estuaries, salt marshes, tidal flats and mangrove swamps
Bays differ in
size, shape, and origin; most importantly, they differ in the behavior
of their water
Bays can be separated
into two general categories: (loosely applied)
- lagoons - quiet
saline bays with no regular freshwater influx; generally protected from
the open ocean by a barrier island, reef or other obstruction that prevents
wave attack and inhibits tidal circulation
- estuaries - turbulent
bays receiving fresh water from rivers and salt water from the ocean
(lagoons will be covered later with barrier islands)
Bay: def. - a concavity
or indentation of the coastline that is smaller than a gulf but larger
than a cove, and that is somewhat protected by open ocean wave energy;
they vary in shape and origin:
- leading edge coasts:
bays are long and narrow; they lie above faults and fractures that allow
sea water to extend inland
- trailing edge coasts:
bays are wide and often cover river valleys that have been submerged by
the rising water from melting glaciers
-
bays in northern latitudes, glaciers have carved deep valleys and water
from melting glaciers have created the scenic elongated bays called fjords
- bays formed as a
result of differential erosion - sandstone carved out by wave action and
held in place by the more resistant granite headlands
Tidal ranges and tidal bores
estuary: def. - an arm of the ocean that is thrust into the mouth and lower course of a river as far as the tide will take it; main sections:
-
head - inland end where the river enters
- main - estuarine
area where fresh water and salt water mix
- mouth - seaward
end at the indentation of the coastline where the ocean enters
Tidal
ranges
estuaries with wide
mouths and narrow heads have a large tidal range due to forcing a given
amount of water into an increasingly narrower part of the estuary; produces
an increase in high tide levels; the ebbing of the same amount of water
results in a similar relative decrease in low tide level
- e.g. Bay of Fundy:
funnel shaped; during flood tide, increases level from 2.4 m at the mouth
to 16.3 m at the landward end - largest tidal range in the world
Tidal bores
an abrupt rise in
the water level at the beginning of flood tide due to the quick reversal
from an ebbing tidal condition to a flooding one; uncommon, forming only
in special circumstances:
- e.g. Truro River
in the Bay of Fundy: bore is 0.5 m high
- e.g. Pororoca River,
branch of the Amazon: bore reaches 5 m high
Estuarine Circulation
The complexity of circulation in an estuary is due to the difference in the two water masses:
-
sea water density is 1.026 g/ml (due to the salinity - total dissolved
solids (tds) of 35 ppt)
- fresh water density
is 1.000 g/ml (STP)
Although the difference in density is small, if there is not a disturbance, the heavier sea water falls below the fresh water
Ways in which the influxes of fresh water and sea water interact in an estuary:
-
stratified estuary: freshwater and saltwater are almost completely separate
- no significant mixing occurs; the flow of each is separate; the incoming
saltwater mass takes the form of a wedge as it proceeds up the slope of
the riverbed; occurs in estuaries with large rivers; e.g. Hudson River
- saltwater wedge can extend 100 km upriver; exist because there are no
physical process such as waves or tidal currents to mix the two waters
- partially mixed
estuary: tidal currents are the dominate factors in circulation; mixing
occurs at the interface between the upper freshwater layer and the lower
saltwater layer; produces a brackish water - 15-20 ppt salinity; form a
gradient from 0 to 35 ppt; e.g. Chesapeake Bay
- fully mixed estuary:
homogeneous profile of water salinities - at any given location, salinity
is the same from the surface to the bottom (water column); conditions:
shallow estuaries (Gulf of Mexico) where wave action extends to bottom
and very strong tidal currents (Delaware Bay and Bay of Fund) or very high
river discharge (Amazon) produce extreme turbulence to mix
types
of estuarine circulation shown by isohalines - lines of equal salinity;
the salinity can vary in as short as a single tidal cycle
some estuaries experience
seasonal shifts: e.g. mixed to a stratified during the summer months when
wave action decreases, or winter storms mix a stratified estuary with increased
wave action
Sediment Deposits
Estuaries are sediment sinks - basins in which large quantities of sediment accumulate; river runoff and incoming tidal currents provide sediment that tends to fill the middle of the estuary; consequently, an estuaries geologic lifetime is short; e.g. port of Hanghou on the Chien-tang estuary had 1 M population 200 years ago - now the estuary has filled in and trade in the region has gone elsewhere; many estuaries have to be dredged periodically to keep them open for shipping
Types of sediment:
-
river-deposited: mixture of sand and mud
- sea deposited: mixture
of sand and marine shell gravel
sand makes up the bed load and mud makes up the suspended load
Types of estuaries based on sediment distribution (energy influence and geomorphology of the bay):
river-dominated
deposition
- weak tidal currents
and small waves caused by sand bars and barrier islands allow rapid deposition
forming a bayhead delta at the river mouth - coastal plain is flat and
the rivers empty abruptly; e.g. Texas coast
- in estuaries with
several rivers sediment is deposited along the banks and shores of the
complex bay; rivers drain hilly terrain and their mouths are long and narrow
- sediment load is deposited gradually; e.g. Chesapeake Bay
tide-dominated deposition
- has no barrier at
its mouth and is funnel shaped - maximize the influence of tides; strong
currents and extreme turbulence totally mix the waters, and the shape of
the bay focuses the flood tide waves; combination of large tidal range
with strong tidal currents results in a sand-dominated estuary floor -
the mud is either swept out to sea or is trapped at the landward limits
of the estuary; e.g. Bay of Fundy
- time-velocity curve:
analyzing the circulation and its sediment transport; asymmetrical
- move great volumes
of sediment back and forth over the estuary floor during each tidal cycle;
sand can picked up by currents flowing faster than 15-40 cm/s; ripples,
sand waves, and dunes appear on the estuary floor
other
biogenic material
deposition - shells and hard body parts from aquatic organisms are added
to the estuarine sediment; filter feeders, such as oysters, worms, and
many bivalves contribute - ingest suspended sediment, extract food particles,
and excrete it all as mud pellets that are too large and cohesive to be
further transported; remove and rework the fine suspended sediment that
might otherwise go out to sea - increase the rate of sediment accumulation
Tidal Flats
tidal flats: def. areas of mud and sand that are exposed at low tide and flooded at high tide; more than half of the coasts of most estuaries are rimmed by tidal flats; extent is determined by the shape of the estuary and tidal range
tidal flat sediment is composed mostly of mud and fine-grained sand and the shells of small aquatic animals; coarser grains settle out in the tidal channels
particles of sediment
transported onto the tidal flats follow a predictable path during a tidal
cycle: (model created by scientists' studies of sediment transport)
- sediment particle
is picked up flood tide reaches high enough velocity to lift sediment grain
(cohesiveness)
- tidal current carries
the particle higher onto the tidal flat
- particle begins
to drop when the tidal velocity decreases below the value required to lift
it
- the particle settles
and is deposited further up the tidal flat when the current continues to
decrease - settling lag (particle always moves beyond the place where it
begins to drop)
- at ebb tide, the
particle is picked up when the current attains the same critical velocity
(a greater threshold velocity than when the particle dropped and by a different
mass of water)
- particle begins
to settle the same way as the ebb tide reaches the same current velocity,
but further up on the tidal flat
- the particle comes to rest at a site further landward than that from which it began due to:
-
because of the settling lag bringing the particle closer to shore, a large
part of the ebb tide will have passed over the particle before the water
reaches the required velocity to pick it up; therefore, there is not as
much time left for the particle to be carried seaward - called scour lag;
settling lag and scour lag accounts for part of the net movement of sediment
particles up a tidal flat
- also, the ebb tide's
velocity is not as high and does not remain at the required transport velocity
as long
Distribution
of particles is controlled by:
- size of particles
- larger particles are transported shorter distances
- time particles are
moved by the tidal currents
- results in a regular decrease in the grain size up the slope of the tidal flat; low tide level is marked by the coarse grains (sand) and high level is marked by the finer grains (mud)
Tidal
bedding: def. - deposited thin, regular layers of sand and mud (few mm
to 1 cm thick)
- determined by rapid
currents, slack high and low tides, spring tides, neap tides, storm tides
- all leave their specific record of sediment accumulation (strategraphic
geological record)
- disrupt the tidal
signature by waves (if wave action is dominate) and aquatic organisms
-
wave action can cause its own bedding
- aquatic organisms
burrow into the tidal flats for protection and food (same creatures that
filter the suspended sediment from tidal waters); organisms avoid the surface
of tidal flats where they can dry out; clams, worms, and a variety of crustaceans
turn over the sediment - bioturbation; can be very plentiful and common
in low-energy estuaries; filter feeder organisms avoid turbulent water
where they could have their siphons clogged by the abundant suspended sediment;
organisms cannot live in an area where a strong tidal action sweeps the
sediment back and forth depriving the organisms of a stable substrate to
burrow and maintain an existence
Salt Marshes
salt marsh: def. - on fringes of estuaries, lagoons, and other bays where sediments are sheltered from wave action, are above the level of neap high tide, and where vegetation eventually takes hold; upper limits coincide with the upper limits of spring high tide - highest level of regular inundation and sediment supply
- an extensive marsh is a sign of a natural estuary that has largely filled with sediment; e.g. State of Georgia coast
Trapping
sediment
the velocity of tidal
current drops drastically between neap and spring high tides - fine sediment
settles without being disturbed by energetic waves; salt tolerant grasses
take hold first which further slow tidal current and traps more fine sediment;
once a stand of grass becomes established, it becomes denser further trapping
sediment
-
important contributors to coastal sediment accumulation
- important as sediment
stabilizers
Marsh grasses
- cordgrass - most
common
- needle rush - located
landward of cordgrass
- reed - where water
becomes freshwater
the development of a salt marsh can be characterized by cordgrass and needle rush
-
young marshes: dominated by cordgrass with needle rush on fringe; many
tidal creeks trapping more sediment
- intermediate marshes:
half and half
- mature marshes:
sufficient sediment collects to support more needle rush; few tidal creeks;
land plants encroach on the outer edges
- more deposition
finally raises marsh above the mean high tide line and terrestrial vegetation
takes over
- in Europe, this process has been hastened: convert marshes to farmland by draining them through a system of dams, dikes, and canals; now the sediment has dried, compacted, and is sinking - need higher dikes for protection; the Netherlands has been trying to prevent further subsidence by flooding selected areas of farmland and returning them to marsh
- marsh development and eventual filling of the estuary has been offset by eroding marshes due to the rising sea level (global warming)
marsh environment is similar to a river and delta floodplain: bordered by natural levees, meandering channels and oxbow lakes
sediment is delivered in two ways:
-
slow flooding by tidal currents
- storm tides that
push large amounts of sediment far into the salt marsh in a short time
Mangrove Swamps
mangroves: def. - woody trees of various species having thick tangles of shrub and tree roots invading the intertidal zones of estuaries in tropical and subtropical climates; grow from 2-8 m high
- cannot tolerate frost
- can tolerate both
salt and fresh water
- grow slowly - may
be out competed by salt grasses
thickets of roots provide
a sheltered habitat; barnacles and oysters encrust the roots and branches;
fish, snails and snakes find protection, nesting sites, and food
Example estuaries - from low to high-energy coastal environment
San Antonio Bay
- Gulf of Mexico
- marginal sea coast
- Matagorda Island
- Guadalupe River
- single largest sediment load into the estuary
- shallow estuary
- 3 m deep; sediment is mostly mud and sandy mud
- tidal range - less
than 0.7 m
- fetch of at least
12 km in any direction allows modest wind-driven waves to provide more
of the energy and to cause complete mixing of the fresh- and saltwater
masses
- the wave energy
and tidal influence is too weak to redistribute the sediment; bay head
delta has developed
- infrequent hurricanes
have influenced sediment distribution by bringing in coarse shell debris
as storm layers on the estuarine stratigraphy
- bay supports a moderate
oyster industry; oyster reefs are perpendicular to tidal currents for maximum
exposure to tidal flow that brings suspended food and nutrients; their
shells and fecal matter are major contributors to the sediment; other estuarine
organisms (worms and bivalves) contribute to a thoroughly bioturbated substrate
- narrow intertidal
environment supports a modest, discontinuous fringe of marsh containing
mostly cordgrass
- analogous to a huge
mud puddle
Chesapeake Bay -
Central Atlantic Coast
- stable trailing
edge of the Atlantic coastal plain
- elongated, complicated
shape is a series of flooded river valleys
- very high energy
mouth has strong tidal currents and shifting sand shoals
- about 300 km; with
the Susquehanna, Potomac, Rapahannock, York, and James Rivers are major
river emptying into the estuary
- tidal range varies
from 2 m at the mouth to 0 m at the bayhead
- wave energy dependent
on wind direction (varying fetches)
- partially mixed
circulation - salinity
- estuary floor resembles
the original flooded valleys with considerable sediment filling in low
areas
- nooks and small
embayments of the eastern shoreline support extensive marshes
- sediment carried
by the rivers is relatively coarse and accumulates close to their mouths;
open part of the estuary has mud
- once had thriving
fish and shell populations - estuary floor has now become overloaded with
sewage, toxic wastes, and heavy metals from industries and cities along
the rivers
Willapa Bay - North
Pacific Coast
- leading edge boundary
of the Washington State coastline
- streams rush down
the near mountainsides directly to the narrow coast and terminate in a
series of small estuaries; modest freshwater input
- deep and high-energy
inlet protected by a large barrier spit
- tidal range is 4
m during spring tides and carries a large sediment load; mud accumulates
in the deeper parts of the basin; tidal channels with large sandy bedforms
are numerous; sand dominates the mouth
- extensive tidal
flat exposes 1/3 of the bay during low tide
- large size (50 km
in long direction) allow modest size waves; waves and large tidal flux
mix the fresh and saltwaters; salinities are high because the tides are
dominate
- much of the bay
is bordered by cliffs; marshes are restricted to those small coves where
the tides have deposited seiment
- oysters thrive
Bay of Fundy - North
Atlantic Coast
- funnel-shaped estuary
cuts inland for 150 km
- highest tidal range;
total mixing; tides are dominant
- steep cliffs line
the estuary
- numerous tree stumps
along the intertidal flats show the area was once terrestrial upland
- tidal range has
been increasing by up to 30 cm each century - due to rebound (isostatic
uplift); the head has uplifted and has forced the same amount of water
into a smaller area
- floor is composed
of coarse sediment and gravel mixed with sand and mud; tides that rush
up the bay form a distinct pattern of decreasing grain size toward the
head of the bay; mud dominates the river mouths
- lack of continuous
marsh growth is due to the absence of flat land combined with the tide-driven
mobility of the sediment substrate
- rolling of the sediment
during tidal movements creates a difficult environment for benthic organisms;
intertidal region is devoid of burrowing creatures and shellfish; the few
inhabitants are mobile creatures that can move away from the tides and
withstand the effects of a mobile sediment