Transcript Estuaries - New Jersey City University [NJCU]

Estuaries
Where the Rivers meet the Sea
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Definition
Types
Circulation
Tides
Peritidal/ Estuarine system
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Marsh
Tidal Channels
Tidal Deltas
Mudflats/ Tidal flats
• Biology
• “Wetlands”
Estuaries
• Where the Rivers meet the Sea
• Semi- enclosed body of water where there
is a mix of river and seawater, and mix of
fluvial and marine processes
• Mouths of many rivers flooded at last sea
level high (last interglacial)
• Boundary between fluvial and marine
processes
– Influenced by tides
Types of Estuaries
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Drowned river valley
Fjord
Bar- built
Tectonic
Tectonic
San Francisco Bay
Fjord
Chesapeake Bay: Drowned
Shoreline
ESTUARY
The Netherlands
Barrier Island
Value of Estuaries
• Greatly influenced by river discharge and tidal
mixing.
– Environmental conditions fluctuate widely.
– Biomass is high because estuaries have high nutrient
levels.
– Species diversity is low because fewer species can
cope with the wide fluctuations in temperature, salinity
etc.
– “nursery grounds”
• young stages develop in the estuary
– (abundant food; low predation
– may move offshore to adult habitats
Estuaries
• greatly influenced by river discharge and tidal
mixing.
– Environmental conditions fluctuate widely.
– Biomass is high because estuaries have high nutrient
levels.
– Species diversity is low because fewer species can
cope with the wide fluctuations in temperature, salinity
etc.
– “nursery grounds”
• young stages develop in the estuary
– (abundant food; low predation
– may move offshore to adult habitats
• TIDES
– tidal bulge results from moon and sun
• neap (sun and moon effects cancel)
• spring (compound)
– diurnal; semidiurnal; mixed semidiurnal
W. W. Norton
• Tidal range
– tides are giant shallow water wave
– at coasts, funnel into bays and other
indentations
– water piles up in enclosed areas
• microtidal- up to 2 m
• mesotidal 2 – 4 m
• macrotidal > 4 m
W. W. Norton
• tidal current
– generated by horizontal movement of water
• most pronounced in mesotidal
• carries great volume in macrotidal
– flood tides
• Incoming
• moves water onshore between low and high
– ebb tides
• Outgoing
• moves water offshore
– Slack
• top and bottom of cycle (when high is in, or low is in)
– bidirectional flow
• leads to bidirectional cross- stratification with mud drapes
– herring bone, mud drape, reactivation surface
Basic Circulation
• Basic plumbing
– water flows in at high tide- fills up
– flows out at low! - empties
– (Variations on the theme)
» may be completely empty or may be wet throughout
cycle
• Harsh environment- desiccation, daily
changes in temperature and salinity
Classification
• One way of quantifying is by comparing
the volume R of freshwater that enters
from the river during one tidal period, with
the volume V of water brought into the
estuary by the tide and removed over each
tidal cycle.
• R is sometimes called the river volume,
while V is known as the tidal volume.
Classification
• Estuaries can be grouped into classes,
according to their circulation properties
and the associated steady state salinity
distribution.
The most important estuary types are
1. salt wedge estuary (R>>V)
2. highly stratified estuary (R>V)
3. slightly stratified estuary (R<V)
4. vertically mixed estuary (R<<V)
5. inverse or reverse estuary (R=0)
Salt Wedge
• River volume R is very much larger than the
tidal volume V, or there are no tides at all.
• The fresh water flows out over the sea water
in a thin layer.
• All mixing is restricted to the thin transition
layer between the fresh water at the top and
the "wedge" of salt water underneath.
• Vertical salinity profiles therefore show zero
salinity at the surface and oceanic salinity
near the bottom all along the estuary.
• The depth of the interface decreases slowly
as the outer end of the estuary is approached
• Mississippi and Congo Rivers
Salt Wedge
Highly Stratified Estuary
• River volume R is comparable to but still larger than
tidal volume V.
• Strong velocity shear at the interface produces internal wave
motion at the transition between the two layers.
• The waves break and "topple over" in the upper layer, causing
entrainment of salt water upward.
• Entrainment is a one-way process, so no fresh water is mixed
downward.
• This results in a salinity increase for the upper layer, while the
salinity in the lower layer remains unchanged, provided the
lower layer volume is significantly larger than the river volume R
and can sustain an unlimited supply of salt water
• Examples of this type of estuary are fjords, which are usually
very deep and have a large salt water reservoir below the upper
layer.
Fjord-Type Estuary
Slightly Stratified Estuary
• River volume R is small compared to tidal
volume V.
• The tidal flow is turbulent through the entire
water column (the turbulence induced mainly at
the bottom).
• As a result, salt water is stirred into the upper
layer and fresh water into the lower layer.
• Salinity therefore changes along the estuary
axis not only in the upper layer but in both
layers
• This type of estuary is widespread in temperate
and subtropical climates
Vertically Mixed Estuary
• River volume R is insignificant compared with tidal
volume V.
• Tidal mixing dominates the entire estuary.
• Locally it achieves complete mixing of the water
column between surface and bottom, erasing all
vertical stratification.
• As a result, vertical salinity profiles show uniform
salinity but a salinity increase from station to station as
the outer end of the estuary is approached
• This type of estuary is found in regions of particularly
strong tides; an example is the River Severn in
England.
Well Mixed
Slightly Mixed
Inverse or Reverse Estuaries
• These estuaries have no fresh water input from
rivers and are in a region of high evaporation.
• Surface salinity does not decrease from the
ocean to the inner estuary, but water loss from
evaporation leads to a salinity increase as the
inner end of the estuary is approached
• This results in a density increase and sinking of
high salinity water at the inner end.
• As a result, movement of water is directed
inward at the surface and towards the sea at
the bottom, with sinking at the inward end.
Components of the Peritidal
System
• Salt marsh
1. High marsh
2. Low marsh
• Tidal channels
• Tidal deltas
– Ebb
– Flood
• Tidal mudflats
Salt Marsh
Salt marshes are vegetated inter-tidal flats.
Salt Marsh
• Marsh divided into high and low marsh
– high is region above high tide
– low is region that’s flooded daily
– each has distinctive vegetations and grain size
• high has coarser sediments
– Is a much more terrestrial-like environment because it is
flooded only at times of extreme high tide.
– Is characterized by a more varied plant community. Spartina
patens
• low has finer sediments
– Is dominated by the smooth cordgrass, Spartina alterniflora.
– cut by channels which funnel water in and out
– may completely empty or may be wet throughout
cycle
Salt Marsh
Salt marsh profile.
Salt Marsh
• cut by tidal channels
– funnel water in and out
– daily influx of water brings muds
• build up mud flats
• Tidal channels
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funnels water into single or multiple channels
currents can scour base
also meander and can cause walls to fail
lag of gravelly deposits (including shell material
and mud clasts
• armored mud balls
– form point bars similar to those in rivers
• BUT have fine material on top of sloping bar surface
• alternations of sand and mud on bar deposits
– tidal channel has lag;
• channels generally cut down a few meters at least
channel abandoned and finer sediments infill
• Tidal flats
– form in proximal areas and between channels
• Tidal mudflats
– away from area of strong tidal currents
– flooded at high; exposed at low
– fine sediments are carried in on high tide and
deposited during slack and as it retreats
– commonly bioturbated
– surface network of channels partially channelize
flow
– allows vegetation to flourish
• PEAT: thick accumulations of dead vegetation
– organisms eat decaying vegetation
– low marsh characterized by this process of daily alternating
currents
– higher areas of marsh are site of accumulation of dead
marsh grass (wrack)
Cores through tidal flat deposits of the Devonian Swan
Hills Formation, Alberta, Canada
Dr. Stacy Atchley, Baylor Univ.
Tidal Flat Colonization—Paleontology and sedimentology of a Late
Cambrian shoreline, where soft-bodied carcasses are stranded and
early crawling organisms left tracks and trails. Dr. Whitey Hagadorn , Amherst Coll
Tidal Deltas
• Ebb
• Flood
Tidal Mudflats
• Flooded at high; exposed at low
• Fine sediments are carried in on high tide and deposited
during slack and as it retreats
– commonly bioturbated
• fiddler crabs, etc
• Surface network of channels partially channelize flow
– allows vegetation to flourish
• PEAT: thick accumulations of dead vegetation
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(organisms eat decaying vegitation)
Marine and transitional marine to tidal flat
Parallel bedded
sandstone and mudstone.
Note flame structures
near base and load casts
near top of core Shallow
subtidal to intertidal
Massive
sandstone
with vertical
burrows
Lenticular and
flaser bedding:
sandstone and
mudstone. Typical
of tidal flats
Biology of Salt Marsh
– Fiddler crabs, snails, mussels, birds and
fish
• The frequency of tidal flooding plays a major
role in determining the activity patterns of these
animals.
– For example, fiddler crabs seek refuge in burrows
during flood tide to escape predators.
– At low tide they leave their burrows and search for
food.
Productivity of Salt Marshes
• The most productive portion of the salt marsh is
the low salt marsh region
– inhabited by Spartina alterniflora.
– little is grazed directly by animals
– most of the production enters the detritus food chain.
• material not consumed by detritivores accumulates in the
sediment
– forms peat or is exported from the marsh in tidal currents
• root systems of salt marsh plants bind the
sediments
– stabilize the substrate
– Reduces erosion of the coastal environment
• Estuarine successions
– if subsidence provides space, sediments can
accumulate (otherwise reworked)
• great LATERAL variability
• Recognizing estuaries
– Looks a lot like deltas!
• BUT delta is progradational
• estuaries are aggradational
– build up within drowned river channel
– base of succession is usually an erosional
surface
Wetlands
• flank estuaries
• since estuaries are natural harbors
and often PORTS
wetlands are often polluted
NYC
Harold Connolly, 2002
NYC
Harold Connolly, 2002