitudinal bar.
Longitudinal bar
http://www.usra.edu/esse/ford/ESS205/fluvial/Rakaia1.jpg
Migration of a longitudinal bar.
Migration of a longitudinal bar.
Longitudinal bar
Longitudinal bar
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Migration of a longitudinal bar.
Migration of a longitudinal bar.
Longitudinal bar
Longitudinal bar
Migration of a longitudinal bar.
Migration of a longitudinal bar.
Longitudinal bar
Longitudinal bar
The deposits of ancient braided rivers are characterized by
horizontal beds of gravel with cross-strata dipping in the direction
of longitudinal bar migration.
Meandering Rivers
The channel occupies a small
part of the floodplain.
Morphological features include:
Levees
Point bars
Chute channels
Oxbow lakes
Meander scars
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Straight channels evolve into meandering channels.
Thalweg:
deepest part of a channel
Thalweg defines a line
that meanders through a
straight channel.
Erosion where the thalweg is close and deposition where the thalweg
is farthest from channel margin.
The boundary shear stress
(τo) at the floor of the
channel determines
whether or not erosion
will take place.
τ o = ρ gDS
Where ρ is the density of
water;
g is the acceleration due to
gravity;
D is the water depth;
S is the slope of the channel.
Where the water is deepest (i.e. along the thalweg) τo is greatest and
erosion occurs.
As the thalweg alternates from side to side along the channel the
sites of erosion and deposition also alternate along the channel.
Over time the channel migrates laterally
into larger amplitude, sinusoidal form.
Click here for a flash animation of a meandering channel.
http://www.wwnorton.com/earth/egeo/animations/ch14.htm
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Channel terminology
Thalweg: the line defining the
deepest part of the channel.
Thalweg
Cut bank
Riffle
Pool
Point
bar
Riffle: regions of shallow, fast
flowing water between
meander bends.
Pool: regions of deeper, slower
flowing water at meander
bends.
Point bar: the inside of a
meander bend.
Pointbars
Cut banks
Pools
Riffles
Cut bank: the outside of a
meander bend.
The extensive floodplain of a
meandering river is almost as
characteristic as the sinusoidal
channel form.
Time 1
During low flow, the river
occupies only the main channel.
During a flood, water levels rise
and pass across the point bar via
the chute channel.
Time 2
Chute Channels, the river is flooded to its banks.
Overflowing water runs down the
slopes, away from the channel
and covers the floodplain.
Time 3
Organic-rich sediment yields
excellent farmland.
Many cities are on the floodplains of rivers (rivers were very
important for transportation).
Many large cities at risk of flood (e.g., New Orleans, Winnipeg).
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Levees: linear mounds of sediment that parallel the
meandering channel.
Floods: water and sediment spread outward from the channel.
Normal flow: water and sediment are restricted to the channel.
Repeated floods build the levees and build the channel vertically.
The deposits form the levees immediately adjacent to the channel.
The channel becomes perched above the floodplain.
Steep slope to the floodplain encourages erosion when the levees are
breached.
Assinaboine River is 8 metres above the surrounding land.
Avulsion: rapid relocation of a meandering river to a new site on the
floodplain.
Time 1. Normal flow conditions.
Time 2. With flooding rapid flow
down the slopes of a levee may
erode a channel that leads out to
the floodplain.
Water from the main river floods
the side of the floodplain
adjacent to the new channel.
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Meandering channels migrate laterally due to the distribution of
erosion and deposition.
Erosion on the outside of a
meander bend.
Time 3. A new channel is cut,
parallel to the original channel.
Deposition on the inside of a
meander bend.
Over time, the new channel takes
more flow from the river while
the old channel is abandoned.
Time 4. The new channel evolves
into a meandering form.
Point bar: depositional body on the inside of a meander bend.
Scroll bars: the remnants of the
tops of ancient point bars.
Flow strength diminishes
along the point bar so that
sediment is deposited.
The point bar “accretes”
in the direction that the
channel is migrating by
erosion of the cutbank.
In ancient meandering river deposits point bars appear as long, low
angle dipping beds of sandstone.
Such deposits are diagnostic of meandering river environments.
From Johnson and Graham (2004); Journal of Sedimentary Research, v. 74, p. 770-785.
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Oxbow lakes form when two cutbanks approach each other.
Ongoing erosion of the cutbanks bring them progressively closer
together.
If the two cutbanks intersect the channel is breached.
Flow in the main channel bypasses the meander and it’s entrance
and exit become plugged with sediment.
The meander becomes isolated from the main channel and forms an
oxbow lake.
Once isolated the abandoned lake diminishes in size, receiving water
only during flooding of the floodplain.
13
Over time the lake fills with sediment and becomes vegetated to
form a swamp.
Once completely filled a meander scar remains.
Oxbow Lake Formation
http://www.mhhe.com/earthsci/geology/mcconnell/streams/channel.htm
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The History of Niagara Falls
Niagara Falls was so well known
among native North Americans
that Jacques Cartier was told
about it 80 years before the first
Europeans visited the site.
The Iroquois name “Onguiaahra”,
meant "the Strait".
The current “natural” discharge
of the river averages 5,760 m3/s
(almost 500 million cubic metres
per day).
(less than 1.5 million toilets)
Ice dams at the head of the river
have reduced discharge to zero.
Winds blowing east across Lake
Erie have increased discharge to
9,760 m3/s.
Today the discharge over the falls is controlled so that some of the
flow is re-routed through turbines to produce electricity
US Hydro capacity: 2,575,000 KW
Canadian capacity: 1,880,000 KW
50% of the discharge of the river
is routed through turbines during
summer daytime hours
Total capacity:
4,455,000 KW
or 4,450 MW
Ontario’s peak consumption: 22,000 MW
75% of the discharge is routed
around the falls during summer
evenings and over the winter
months (only 25% passes over the
falls).
Niagara Falls exists because the
Niagara River passes over the
Niagara Escarpment.
About 12,500 years ago the Falls was
located where the River intersects
the Escarpment.
Through the erosive action of the
water flowing over the Escarpment
the location of the falls receded
southward.
Total US and Canadian
hydro production at
Niagara Falls is 20% of
Ontario’s peak
consumption
The position of the falls has receded to
its present location by steady erosion
that also formed the Niagara Gorge.
Recession of the falls took place at an
average rate of 1.5 m/yr.
The current rate is 0.1 m/yr due to
reduce flow over the falls and other
measures to stabilize its position.
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Prior to the last glaciation of the area
the Niagara River flowed further west
of its current position, passing
through the escarpment at St. Davids.
The falls was a few kilometres
downstream of its current position.
The glaciers receded from the area
12,500 years ago.
Sediment from the glacier plugged the
old gorge and the Niagara River
followed its present course.
The waters fell over the Escarpment
above Lewiston.
The old gorge passed out of the
modern gorge at the Whirlpool.
Initially discharge through the river
was very large due to the glacial
meltwater.
Recession rates of the falls were
relatively rapid due to the large
volumes of water flowing off the
retreating glaciers.
By 10,500 years ago the falls had
receded 2.5 km southward from the
Escarpment.
Recession slowed markedly as
discharge through the river was
rapidly diminished.
Over the next 5,000 years the falls
receded only 1.5 km southward
As the glaciers retreated north of Lake Huron they left the crust
“isostatically depressed”.
The weight of 2 km of ice had pushed the crust
downward so that the land surface was lower than
today.
Waters from Lakes Superior, Michigan and Huron flowed through
the depressed region known as the Nipisssing Spillway.
The Niagara River
only received
discharge from Lake
Erie.
Over the next 5,000 years the falls
receded only 1.5 km southward
… to a position just northeast of the
Whirlpool
Over this 5,000 years the land surface
to the north slowly rebounded and
eventually shut off the Nipissing
Spillway.
Discharge returned to the Niagara
River and recession rates rose to
levels that are comparable to today.
16
Within approximately 1,000 years the
Falls reached the Whirlpool.
Within approximately 1,000 years the
Falls reached the Whirlpool.
Just south of the Whirlpool the preglacial gorge remained plugged with
sediments deposited by the glaciers.
Within a few days to a couple of weeks
the river scoured through the soft
sediment until it reached the location
of the pre-glacial falls.
Subsequent recession of the Falls
through the resistant bedrock was
much slower and continued to today.
The Manitoba Flood of 1997
Spring of 1997 saw unusually rapid melt of a thick snowpack over
much of the Red River Basin.
Eventually the falls will recede back to
Lake Erie.
The lake may drain rapidly and
eventually will only contain an
extension of the St. Claire River.
Under natural conditions this could
take place within 20,000 years.
The Flood Recurrence Curve for Fargo indicated that this was the
200 year flood.
At Fargo the water height was almost 25 feet above low flow.
Floodwaters rose sharply through April, peaking by mid-May.
Discharge at
Fargo, N.D. were
over 30 times
normal flow for
the river.
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At Winnipeg the discharge was 138,000 cubic feet per second.
South of Winnipeg the flood waters spread out over the Red River
Floodplain, forming a shallow lake up to 40 km across.
South of Winnipeg the town of
Morris, Manitoba, remained
relatively dry due to the
construction of dykes.
Elsewhere single homes tried to keep
the water out with sandbag dykes
around their homes.
Dykes around the southern perimeter
inhibited the floodwaters south of
Winnipeg but the river flows through
the city.
The city was saved by the 47 km long
Winnipeg Floodway which
redirected 59,000 cfs around the city.
The diversion lowered the level
of floodwaters by 15 feet in
Winnipeg.
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