Water Movements in Lakes

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Water Movements in Lakes
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What causes water movements in lakes? Tides Meas
urable in large lakes, but small. e.g., 20 cm
max in Lake Superior Earthquakes Tsunamis can
cause major devastation in the ocean. Volcanic
explosions Not usually important in
lakes. Influx of water Important in lakes with
short hydraulic residence time --
reservoirs. Wind
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Influx of water Important in lakes with short
hydraulic residence time -- reservoirs
River water entering a reservoir will sink to its
density Overflow -- river water less dense than
epilimnetic water. E.g., freshwater entering a
saline lake.
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Overflow in a reservoir
warm
cold
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Underflow -- river water more dense than
hypolimnetic water. E.g., salt water entering a
freshwater lake or cold river water entering a
warm lake.
cold
warm
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Interflow -- river water has an intermediate
density due to temperature, salinity, or sediment.
Hypolimnorium
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What would a light extinction curve look like in
a reservoir like this?
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(No Transcript)
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In large lakes, currents can be deflected by the
Coriolis force.
N
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Wind Progressive waves Langmuir
circulation Seiches surface seiche internal
seiche
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Progressive waves Height depends on the strength
of the wind and length of fetch Transfer of
momentum with little downwind water
movement Major effect is where they hit the
shore
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Breaking waves
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The downwind movement of water occurs in a helix,
called Langmuir circulation.
Wind
Waves
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Langmuir circulation
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Flotsam will accumulate in the surface
convergence zones, creating streaks on the
surface
convergence zone
convergence zone
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Langmuir circulation is a major mechanism of
surface mixing. Depending on the strength of the
wind, mixing can occur several meters down.
Convergence zones
End-on view
Yellow Bay
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Seiche -- an oscillation of the entire water
body Surface seiche
Wind
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Wind
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Surface seiche
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Period of surface seiche depends on length and
depth of lake (Table 7-2).
90.0

55.8

O
ntario, USA
--
Canada

311

86.0

304.0

Huron, USA
--
Canada

444

76.0

400.0

Erie, USA
--
Canada

400

21.0

786.0


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Height of surface seiche is usually quite small,
a few mms, but can be higher. Seiche in Lake
Erie of 2 m with a period of 14 h.
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Internal seiche -- wind piles up the epilimnion
on one end of the lake. When the wind stops, get
a rocking of the thermocline
Wind
thermocline
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Internal seiche
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Period of internal seiche (Table 7-3).
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The height of an internal seiche is generally
much more than that of surface seiches
Internal seiches 1 m in Lake Mendota gt10 m common
in Loch Ness and Lake Michigan
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Complications Circular movement of seiches in
very large lakes -- due to Coriolis
force Generation of internal progressive waves
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Internal progressive wave
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Would you hold that meter still! The temperature
keeps changing.
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Why are waves important? MIXING