I. Introduction

A.  An important coastal environment frequently associated with barrier  islands is the estuary that separates the mainland  from the islands. An estuary is a semi-enclosed portion of ocean that is somewhat isolated by land, and in which freshwater and salt water mix.

B. A glance at a globe demonstrates how vital the influence of estuaries has been on the location of large and small cities throughout the world (London, New York, Boston, Norfolk, Tampa- St. Petersburg, Seattle, Baltimore, Tokyo, Hong Kong, etc.).

C. For humans estuaries provide:

     1. Navigable harbors and waterways for the transfer of:

          a. Food

          b. Raw materials

          c. Manufactured goods

     2. This activity is economically indispensible to a

     country's commercial and industrial practices.

     3. Recreational sites

     4. Commercial fishing

D.  For  years the wetlands along the fringes of estuaries were considered useless and served as convenient repositories for the disposal of industrial and domestic waste.

E. Now, cleaned up and reclaimed by landfill, this valuable land provides opportunities for urban expansion into the sea. This, in itself, is a major threat to the environment.

F. These increasingly stressed estuarine and wetland environments are among the most biologically productive earth environments.

     1. The organic production of marshes, increased by

Nutrients supplied by rivers, can be as high as 5-10 tons per year of organic matter per acre, compared with 1 ton per year per acre for a carefully fertilized and tended wheat field, or less than 0.5 tons per year per acre for the open ocean or a desert. (unit= gms Carbon/m2/year)


II. How do estuaries differ from other oceanic environments?

A. The shallowness of the water differentiates them from other non-coastal, marine environments, but two other factors separate them even from other coastal environments:


B. DEPTH - Estuaries are very shallow compared to open ocean.

1. Changes in the nature of water with respect to distance and time are much greater in these shallower coastal waters so river runoff and tidal currents have a very significant effect on the nature of the water.



1. River runoff has the direct effect of reducing salinity in the surface layer in areas where mixing is not significant, and throughout the water column where mixing occurs.

2. In temperate and tropical climates the runoff of the rivers will be at the maximum during the rainy season and will decrease the salinity significantly below the oceans average of 35 o/oo.

3. Varies with tide and seasons



1. The shallow water permits very great ranges in temperature to occur on a yearly and even a daily basis. Sea ice forms in many estuaries in high latitudes.

2. Seasonal changes in temperature can be most easily detected in the coastal regions of the mid-latitudes, where surface temperatures are at a minimum in winter and reach maximum values in late summer.

3. Strong thermoclines may develop in areas where mixing does not occur. 

a. Here very high-temperature surface water may form a relatively thin layer.  Mixing reduces the surface temperature by distributing the sun's heat through a greater vertical column of water, thus pushing the thermocline deeper and making it less pronounced.

4. Both tidal currents and prevailing winds can have a significant effect on surface temperatures.


III. Origin and Classification of estuaries

A. The word estuary comes from the Latin word aestuarium meaning tidal.  It refers to a semi-enclosed coastal body of water in which sea water is significantly diluted by fresh water from land runoff.

B. Origin of estuaries

     1. Essentially all estuaries exist today due to the fact

     that in the last 18,000 years sea level has been raised

     approximately 120 meters (394 ft) owing to the melting of

     much of the major continental glaciers covering portions

     of North America, Europe and Asia during the Pleistocene

     Epoch (1.6 - 0.01 mya).

     2. Geologically estuaries are  short-lived, temporary

     features because their basins are quickly destroyed by

     sediment influx from the rivers and surrounding land mass.

          a. Different processes are responsible for this

              1) Freshwater marshes slowly extend out into the

              river-end of the estuary and saltwater marshes

              and bars tend to close the seaward part.

              2) Estuaries and fjords that are cut into solid

              rock are less likely to be affected by these


              3) Deltas at river mouths in the estuary often

begin the process of infilling the estuary.

              4) In addition, large quantities of sediment can

              be carried into the estuary from the offshore


               5) The cutback of the coastline by marine erosion

slowly eats away at, and will eventually destroy

an estuary.

          b. If sea level were to lower, most estuaries would

          rapidly disappear, to be replaced by rivers cutting

          into their deposits.  If sea level were to remain con-

          stant or rise just a few meters, the processes of des-

          truction would not be stopped. Only a considerable

          rise in sea level would maintain or rejuvenate present


C. Classification of estuaries by origin:

     1.  Coastal Plain - low-lying coasts with drowned river


              a. Chesapeake and Delaware Bays

              b. Georgia, South Carolina

              c. Ireland

     2. Fjord - glaciated valleys that are U-shaped with steep

          walls.  They usually have a glacial deposit forming a

          sill near the ocean entrance.

              a. Scandinavia, Canada and New Zealand

     3. Bar-built - shallow estuaries separated from the open

          ocean by bars composed of sand deposited parallel to

          the coast by wave action.  Lagoons separating barrier

          islands form the mainland are bar-built estuaries.

              a. Laguna Madre, Texas

              b. Pamlico-Albemarle Sound is a combination of

              this and a coastal plain estuary.

              c. Mobile Bay, Alabama

     4. Tectonic - produced by faulting or folding which causes

          a restricted down-dropped area into which rivers flow.

              a. San Francisco Bay

              b. Juan de Fuca Strait

c. These are typically deeper than the estuaries

on a flooded coastal plain.

     5. Chesapeake Bay has also been affected by a meteorite

     (bolide) impact that occurred during the Eocene (~ 35 mya).

     The crater (1-2 km deep) and breccia generated by this

impact were discovered from cores drilled down into rock

layers in the region. Debris blasted from the crater up

into the atmosphere has been found in the coastal Atlantic

Ocean as far away as New Jersey. Results of such an impact

would have included: seismic sea waves, acid rain, blast

waves, forest fires, toxic gas clouds, flooding, etc.


IV. Circulation and mixing of water in estuaries

A.  Mixing of fresh and saltwater in estuaries results in long-term average flow seaward in the surface layers and a net landward flow along the bottom.  This is called an estuarine circulation. We are concerned about circulation in estuaries because it moves organisms and pollutants around. Salinity differences resulting from different circulation patterns also affect the environment for the organisms. Now we'll look at how it develops and how estuaries can be classified based on the nature of the salinity distributions within the estuary.

     1. There is a continuum (a continuous extent, series or whole with no sharp clear-cut

boundaries) of circulation patterns in estuaries.

B.  In an ideal estuary (no friction between water masses, no tides, and no winds), low-salinity river water flows in at the head of the bay and spreads out over the seawater beyond because it is less dense.


     Density =--------


     1. This generates a more-or-less horizontal pycnocline.


C. The subsurface saltwater in such an estuary forms a wedge with its  thin end pointed upstream, so in its simplest form  this  is called a salt-wedge estuary.

     1. The simplified picture is modified by several factors:

a. Friction between seaward-moving fresh water and the underlying seawater causes some underlying water to be dragged up from below and mixed with the surface layer.  This process is called entrainment. This newly mixed water cannot reenter the lower layer because its reduced salinity makes it less dense than seawater.  Therefore, ocean water flows into the estuary along the bottom to replace that drawn seaward by surface flows. Landward flow along the bottom is much greater in volume than the flow of river water into the estuary.

     2. Occurs where river flow is large and tidal range is low.

          a. Mouths of the Mississippi or Columbia rivers during floods.

              1) In the Columbia River the salt wedge moves as far as 15 miles upstream

D.  As fresh water discharge into an estuary decreases, tidal effects become more important. The estuary may act like a salt-wedge estuary only during floods. During low-flow periods, the same estuary is usually strongly influenced by tides and tidal currents.  Then it departs substantially from the pattern of a simple salt-wedge stratification and becomes a moderately stratified or partially-mixed estuary.


1. Tidal currents cause turbulence (random movements) throughout the water column, which, in turn, leads to mixing of salt and fresh water masses. Consequently, more salt water is transferred from the subsurface to the surface and some fresh water from the surface also mixes downward.

2. Generates a very different salinity pattern than the one observed in a salt-wedge estuary.

3. Salinity decreases in a landward direction both in the surface and the subsurface layers.

4. Found in deeper estuaries like Puget Sound and San Francisco Bay.


E. Tides dominate currents in most estuaries. Where tidal effects are relatively strong, waters in the estuary are less stratified.  In  an  estuary with small river flow but large tides  and  tidal currents,  the waters may be mixed almost completely from top  to bottom.  These are called well-mixed estuaries.


     1. Many small estuaries along the coast of South Carolina

     2. River discharge forms cloudlike parcels of low-salinity

     water, which, when turbid with suspended sediment, are

     easily recognizable from the air.

     3. Pamlico Sound is a well-mixed estuary, but not because

     tidal currents are so strong, Instead, because Pamlico

     Sound is so shallow (nowhere much deeper than 30 feet)

     winds mix the water column all the way to the bottom.


F. Because of their great depths and irregular bottom topography, fjords, have complicated circulation systems. Many fjords have a sill or submerged ridge of glacial sediment deposited at their mouths which cuts off most of the deeper water from communication with the adjacent ocean.


     1. Fresh water flowing into a fjord forms a low-salinity

     surface layer that moves seaward, in a typical estuarine


     2. The deeper waters of a fjord may be almost completely

     isolated from the surface circulation and may become ser-

     iously depleted in oxygen due to lack of downward mixing of

     oxygenated surface waters.



V. Environmental Issues in Estuaries

A. Estuaries are extremely vulnerable to the activities of man

because of their shallowness and nearness to land. They are

significantly influenced by:

     1. Nutrient-rich agricultural run-off, causing O2 depletion

     2. Sewage and industrial pollution

3. Temperature changes induced by industries that use

large amounts of water for cooling or during their

processing (i.e., nuclear power)

4. Changes in salinity induced by man (i.e., PCS Phosphate

pumps 65 million gallons/day of freshwater into the

Pamlico to dewater their mine).

5. Changes in turbidity (i.e., sediment load) of water

caused by clearing forests for agriculture and towns.

6. Real-estate development after draining and filling

     of wetlands

7. Waste disposal

8. Commercial fishing

9. Dredging

10. Shipping

11. Habitat destruction

B. Evidence of Man’s Impact

     1. 70% of the US population live within a day’s drive

     of the coast

     2. Only about 6% of the US shoreline used for recreation

     in under public ownership; the rest is privately owned.

     3. In the mid-1098’s about $4.2 billion in retail sales

     were generated in the coastal zone and there were over

     10 million boats, 80% of which were in the 30 coastal


     4. 7 of the largest cities in the US are on the coast

     5. About 33% of US population lives in coastal counties

     6. Shoreline property continues to increase in value

     7. About 40 % of US manufacturing is in coastal counties

     8. Many large energy facilities and oil refineries are

     on or very near the coast

     9. Coastal developments have already destroyed or built

     over 20-50% of coastal habitats.

C. Biological activity in estuaries

     1. Estuaries are an incredibly rich environment in terms

     of the diversity of organisms. This diversity is due to

     the high level of nutrients (nitrate and phosphate, etc.)

     dissolved in the water. Nutrients collect in estuaries

     because the denser salt water  flows in along the bottom

     bringing with it organic debris from the shallow ocean.

     The river water also brings in nutrients from the land

     masses, which are the major nutrient source to the ocean.

     2. Because most estuaries are shallow the light penetrates

     all the way to the bottom contributing to abundant plant

     growth throughout the water column, but especially on

     the bottom where sinking generates the greatest supply

     of nutrients. Because plants are at the base of the food

chain here, as they are nearly everywhere on Earth, this

leads to a rich source of food for all kinds of biota.

The abundant plant life also provides shelter for animals.

3. These precious habitats, which are the most productive

of all oceanic regions serve as the spawning and breeding

grounds to support much of the marine life in deeper parts

of the estuary. This in turn supports much of the marine

life in the rich fishing grounds of the continental shelves

a.Human impacts can destroy these breeding grounds by

throwing off the carefully-tuned cycle of the organisms

that live in the very stressful environment of the estuaries.

          1) Natural stress = drastic salinity and

          temperature changes

          2) Human activities significantly increase

          the stress due to the impacts described earlier

     b. The US fish and Wildlife Service estimates that

     from 50-75% of all commercially-caught marine fish are

     “born and raised” in estuaries.

4. Altering one component of these complex food webs can affect

the entire ecosystem.

     a. Many of the pollutants adhere to particles, sink to

          the bottom and are, therefore, removed from the water

          column.  However, they may be released back into the

          system by the action of bottom feeders and dwellers,

          strong currents, and trawling that stir up bottom


D. Flushing Time

     1. Most estuarine pollutants are flushed out to sea

     2. Flushing time is a measure of the turnover time for

     fresh water in an estuary that is calculated by comparing

     the total fresh water content of the estuary to the rate at

     which fresh water enters. If it were possible to remove all

     the freshwater from an estuary and then record how long it

     took the rivers to replace this water, the flushing time

     would be determined. Many flushing times are required to

     remove old pollutants.

          a. During the rainy season flushing time is on the

          order of weeks to months for the Pamlico.


VI. Coastal Seas

A.  Some coasts border on seas rather than directly on the open ocean.

These coastal seas differ in their degree of communication with the

open ocean.  The Gulf of Mexico and the Caribbean Sea communicate

substantially with the Atlantic Ocean so that open ocean processes

dominate their current systems. Places such as the Black Sea,

however, have such limited communication with the open ocean that

the ocean has no direct influence at all. Intermediate between

these two situations is that of the Mediterranean Sea.

B. I'm going to discuss the two general types of circulation patterns

that result in these seas with restricted communication to the open

ocean, using the Mediterranean and the Black Seas as examples.

1. Evaporation dominates precipitation = Mediterranean

     a. Results in considerable vertical circulation

     because the surface waters become more dense as

     evaporation proceeds and therefore sink down

     into the underlying waters promoting vertical

     mixing. This sinking surface water carries oxygen

     with it and promotes animal life at depth. The

     sinking water is replaced by surface water that

     flows into the sea through the Strait of Gibraltar

     The denser bottom waters flow out of the Sea at a

     Depth of 200-600 meters through the Strait.

2. Precipitation dominates evaporation == Black Sea

     a. Here the surface water is not made more dense

     by evaporation so it remains on the surface and

     does not sink into deeper parts of the basin

     carrying oxygen with it. The result is that the

     bottom is not flushed and becomes stagnant. The

     bottom of such seas can not support much life,

     which may be limited to anaerobic bacteria.