Along active collisional margins, tectonic processes have uplifted and deformed rocks to create rugged landscapes with very little sediment input because of immature drainage basins along such geologically youthful landscapes. The presence of recently uplifted mountainous areas can also act as a barrier and prevent river systems from flowing, and thus carrying sediment, to the coast. Moreover, as you learned in Module 2, the relatively narrow and steep continental shelves that are characteristic of active tectonic margins do not help to dampen waves that move onshore from the open oceans. The result is that along active tectonic margins there is little delivery of sediment that could accumulate, and that sediment that does make it to the coast can be easily eroded and transported away by the high wave energy. Globally, there are many different types of morphologies along rocky coasts because of a wide range of rock types, styles of tectonic deformation, hydrographic regime, and styles of weathering.

Some of the most spectacular features of rocky coasts include sea arches and sea stacks, which are produced by the constant erosion of waves. A sea arch develops when a headland protruding into the ocean causes waves to refract around it. This refraction of waves concentrates their energy in specific locations along the headland, causing particularly rapid erosion if weakness such as faults and fractures are present in the rocks. In other cases, the waves may simply begin to erode into rock that is less resistant to erosion than the surrounding rock. Either way, the erosion leads to the development of small caves that may eventually meet below a promontory, leaving an arch above. The erosion continues and, for this reason, sea arches are very ephemeral coastal features. When they ultimately collapse, the remnants of the arch are called sea stacks.

For more information on sea arch formation and sea stacks, check out the following videos:

Video: West Wales - Sea Arches & Stacks (3:04)

Video: How Caves, Arches, Stacks, and Stumps are Formed (1:58) (Video is not narrated.)

Take a few minutes to think about what you just learned, and then answer the questions below.

The vast majority of large reefs created by corals in shallow, sunlit waters (< 50 m water depth) are located within a tropical zone located between 30º N and 30º S latitude with a preferred temperature range of approximately 22º to 29º C. Corals also grow best in areas with little suspended sediment in the water, so large coral reefs systems are not common to locations where there is a large input of sediment to the coastal zone by river systems. Although there are cold, deep water types of coral present in the ocean basins, they do not create large nearshore reef structures that affect adjacent coasts.

Red dots indicate the global distribution of coral reefs. Reef systems built primarily out of calcium carbonate secreting organisms.

Credit: NASA: Wikimedia Commons: Coral reef locations (Public Domain)

In total, there are three main types of shallow water coral reef structures: 1) barrier reefs, 2) fringing reefs, and 3) coral atolls. These three types are differentiated on the basis of proximity to land, the overall scale of the reef structure, and the shape of the reef.

1. Barrier reefs are typically large-scale, linear features that extend parallel to a shore, with a lagoon between the reef and the mainland.
2. Fringing reefs are directly attached to the shore, with no well-developed lagoon between the reef structure and the mainland.
3. Coral atolls are circular reefs that often start out as fringing reefs attached to a volcanic island. As the volcanic islands subside, the reef grows upward and a lagoon develops behind the reef and inside the submerging island. Eventually, the island can subside below the water level, and a ringlike coral reef structure remains.

Satellite image of a portion of the Great Barrier Reef on the east coast of Australia. Note the parallel trend of the reef to the coast and the lagoon that separates the reef from the mainland. The reef can only be reached by boat from the coast. The arrows identify sediment plumes that are entering the lagoon by the inflow of water. There are substantial concerns because such plumes can also deliver excess nutrient and pollution loads that are deadly to the coral organisms.

Credit: NASA: EO: River Plumes Threaten Great Barrier Reef (Public Domain)

Australia’s largest fringing coral reef, Ningaloo Reef, on the western shore of Australia. Close examination of the image reveals that the reef is in close contact to the land, and there exists a little lagoon between the reef and the mainland. You can swim from the beach to the reef.

Credit: NASA: EO: Ningaloo Reef (Public Domain)

Wake Island atoll in the central Pacific Ocean is located 4,000 km west-southwest of Hawaii. The central blue lagoon area is approximately the outline of the now submerged crater of the volcano around which the atoll developed. Note the aircraft landing strips and other structures that are maintained by the U.S. Department of the Interior for U.S. Air Force and Army operations in this remote Pacific location.

Credit: NASA: Wake Island, Pacific Ocean (Public Domain)

Take a few minutes to think about what you just learned, then answer the questions below.

An examination of nearshore zones on a global basis reveals that they are characterized by a wide variety of morphologies, which are dependent upon the wave and tidal conditions as well as the size of sediment that is present. Overall, it is a zone in which there is generally the capability for significant sediment transport both perpendicular to the beach as well as alongshore because of longshore sediment transport by wave action. In some nearshore systems, there may be a variety of alongshore bars or ridges of sediment that can eventually migrate landward and attach to the beach and thereby contribute to the seaward extension of the beach.

Nearshore and beach system in Portugal during low tide. The nearshore in this example would extend offshore from the outermost limit of breaking waves, and the surf zone would include the zone between the breaking waves.

Credit: Wikipedia: Littoral Zone is licensed under CC BY-SA 3.0

The beach is located landward of the nearshore zonation and can consist of a wide range of sediment types including pebbles, gravel and even sediment particles that are as large as boulders. The composition of a beach can also be highly variable, and sediment on a beach can be anything from grains of the mineral quartz to fragments of calcium carbonate shell produced by shelled organisms. The size of the sediment as well as the composition of the sediment that is located on a beach is wholly a function of the character of the sediment that is available at that location. Because beaches are constantly being influenced by the impact of waves and by changes in the elevation of the water levels because of tides, they can be extremely dynamic environments, capable of undergoing drastic changes in morphology within a timescale that ranges between nearly instantaneous to seasonal or longer.

During high energy events, such as large storms or tropical cyclones, waves increase in size and therefore are more capable of eroding sediment from a beach and transporting it either farther offshore or along the shoreline to a new location in very short periods of time. During longer time periods, similar results can occur even during fair weather when low energy waves are affecting the beach. In general, however, most beaches undergo the most substantial erosion and loss of sediment during storm events and then experience a period of recovery to regain a cross-sectional profile that is in equilibrium with the wave and tidal conditions.

The slope of beaches can also be highly variable and is a function of the composition of the beach and the characteristics of the waves impacting the beach. Consider what happens as a wave breaks and the swash of the wave runs onto the beach and then returns. As a wave breaks onto a beach, it rushes up the slope of the beach and carries some sediment with it. The energy of the wave is expended due to friction with the surface and the pull of gravity downslope on the beach. As the water of the wave returns downslope, referred to as backwash, it still carries some sediment with it. However, if the beach is very coarse-grained, then the backwash can rapidly percolate into the subsurface and very little water remains to carry sediment back down the beach to the sea. In this case, any sediment carried up the beach by the breaking wave becomes stranded at the landward limit of the breaking wave, and no backwash is available to carry sediment back down to the base of the beach. Alternatively, if the beach is fine-grained and saturated with water, then the backwash cannot percolate into the subsurface, and as the water returns to the sea it carries relatively more sediment with it, leading to a gentle gradient. As a result, coarse-grained beaches with a significant amount of percolation have overall steeper gradients than fine-grained beaches under similar wave conditions.

Sandy beach along northwestern California. Note the transition from the beach to a backshore dune environment where permanent vegetation is established.

Credit: M. Kulp © Penn State is licensed under CC BY-NC-SA 4.0

Steep coarse-grained beach at Chedabucto Bay, Nova Scotia. Notice the generally smooth and rounded characteristics of the sediment on the beach, which has resulted from reworking by waves and collision of individual grains with other sediment grains.

Credit: S. Wilde

Cobble-boulder Beach on the southeast side of the Otago Peninsula, New Zealand. Note the relatively steep gradient of the beach because of the rapid percolation of breaking wave water into the coarse-grained beach.

Credit: Stug.stug at the English Wikipedia, is licensed under CC BY-SA 3.0, via Wikimedia Commons

Coastal dunes, often referred to as sand dunes, form where there is a readily available supply of sand-sized sediment and are located landward of the beach in the supratidal zone. The size of the sediment, duration, velocity, and direction of winds in the coastal zone, as well as the size and extent of vegetation, are fundamental properties that govern the size and shapes of dunes in coastal settings. The development and growth of dunes derive from the beach when the wind is blowing in an onshore direction. When the wind is blowing offshore, sediment in the dunes is transported back onto the beach or offshore into open water where it may later be carried back to the beach by waves.

Sand accumulates to create a dune system when the wind carrying the sand encounters an obstacle. Pieces of driftwood, trash, or piles of seaweed can all provide such an obstacle, causing the velocity of the wind to locally decrease, at which point the transport of the sand ceases and it is deposited. Most often, the obstacle that creates large continuous sand dunes is salt-water tolerant vegetation, either beach grasses or shrubs and trees depending upon the climate of the region. Vegetation, therefore, promotes the deposition of sand and acts to stabilize the dune system because of rooting.

During fair weather conditions, the base of the dunes is not affected by wave energy when a beach is in equilibrium with the prevailing conditions, because the waves dissipate on the beachface. During a storm, when water levels are elevated because of storm surge and large waves are being produced, a dune system may, however, be subjected to breaking waves that can cause erosion and the removal of significant volumes of sediment from the dunes. In extreme situations, dunes can be completely washed over by storm waves, completely flattened, and the sediment that was removed can be carried offshore, alongshore, or farther inland to create a wash over platform.

Overall, the presence of well-established dune systems act as barriers against storm waves and, thus, help to protect infrastructure that is located landward of the dune systems. For this reason, as well as the unique coastal ecosystems that they provide, dunes are very often protected environments. Fences around dunes and walkways above dune vegetation are common features in high traffic areas, and walking through dunes or riding motorized vehicles across them can be met with hefty fines and punishment because of the damage these activities can cause to the dune vegetation.

Beach grass growing Dune system at Ocean City New Jersey, United States.

Credit: Beach Grass, Ammophila Arenaria, by Ellywa, "Helmgras kijkduin februari 2005" is licensed under CC BY-SA 3.0

Picture showing the erosional effects of Hurricane Isabel (2003) to a dune system in Nags Head, North Carolina. In this case, the erosive power of the hurricane storm surge and waves removed the entire front half of the dune system.

Credit: USGS: St. Petersburg Coastal and Marine Science Center (Public Domain)

Vegetated and protected dune system at Ocean City, New Jersey, United States. The purpose of the fencing along the backside of the dune is to prevent people from walking through the sensitive vegetation of the dunes and to help trap sand that is being moved or carried by the wind so that it is not lost to areas outside of the dunes.

Credit: M. Kulp © Penn State is licensed under CC BY-NC-SA 4.0

Please take a few minutes to think about what you just learned then answer the questions below to test your knowledge.

Because barrier islands are built by waves, they do not develop in tide-dominated environments. Waves are responsible for the longshore transport of sediment, and it is this transport that drives the deposition of sediment to create elongated features consisting of sandy barrier islands. There are, however, numerous examples of barrier islands within mixed-energy as well as wave-dominated environments. In fact, the two fundamental types of barrier islands are recognized as wave-dominated and mixed-energy barriers.

Wave-dominated barrier islands are long, narrow barrier islands with typically widely spaced tidal inlets. Because of the wave dominance, the capability for longshore transport is high, and tidal inlets in this type of system are relatively narrow because longshore transported sediment acts to fill in the inlets and restrict their widths. The tidal deltas on the seaward side of the inlets, ebb-tidal deltas, also tend to be small compared to mixed energy barrier systems because waves tend to limit the distance that ebb-tidal deltas can migrate seaward.

Close up view of the wave-dominated barrier islands of the North Carolina Outer Banks along the mid-Atlantic eastern coast of the U.S.A. Note the long, thin morphologies of the islands and the sparseness of tidal inlets that connect the backbarrier environments to the nearshore and beyond.

Credit: NASA (Public Domain)

Mixed-energy barrier island systems are typically short and wider at one end than the other end. Historically, this type of morphology has been referred to as a drumstick barrier island because of its approximate similarity in shape to the drumstick of a chicken leg. The tidal inlets between these barriers are large because of the relatively higher tidal energy. Compared to wave-dominated barriers, they also have large ebb-tidal deltas because the strength of the tidal currents is able to transport sediment seaward in a regime of relatively low wave energy. The relatively wider end of the island is the result of the accretion of sediment as waves refract around the edge of the ebb-tidal delta, causing a localized reversal in the longshore transport pattern and leading to sand accumulation.

Several hundred kilometers farther south of the North Carolina coast along the eastern U.S.A. is the coast of the state of Georgia with barrier islands that are different from the barrier systems of North Carolina. Because the Georgia barrier islands here are in a mixed-energy environment, waves are not the dominant forcing mechanism of coastal change, and the barriers are shorter with more tidal features such as closely spaced tidal inlets that allow the exchange of large volumes of seawater between the backbarrier and open ocean.

Credit: NASA (Public Domain)

In recent years, there have been numerous studies investigating how barrier island systems respond to change in sea level and impacts created by storms as they erode sediment and destroy coastal property. Because barrier islands form a true barrier along many inhabited coastal zones, they represent a line of defense for inland communities from the destructive power of storm surges and waves that are driven by large storms. Unfortunately, however, there are global trends in barrier size reduction because of reduced sediment input caused by damming rivers, human modifications to coastal systems, storm-driven erosion, and relative sea level rise.

In future modules, we will explore the dilemmas facing communities located on barrier islands and the question of whether these communities can expect to survive.

Please take a few minutes to think about what you just learned, then consider how you would answer the questions on the cards below. Click "Turn" to see the correct answer on the reverse side of each card.

Variations in delta morphology tell us something about the processes that cause and drive the evolution of deltaic environments. Globally, it is widely accepted that there are three end-member morphologies of deltas that reflect the relative influence of wave energy and tidal energy in the receiving basin or sediment input by the source river into the receiving basin. On a ternary plot, these three end members each represent one apex of the plot, and all deltas fall somewhere on this plot. Deltas that are primarily the result of high rates of sediment input tend to be elongated because of their rapid outbuilding associated with high rates of deposition into the receiving basin. Wave-influenced deltas have smooth, often arcuate shorelines with numerous ridges that reflect the longshore transport of river-delivered sediment by the high wave energy. Tidally influenced deltas have numerous shoreline perpendicular tidal passes and tributaries, with sediment bodies aligned parallel to the direction of tidal exchange.

Ternary plot showing the relative influence of sediment input, wave energy, and tidal energy on delta morphology and where some of the Earth’s deltas plot within this scheme.

Credit: Galloway, 1975, as cited in Delta Morphologies and Driving Process, licensed under CC BY-NC-SA 4.0

Satellite image of the Mississippi River delta that has built into the northern Gulf of Mexico along the south-central United States. The elongated, digitate form of the modern locus of deposition is the result of high sediment supply by the Mississippi River and low wave and tidal energy in the Gulf of Mexico. Like many other deltas, the Mississippi River is currently in a state of deterioration because of relative sea level rise and reduced sediment loads resulting from dams that capture sediment farther upstream.

Credit: Visible Earth, NASA; NASA image created by Jesse Allen, using data provided by the University of Maryland’s Global Land Cover Facility. (Public Domain)

Satellite image of the Danube River delta where the Danube River, the largest European river, empties into the Black Sea. The delta has been occupied by humans since the end of the Stone Age. Note the generally smooth coastline of the delta and the presence of ridges (lower half of delta) and barrier islands that suggest the front of the delta has been shaped by wave processes. The ridges represent alongshore accumulation by longshore transport processes.

Credit: NASA (Public Domain)

Rotated view of the Fly River delta of Papua New Guinea in the lower right-hand corner, where the Fly River empties into its receiving basin. The delta morphology is characteristic of tidally influenced deltas. This delta is mesotidal with a tide range of approximately 3.5 m at the mouth to 5 m farther inland.

Credit: NASA (Public Domain)

During the last decade, a substantial amount of concern has arisen regarding the health of the planet's major deltas. Over-exploitation of deltaic resources by humans, the introduction of pollutants, and excess nutrients to the rivers, as well as the management of river water that feeds deltas has severely damaged the sensitive environments of many deltas. Additionally, reduced sediment loads in many deltas, because of the construction of dams, coupled with global sea level rise and/or local land subsidence has resulted in widespread loss of deltaic wetlands and fronting sandy barrier shorelines. Because deltaic plains are so heavily relied upon by humans and, in some cases, are densely inhabited by humans, there are, for some deltas, widespread efforts to try to halt coastal erosion and environmental damage.

For example, the wetlands of the Mississippi River delta have undergone substantial change during the last century, with large areas of wetlands converted to open water because of relative sea level rise and erosion by storms. The rate is just staggering, with a football field of wetlands vanishing into open water every 30 minutes! The loss of wetlands across the delta is so severe that communities and infrastructure that were once separated from the open Gulf of Mexico by wetlands are now exposed to open marine water and have become more vulnerable to the damaging effects of storm surge. As a result, the state of Louisiana has developed a series of plans to build new land and infrastructure that would help reduce the net loss of land. The staggering change between 1932 and 2011 can be seen in the two satellite images below.

Coastal Louisiana, 2011.  Based on a NASA satellite image, gray and white areas show land and blue indicates open water. New land—mainly coastal improvements such as shoreline revetments and enriched beach areas—that built up since 1932 is shown in green.

Credit: NOAA (Public Domain)

Coastal Louisiana, 1932. The image combines the 2011 satellite image with a U.S. Geological Survey map, in which land areas that were present in 1932 are light gray. Since the 1930s, Louisiana's coast has lost 1,900 square miles of land, primarily marshes. Comparing the two maps reveals the dramatic coastal change.

Credit: NOAA (Public Domain)

Please take a few minutes to think about what you just learned, then answer the questions below.

Different ways that estuaries can form include:

1. Ria Estuaries: rising sea level fills an existing river valley, such as what happened to create a special case of Ria known as the Coastal Plain Chesapeake Bay Estuary of the eastern U.S.A.
2. Tectonic Estuaries: tectonic deformation of the Earth’s crust, such as faulting, creates a localized depression that fills in with marine waters such as San Francisco Bay. Water depths in tectonic estuaries can be highly variable.
3. Bar Built Estuaries: migrating barrier islands or spits extend across the mouth of a bay and restrict the amount of marine water entering the bay to create an estuary. They tend to be relatively shallow and typically less than 10m of water.
4. Fjord Estuaries: glacially carved, U-shaped valleys that filled with marine water since the end of the last ice age. They can extend long distances 10s to 100s of kilometers and as deep as several hundred meters.

The relatively higher density of saltwater compared to freshwater means that freshwater will float on top of saltwater. In some cases, there may be mixing between the two water masses but, in other cases, little to no mixing may take place to produce a highly stratified water column with a layer of buoyant freshwater situated above a layer of saltwater. The development of a well-mixed estuary or a highly stratified estuary is a function of the estuary morphology, magnitude of freshwater input, estuary-mouth tidal range, and how the tide propagates up the estuary. A classic comparison is that relatively shallow, bar-built estuaries are often well-mixed, and, alternatively, deep, fjord-style estuaries are highly stratified. It is also important to note that besides there being a vertical profile of salinity variation possible, there are also variations along the length of the estuary. Near the mouth of an estuary, one would expect to generally find the highest salinity water where the influence of the sea is the greatest and the most freshwater at the most inland extent where freshwater river systems enter into the estuary. These variations in salinity structures lead to differences in the distributions of salt-water and fresh-water tolerant plant and animal species within an estuary and how organisms use the estuary environments.

Salinity in Estuaries.

Credit: Creative Commons Attribution 3.0 Australia license OzCoasts (Geoscience Australia) 2012

Patterns of estuarine circulation play an important role in sedimentation within the estuary and the distribution of different ecosystems in the estuary. Fundamental control on the types of circulation patterns can be linked to the relative relationships of freshwater input, the tidal range, velocity, and direction of winds, and currents created by waves. Each of these is capable of creating currents within the estuary and thus control how freshwater and saltwater interact in the estuary and where sediment can accumulate within an estuary. For example, during periods of low river inflow to an estuary, wind-driven saltwater may move farther into an estuary than it would during high river discharge into the estuary. During intervals of high freshwater runoff, freshwater, and sediment may extend well into the estuary and lead to sedimentation near the mouth of the estuary in a location where sedimentation by rivers is typically low.

The complexity of natural systems and the complexity of the interaction between the various spheres of the Earth is extremely evident when considering coasts and coastal systems.

Consider the potential for the complexity of salinity and circulation within an estuarine system, a partially enclosed coastal body of brackish water that has freshwater input by rivers and saltwater input by marine processes. The circulation of an estuary can vary substantially across a range of time scales such as daily, seasonally, or annually. Changes in freshwater input during periods of heavy rain, fluctuations in tides, or changes in the direction that the wind is blowing can all contribute toward how water moves around within the estuarine system. During periods of heavy rainfall, freshwater input may result in a much-lowered salinity in the estuary, whereas during periods of drought and strong onshore winds, salty water can be pushed into the estuary by waves and the salinity of the estuary increases. Complexity in an estuarine system exists because of the numerous interacting physical processes of the hydrosphere and atmosphere that act on the system.

Now, consider how this complexity may affect the anthrosphere or the part of the environment that is made or altered by humans for human activities or habitat. Suppose you were making a viable living, fishing oysters for commercial resale. Most oysters are relatively environmentally restricted, and too much freshwater can kill them, as can too much saltwater. Perhaps, one year, you are able to very successfully oyster fish an area of the estuary because the freshwater and saltwater input has been perfectly balanced to create optimal conditions for oysters very close to your home. The following year, there is so much freshwater runoff into the estuary that all of the oysters near to your home die or do not grow to the adequate size. As a result, you would have to travel to new locations in the estuary to find the optimal oyster growing conditions, something that might mean longer boat travel time for you and more fuel to get onsite, thereby reducing your net profit. So, the complexity of natural processes interacting with each other generates complexity for how society deals with these changes.

The complex interaction of estuary morphology, freshwater influx, waves, and tides leads to the diverse range of estuarine conditions and environments that exist on a global basis and that may exist daily to seasonally in an individual estuary. The high primary and secondary productivity of estuaries and the fact that they can provide an important buffer to inland communities and infrastructure against the open ocean make estuaries a critically important coastal system. Yet, many are at risk because of exploitation. The Chesapeake Bay estuary, for example, has experienced widespread degradation of fisheries because of damage to bay habitat from the introduction of excess nutrients and pollution, and extensive programs are underway to mitigate against the substantial damage that has already been done to the system.

Please take a few minutes to think about what you just learned then answer the questions below.

The term coastal wetlands defines an area of land that is permanently or seasonally inundated with fresh, brackish, or saline water and contains a range of plant species that are uniquely adapted to the degree of inundation, the type of water that is present, as well as the soil conditions.

In some cases, coastal wetlands can extend across extremely large areas, as is the case for southern Louisiana along the north-central Gulf of Mexico. The importance of coastal wetlands is well known in southern Louisiana because there, like many other places, the coastal wetlands provide important habitat for a wide range of organisms. They also protect inland communities from the large storm surges that tropical cyclones can produce, by creating friction against an incoming storm surge, resulting in a reduction of the magnitude and extent of inland flooding during tropical cyclones. The greater the width of intact wetlands, the less likely it is that more inland areas will experience the full force of a tropical cyclone. Southern Louisiana has been, however, experiencing drastic loss of wetland at rates that for the last several decades have been equivalent to the loss of a U.S. football field every approximately 30 minutes.

Coastal Wetland Examples

Coastal wetland is used broadly here to identify areas where wetland plants inhabit the coastal zone, in either freshwater or saltwater environments of the coastal zone. For this reason, along the continental U.S. coastal zones, it includes vegetated environments such as salt marshes, fresh marshes, bottomland hardwood swamps, and mangrove swamps. In the United States, coastal wetlands extend across nearly 40 million acres and constitute approximately 38% of the total wetlands in the conterminous U.S.

• Marsh: A marsh is a type of wetland that consists of herbaceous plants (plants with leaves and stems). Typically, there is a period of annual dieback or at least a resting period from growth, and the system can be either salt or freshwater in nature.
• Salt Marsh: True salt marsh is strongly affected by the tides of a given area on a daily basis because they are located within the intertidal window of elevation. Along the coast of the U.S., all salt marsh experiences a severe to slight dieback during the winter, but begins growing strongly the following year with the return of warmer temperatures. Typical salt marsh structure includes tidal creeks through the marsh platform and localized ponds that may hold some water even during very low tides.
• Fresh Water Marsh: On a global basis, the distribution of coastal-zone freshwater marsh is most closely tied to river systems that enter into coastal zones. The steady influx of freshwater into coastal rivers provides an opportunity for fresh-water vegetation to dominate and prevents the incursion of flora that requires some level of salinity. The Florida Everglades of Florida in the U.S. represent some of the largest, and perhaps the largest, freshwater marsh in the world.
• Swamp: Forested wetlands with little circulation to nearly stagnant conditions are characteristic of swamps. Freshwater, brackish, and saline water are all possible environmental conditions in a swamp.
• Bottomland Hardwood swamps: In the coastal zone, bottomland hardwood forests are closely linked to the availability of freshwater. As a result, most extensive bottomland hardwood swamps are in low-lying river flood plains. Occasional flooding of these environments provides sediment to help anchor the vegetation and nutrients that are critical to growth. Excessive flooding or the introduction of saline waters can have serious effects on the health of such systems.
• Mangrove Swamps and Forests: Mangrove swamps are distributed through tropical and subtropical regions. There are numerous species of mangroves, but they all represent a plant that is halophytic or salt-loving and is associated with other trees and plants that grow within brackish to saline tidal waters. They may consist of a very complicated maze of woody roots and limbs. Depending upon the species of mangrove, very complicated interwoven root structures can develop and provide a significant buffer against erosion and inland progressing waves and storm surges during high-energy events. Across the U.S., they are restricted to low-latitude environments because of the species' intolerance for cold temperatures. Three different species exist from south Florida along to the Texas Gulf Coast, and, in fact, one of the largest mangrove swamps in the world is on Florida's southwest coast where Red Mangrove form structurally resilient coastal environments because of their interlocking woody growth patterns.

Maritime forests are coastally located areas of woods that develop on elevations and topography that is higher than that of coastal wetlands. They primarily rely upon shallow freshwater and cannot tolerate long exposure to salty water; even salt spray can be detrimental to some species of trees that establish maritime forests habitats. Such systems are discontinuously distributed along the U.S. Atlantic coast, but can be found elsewhere as well. In most cases, the soil composition promoting the growth of these forests is sand, either derived from sedimentary units deposited in the past or more recent deposits, such as beach ridges.

Picture of a maritime forest along the Georgia Coast, U.S.A. Note the relatively smooth canopy that helps to shield the interior plants from coastal winds and salt spray.

Credit: USGS: Patuxent Wildlife Research Center (Public Domain)

Coastal wetland and maritime forests represent unique provinces of vegetation that are uniquely adapted to the harsh conditions of the coastal zone. They provide a range of different habitats for coastal animals and simultaneously provide an important coastal barrier to more inland-located environments, as well as people and infrastructure. There is, however, much concern for the future of our modern coastal vegetation as the different provinces of coastal vegetation face continued exposure to pollution and excess nutrient inputs, changes in the elevation of sea level, and repeat large-scale storm impacts.

Barren areas of salt marsh loss on Cape Cod, Massachusetts, U.S. In the midst of extreme storm events, the introduction of pollutants and nutrients from runoff, and the likelihood of future sea level rise, there exists a wide range of concern for the future of vegetated coastal environments.

Credit: Salt Marsh Die-off National Park Service (NPS) (Public Domain)

Please take a few minutes to think about what you just learned and then answer the questions below.