Shorelines Unit (Topic 6B) – 1
Name:
Shorelines Unit
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Ocean
Estuary
Land
Estuary
Barrier Islands
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.
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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 more. The
mouth of a river, harbors, and marinas are all examples of estuaries
as well.
Barrier Islands
Estuaries
Estuary
Section:
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Unlike the rest of the coast, waves are not responsible for much of the water motion in estuaries,
because the partially-surrounding land blocks waves coming from some (or most) directions.
Instead, tides play an important role in creating currents in many estuaries. As sea level rises
towards high tide in 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.
1. What is an estuary?
2. Give at least 3 examples of words that we use to describe estuaries.
In other words, what words do we use to “label” or “name” estuaries?
3. What is the major cause(s) of currents in estuaries?
4. When does water flow into an estuary, when the tide is rising or falling in the ocean?
When does water flow out of an estuary, when the tide rising or falling in the ocean?
Shorelines Unit (Topic 6B) – 2
The Value of Estuaries
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).
5. Where is the water often fresher, in estuaries or in the ocean outside estuaries?
6. Where does the fresh water come from?
7. Where are there typically more nutrients, in estuaries or in the ocean outside estuaries?
8. Where is there typically more life, in estuaries or in the ocean outside estuaries?
9. What benefits do estuaries typically provide to people?
How do we use estuaries? What do we use them for?
Shorelines Unit (Topic 6B) – 3
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 at high tide, though
freshwater runoff can play a role as well
when it rains. Wetlands are often found
along the borders (edges) of estuaries.
Salt Marsh
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
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
Mangrove Forest
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).
10. What are wetlands?
11. Why are coastal wetlands often “wet” (covered by water)?
12. List the 2 kinds of marine (coastal) wetlands.
Shorelines Unit (Topic 6B) – 4
Life in Wetlands
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.
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).
Fish hiding in
mangrove roots.
13. Where is there typically more life, in wetlands or in the ocean outside wetlands?
14. What do most animals living in wetlands eat?
Shorelines Unit (Topic 6B) – 5
The Value of Wetlands
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. Moreover, wetland plants 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
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 lowoxygen 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: they must build a man-made wetland or help extend a
natural one to compensate.
15. What benefits do wetlands typically provide to people?
How do we use wetlands? What do we use them for?
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 thanks to all the nutrients in the river water. Many
great ancient civilizations (e.g., Egypt, Mesopotamia, China,
India) started in deltas because of the rich farmland.
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Ocean
Estuary
Land
Estuary
Barrier Islands
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.
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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.
Barrier Islands
Deltas
Estuary
Shorelines Unit (Topic 6B) – 6
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Delta
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 a shoreline, because new sediments are
continually brought by the river and its distributaries. If the river is dredged and controlled by
man-made levees so that it 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 to Hurricane Katrina.
Shorelines Unit (Topic 6B) – 7
16. What is a delta?
17. What typically takes sediments away from the ends of rivers and down the coast – but
does not do so fast enough at deltas?
18. Is the soil in deltas good for farming or poor for farming? Why?
19. Give an example of where a delta can be found in the United States.
20. Is New Orleans rising or sinking?
Shorelines Unit (Topic 6B) – 8
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
Barrier Island
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 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.)
21. What is a barrier island?
22. Where are barrier islands common along the coast of the United States?
Shorelines Unit (Topic 6B) – 9
Headlands and Coves
Coves
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). (Sooner or
later the wave-resistant rock will
completely erode away. It just erodes
slower than the less-resistant rock.) Along
the coast of California, igneous rock –
cooled lava – tends to resist erosion better
than sedimentary rock – rock made of
stuck-together sediments.
At First
#2
#3
Land
Headlands
Ocean
Headland
black = wave-resistant rock
Cove
#4
#5
Land
Wave Crests
Ocean
23. What is a headland?
24. What is a cove?
25. What is igneous rock made from?
26. What kind of rock is more wave resistant, igneous rock or sedimentary rock?
27. Where is the rock eroding faster, at the cove or at the headland?
Shorelines Unit (Topic 6B) – 10
Sea-Arches and Sea Stacks
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 until the rock on either side
cannot support the weight of the arch and the arch collapses. The fallen rock is made of the lessresistant rock, so it is ground up into sediments by the waves and carried away. All that remains
is the sea stack (the wave-resistant rock).
Headland
Sea Arch
Sea Stack
Shorelines Unit (Topic 6B) – 11
Wave-Cut and Marine Terraces
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
Wave-Cut Terrace
Marine Terrace
Wave-Cut Terrace
Marine Terraces
28. What is a wave-cut terrace?
29. What is a marine terrace?
30. What creates the flat wave-cut terrace?
31. How or why does a wave-cut terrace become a marine terrace?
32. What can lift the marine terrace out of the water?
33. Does sea level rising or sink during ice ages?
Shorelines Unit (Topic 6B) – 12
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: more of
the water that evaporates from the ocean does not return to the ocean, because more of it falls as
snow and becomes ice on land – less falls as rain and flows back into the ocean).
Rivers
and Runoff
Coastal
Cliffs
Layers
Uplift
Fault
34. What kinds of sediments are coastal cliffs
made of or from?
35. Where to the sediments pile up?
36. How do the sediments become cliffs
on dry land along the coast?
Layers in the Cliffs. Courtesy of Dr. Douglas Neves.