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Ocean
Estuary
Land
Estuary
Estuaries can form along the shoreline in several ways. When sea
level was lower, rivers and glaciers (huge blocks of ice that flow
down from the mountains) carved out valleys at the location of the
present shoreline. When sea level rose, the ocean flooded the
valleys, and they became estuaries. In other places, sand piles up
offshore, becoming a barrier island (or barrier bar) which partially
cuts off a body of water from the ocean. Also, earthquakes can
raise the ocean floor or lower the land beneath sea level (30 feet or
more in some cases!) to create an estuary. This is how the
southern end of the San Francisco Bay formed.
ar
tu
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Estuaries are places where the ocean is partially surrounded by the
land. They go by many names: bay, sound, lagoon, and so on.
The mouth of a river, harbors, and marinas are all examples of
estuaries as well.
Barrier Islands
Estuaries
Barrier Islands
Shoreline Features
Estuary
Oceanography 10, T. James Noyes, El Camino College
r
Rive
Unlike the open ocean, tides play an important role in creating
currents in many estuaries. As sea level rises towards high tide in
Delta
the ocean, water flows into the estuary, and when it sinks toward
low tide, water flows out of the estuary. In estuaries that are large
and have narrow or shallow connections with the ocean, it can
take a long time for the water to flow in or out. The greater the size of the estuary and the
smaller connection with the ocean, the faster the water must move into or out of estuary.
Tidal currents can be quite treacherous and an important factor in causing mixing in an estuary.
Estuaries
National Park Service
Courtesy of
Doc Searls
(CC-BY-SA 3.0)
Earth Sciences and Image
Analysis Laboratory,
NASA Johnson Space Center
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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Ocean life is especially abundant in most estuaries. Estuaries get a lot of freshwater runoff from
the surrounding land (particularly if a river flows into them). The fresh water itself is bad for
most marine organisms, but most estuary animals and algae have adaptations that help them
survive in fresh water. The real benefit of the freshwater runoff is that it carries nutrients from
the soil into the ocean. Because land surrounds the estuary, the estuary gets extra nutrients (more
nearby land) and the nutrients tend to be “trapped” (they do not drift out to sea as easily). This
leads to abundant algae, both macroalgae (seaweeds) and phytoplankton, which in turn are food
for animals. Estuaries are often called “nurseries” for ocean animals, because many ocean
animals go to estuaries to breed or lay their eggs. The juvenile (baby) animals can find abundant
food and hiding places (e.g., among the seaweed or nearby wetlands, and in the mud on the
bottom). Also, many large ocean predators do not like shallow water or are not well adapted to
tolerate a mix of fresh and salt water, unlike the juveniles (who may lose this ability as adults).
Estuaries are valuable to people, because they provide food (remember: many ocean animals need
estuaries to reproduce effectively, even if they don’t live there). In addition, they are important
for trade, so much so that we actually build estuaries. For example, over $200 billion in goods
pass through the ports of Los Angeles and Long Beach each year, and such trade is linked to over
800,000 local jobs worth $39 billion in wages. These ports are the busiest in the United States
(8th busiest in the world); over 40% of all the goods that we import come through these ports.
Estuaries are also valued for recreation (e.g., sailing, swimming) and by some industries (e.g.,
“fish farming,” power plants use the water too cool down their turbines). Estuaries can be loved
to death by humans, who often pollute them intentionally (e.g., sewage pipes, industrial
chemicals) or unintentionally (e.g., storm drains, toxic chemicals leaking from boat paint).
USGS
Wetlands (and Mudflats)
Wetlands are marshy or swampy areas that are out of the water part of the time and covered by
water part of the time (hence the name “wetlands”). In coastal areas, wetlands are covered by
water about half the time (e.g., at high tide), though freshwater runoff can play a role as well.
Wetlands are often found along the borders (edges) of estuaries.
There are two kinds of coastal wetlands, salt marshes and mangrove forests. Both are covered by
special, salt-water-tolerant plants (not algae!), but salt marshes are dominated by grasses (reeds)
while mangrove forests have mangrove trees. Some plants have thick outer layers to keep salt
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
water from getting into them and
fresh water from leaking out.
Other plants excrete salt crystals,
letting rain wash them away, or
concentrate the salts in their
leaves and drop the leaves to keep
the salt from getting back into
their bodies. In addition, these
plants have to be able to survive
in low-oxygen conditions. This
may seem strange, given that
plants can make oxygen, but
plants do not make oxygen at
night (they need the oxygen to
“burn” the food that they made
during the day). Plants can extract
oxygen using their roots, but
wetland plants’ roots are buried in
thick mud with little oxygen in it.
Some plants have wild root
systems, the most famous being
those of mangroves, that actually
stick out of the water so that the
plant can “breathe.” The lack of
oxygen is related to the “stinky”
smell of some wetlands: the smell
is sulfur in gases given off by
special bacteria living in the mud
who can survive without oxygen
(in fact, oxygen is bad for them,
which is why they like the mud).
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Saltmarsh
Mangrove Forest
Like estuaries, life is especially abundant in wetlands. In fact, more photosynthesis takes place
per square foot in wetlands than in any other ocean environment. In other words, more food is
made in wetlands than anywhere else in the ocean. (This is what scientists mean when they say
that wetlands are the most “productive” places in the ocean.) Like estuaries, there is more
photosynthesis happening, because of lots of nutrients being washed off the land. Wetland plants
are good at absorbing the nutrients before they enter the ocean.
Ironically, very few animals actually eat the living plants in wetlands. (Presumably because of
the thick outer layers that keep out salt and keep in the oxygen.) Most animals at the bottom of
the food chain wait for the plants to die, and then eat the dead plant matter after it has been
partially decomposed by bacteria (or eat the bacteria!). Like estuaries, wetlands also serve as
nurseries for ocean animals, providing food and hiding places (e.g., in the small places between
the plants). Many animals hide beneath the mud of the wetlands or the neighboring mudflats,
places where the muddy bottom of the estuaries is exposed during low tides.
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
Some animals (e.g., worms) devour
dead material than has fallen into the
mud (“deposit feeding”). Other
burrow up to the surface when water
covers the mudflat and strain
phytoplankton and zooplankton out of
the water (“suspension” or “filter
feeding”). Birds in particular love
wetlands for the abundant food in the
water and mud, and protection from
land predators which cannot swim
(e.g., cats).
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Fish hiding in
mangrove roots.
Aside from a food resource (remember: many ocean animals need wetlands to reproduce
effectively, even if they don’t live there) wetlands provide many other benefits, and should not
only be thought of as smelly, insect breeding grounds. The wetlands slow down the water
flowing from the land into the ocean, allowing small sediments to settle which builds up new
land by the coast. In addition, the roots of wetland plants hold onto the sediments, helping
reduce coastal erosion. Wetlands are also good at absorbing water, keeping it from flooding our
homes and businesses when heavy rains or storm surge (sea level rises during storms) cause
coastal flooding 1. Moreover, wetlands absorb excess nutrients in freshwater runoff from
fertilizers and sewage, keeping them from entering the ocean where they can lead to blooms of
toxic algae, disease-causing bacteria, or “dead zones” (bacteria remove all the oxygen from the
water when decomposing dead algae, so the animals cannot breathe). In addition, some toxic
chemicals biodegrade faster in the low-oxygen soils of wetlands. It is far better for these
chemicals to be in the wetland sediments than in ocean water, where they are far more likely to
get into the food chain. Thus, wetlands serve as important filters, keeping pollutants from
entering the ocean. In fact, some cities have built wetlands to serve as “treatment facilities” for
wastewater (in some cases, after destroying the natural ones = big waste of $). Man-made
wetlands have a mixed track record and most are not as beneficial as natural wetlands.
We have destroyed about 50% of our wetlands in the United States. The Los Angeles area once
had extensive wetlands. For example, the entire area of the ports of Los Angeles and Long
Beach used to be covered by wetlands. These wetlands extended all the way to Madrona Marsh
by the Del Amo Mall. (Imagine. There was a time when you could paddle a canoe all the way
from San Pedro to Torrance!) In California, only 10% of our wetlands remain. Developers can
still destroy wetlands in California, but the laws of the state now make them perform a
“mitigation project” to reduce the damage:
A member of the George W. Bush administration
they must build a man-made wetland or
caught some flak for claiming that under their watch,
help extend a natural one to compensate.
the amount of land covered by wetlands had gone up.
She was accused of being disingenuous, because she
was including wastewater treatment facilities, many of
which are significantly different from natural wetlands.
1
New Orleans would be better protected from hurricanes if government officials had not encouraged economic
development policies that harmed the neighboring wetlands.
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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NASA
Deltas
A delta develops when sediments pile up at the end
of a river because waves are unable to carry all of the
sediments away down the coast (longshore transport of
the sediments). Big deltas typically form at the ends of
rivers that carry a lot of sediments (i.e., big rivers) in
places where waves are small.
As more and more sediments pile up, they can block the
end of the river, causing it to split into smaller rivers
called “distributaries.” The distributaries spread out, each
finding its own way through the sediments and down into
the sea, sometimes giving the delta a triangular-shape. (The Greek letter “delta” has the shape of
a triangle.) If enough sediments block the river, the river itself can actually shift its course.
One famous example of a delta is the Nile Delta in Egypt. When you think of Egypt, you
probably think of pyramids and a barren place covered by sand, but the delta is a green place
with lush vegetation. Many great ancient civilizations (e.g., Egypt, Mesopotamia, China, India)
started in deltas because of the rich farmland.
Many people still live in deltas, but unlike ancient peoples, we have the technology to keep the
delta from changing. For example, we remove sediments from the bottom of the river and
distributaries (“dredge” them) to make it safe for ships to travel without running into the bottom,
and build levees (big piles of sediment) to keep our homes and business from being flooded by
the river or distributaries. In California, we have a delta at the north end of San Francisco Bay,
the Sacramento-San Joaquin Delta. Levees broke there not long ago (June 2004), flooding
people’s home and fields. Governor Schwarzenegger asked for federal emergency relief funds
(rather than using California’s tax dollars) to help shore up the delta’s levees, an interesting
strategy since typically these funds are used to clean up after a natural disaster, not prevent one.
(It would certainly cost less to prevent the problem then to clean it up.)
Natural deltas are places where land is growing along
Think of the sediments of the delta as
a shoreline, because new sediments are continually
a sponge filled with water. If you put
brought by the river and its distributaries. If the river is
an object on the sponge, the sponge
dredged and controlled by man-made levees so that it
sinks and the water is squeezed out.
cannot change course periodically and dump sediments
all along the coast, then most of the sediments never reach the coast or are funneled out into the
ocean. This allows waves to begin to erode the land of the delta, and the land of the delta sinks:
water in the deeper sediments is slowly squeezed out by the weight of the sediments above, and
the sediments above sink down and fill in the resulting space. A good example of land sinking in
a delta is the city of New Orleans which is located in the Mississippi delta: New Orleans is now
about 8 feet below sea level, on average (in places, it is over 20 feet below sea level). The levees
are the only thing
keeping the river and
the ocean out, as we
are all aware of due
Levee
to Hurricane Katrina.
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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Barrier Islands
Barrier islands are long, thin
piles of sand that run along
(“parallel to”) the coast. The
water trapped between the
island and the coast is an
estuary (lagoon) and wetlands
grow along its edges. The side
facing the ocean endures the
pounding of ocean waves, thus
protecting the mainland from
wave erosion (and flooding by
storm surge, the rise in sea
level caused by storms).
Because barrier islands are
essentially large piles of sand,
they shift as waves push sand
from one place to another. In
addition, large storm waves
smashing into the islands can
push barrier islands back,
toward the land. This is called
barrier island “migration” and
has been well documented.
Barrier Island
Barrier islands are common along the eastern and southern (Gulf) coasts of the United States,
apparently due to their shallower, flatter continental shelves (easier for piled-up sand to reach the
surface) and more sediments leftover from previous ice ages (when sea levels where lower, sand
traveled farther out). Several processes have been suggested to explain the origin of barrier
islands. One is that they form like a summertime beach: sand from the ocean floor is pushed
towards land by waves and piles up. (This has been observed.) Another is that rising sea-level
flooded the coast, leaving only the dunes on the high berm of the original shoreline’s beach
above water. (Not seen yet, but sea level has not risen that much.) The last suggestion is that a
beach literally leaves the coast at a “bend” in the coastline. (This has actually been observed, but
does not explain the origin of most barrier islands.)
Barrier Islands along the Gulf (southern) coast of the United States. NASA image created by Jesse Allen,
Earth Observatory, using data provided courtesy of Laura Rocchio, NASA Landsat Project Science Office.
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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Headlands and Coves, Sea-Arches and Sea Stacks
Headlands are places where the land extends
out into the ocean (small peninsulas). Coves
are the places between headlands, where the
ocean pushes into the land. When waves
erode a shoreline, they encounter some
rocks that are more resistant to wave erosion
than others. The places where the rock is
less resistant (“softer”) erode faster and gets pushed back faster
(the cove), leaving the wave-resistant rock sticking out into the
ocean (the headlands). Along the coast of California, igneous
rock – cooled lava – tends to resist erosion better than
sedimentary rock – rock made
black = wave-resistant rock
of stuck-together sediments.
At First
#2
Coves
Headlands
Ocean
Sooner or later the waveresistant rock will
completely erode away.
It just erodes slower than
the less-resistant rock.
#4
#3
Land
#5
Land
Wave Crests
Ocean
Headland
Headland
Cove
Courtesy
of Forest
& Kim Starr
(CC-BY-3.0)
Courtesy of Tatters
(CC-BY-3.0)
Headland
Cove
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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Often a small amount of wave-resistant (“harder”) rock protects “softer” rock behind it. As the
shoreline on either side retreats more and more, the waves begin to refract, attacking the “softer”
rock behind the end of the headland and eroding it. Eventually, the wave-resistant rock is left
standing alone: it has become a sea stack.
Sometimes the waves erode all the way through the less-resistant rock behind the end of the
headland, but the end of the headland and the mainland support the rock above. This feature is
called a sea arch. The space beneath the arch continues to widen
Note: Sometimes students
until the rock on either side cannot support the weight of the arch
tell me that the rocks of
and the arch collapses. The fallen rock is made of the less-resistant the fallen arch pile up,
rock, so it is ground up into sediments by the waves and carried
forming a “sea stack.”
away. All that remains is the sea stack (the wave-resistant rock).
This is incorrect.
Headland
Sea Stacks. NOAA
Sea Arch
Sea Stack
Sea Arches. Courtesy of John Allan (CC-BY-SA-2.0).
Sea Stacks. NOAA
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
Wave-Cut and Marine Terraces
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A terrace is a flat area, sometimes elevated like a balcony.
When waves erode a shoreline, they push back
the cliffs, leaving behind a broad, flat area called
a “wave-cut terrace” (also known a wave-cut
bench or platform). The waves push forward,
not down, so the shoreline retreats but the ocean
does not get deeper. If sea-level falls (e.g.,
during an ice age) or an earthquake lifts the land
upward, then the flat area is no longer underwater and we call it a marine terrace. As you can
see in the photograph below, Palos Verdes has
several marine terraces. People often say that a
series of marine terraces resembles the steps of a
staircase.
Before Uplift
Wave-Cut Terrace
Marine Terrace
After Uplift
After Uplift & More Erosion
Marine Terrace
Wave-Cut Terrace
Wave-Cut Terrace.
Marine Terraces.
Marine Terraces.
Oceanography 10, T. James Noyes, El Camino College
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Oceanography 10, T. James Noyes, El Camino College
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Layers in the Cliffs
Look closely at our coastal cliffs (where they are not covered up by vegetation or erosion), and
you will see that they are composed of layers (“strata”) of rock. This is one indication that the
rocks are made of sedimentary rock: sediments which have become stuck together to form solid
rock.
Sedimentary rock usually forms in valleys on land or on the bottom of the ocean. Sand and mud
washed off the land by rain and tiny plankton shells build up on the ocean floor. Different kinds
of sediments pile up at different times, resulting in different layers. The weight of the sediments
above squeezes everything below, forcing out the water. Chemical residues left behind by the
water “glue” the sediments together into solid rock. The sedimentary rock becomes part of
coastal cliffs if an earthquake lifts up the land or sea level sinks (e.g., during an ice age 2).
Rivers
and Runoff
Coastal
Cliffs
Layers
Uplift
Fault
Layers in the Cliffs. Left: Courtesy of Dr. Douglas Neves. Right: Courtesy of PDPhoto.org (public domain).
2
Less water that evaporates from the ocean returns to the ocean, because more of it falls as snow and becomes ice
on land.
Oceanography 10, T. James Noyes, El Camino College
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