U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3022

April 2018

Coastal Estuaries and Lagoons: The Delicate Balance

at the Edge of the Sea

Printed on recycled paper

Introduction 

Coastal communities are increasingly concerned about the 

dynamic balance between freshwater and saltwater because of 

its implications for societal, economic, and ecological resources. 

While the mixing of freshwater and saltwater sources defines 

coastal estuaries and lagoons, sudden changes in this balance 

can have a large effect on critical ecosystems and infrastructure. 

Any change to the delivery of water from either source has 

the potential to affect the health of both humans and natural 

biota and also to damage coastal infrastructure. This fact sheet 

discusses the potential of major shifts in the dynamic freshwatersaltwater balance to alter the environment and coastal stability.

Dynamic Balance

The distributions of flora and fauna in coastal marine 

environments are dependent on the freshwater-saltwater balance 

created by the pattern of freshwater and saltwater mixing in 

the estuary. The location of these interfaces is important to 

ecosystem function, as determined by the magnitude of river 

flow to the coast (fig. 1).

Freshwater Delivery

Many factors influence freshwater delivery to the coast 

including drought, flood, groundwater seepage, precipitation, 

evapotranspiration, streamflow regulation, and water extraction 

for agriculture, industry, and public water supply. In any given 

location in an estuary, salinity is lower during periods 

of medium and high streamflow. During low streamflow 

and drought, saltwater intrudes into the system, and 

the freshwater-saltwater interface moves upstream 

(figs. 2 and 3).

Coastal Marine Water Levels

Marine water levels fluctuate in response to many 

temporal factors. Coastal water levels rise and fall daily 

during tides. Onshore and offshore winds can either 

increase or decrease tidal water levels. High tides are 

exacerbated during tropical cyclone events, when strong 

winds push higher saline water towards the shore and 

inundate freshwater areas with saltwater. Over longer 

time scales (decades to centuries), sea-level rise in 

combination with storm events can increase 

saltwater inundation in low-lying areas of 

estuaries and lagoons.

Environmental Response and 

Coastal Resilience

Vegetation

Certain vegetation zones along estuarine 

gradients are tolerant of a gradient of salinity, but 

episodes of hurricane, drought, and flood can expose 

ecosystems to salinity extremes that result in change to 

0 psu 5 psu 20 psu 30 psu

Freshwater delivery Marine water level

Salinity concentration, in

practical salinity units (psu)

Time Distance

Flow

Time

Height

Freshwater

Saltwater

Observed

water

level

Predicted

water

level

Figure 1. Salinity is balanced in coastal estuaries and lagoons by deliveries of freshwater versus saltwater.
1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

Date

0

5,000

10,000

15,000

20,000

Daily maximum specific conductance,

in microsiemens per centimeter

Daily mean streamflow,

at 25 degrees Celsius

in cubic feet per second (f3/s)

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

Daily mean streamflow—Savannah River near Clyo, Georgia

 (USGS 02198500)

Daily maximum specific conductance—Savannah River 

 (Interstate 95) near Port Wentworth, Georgia (USGS 02198840)

EXPLANATION

6,000 f3/s—Streamflow value below which specific

 conductance level in the river increases

USGS streamflow-gaging station and identifier

USGS surface-water-quality site and identifier 02198840 USGS, U.S. Geological Survey

02198500

ATLANTIC OCEAN

Savannah River

02198840

02198500 02198500

GEORGIA

SOUTH

CAROLINA

Port Wentworth

Clyo

Figure 2. Relation between stream discharge and specific conductance along a portion of the Savannah River in Georgia. When 

freshwater discharge at Clyo is higher than 6,000 cubic feet per second in the high tides during the spring tide of the new moon, specific 

conductance is low at Port Wentworth (U.S. Geological Survey [USGS] gages 02198500 and 02198840, respectively; Conrads and others, 

2013). Note that the data from these two nearby gages are combined because a full set of discharge and conductivity data is not available 

for these individual gages across the range of years (USGS, 2017).

coastal environments. If such salinity intrusion events become 

more frequent and intense (Knutson and others, 2010), coastal 

vegetation may shift dramatically in type. In particular, tidal 

freshwater communities may shift to more salt-tolerant types of 

vegetation or to open water.

Although vegetation can be resilient to short-term salinity 

changes, zones related to salinity tolerance may move landward 

if the salinity limitations of species are exceeded over long 

periods of time (fig. 3). In other cases, coastal wetlands may 

transition overall to a more salt-tolerant species, or they may die 

off (fig. 4). As an example of vegetation transition in an estuary, 

in the Mississippi River Delta, a multiyear drought with higher 

levels of salinity caused a freshwater marsh to transition to an 

intermediate marsh.

Fisheries

For fisheries, increases in freshwater flow can have either 

positive or negative effects. For example, annual Callinectes 

sapidus (blue crab) landings are correlated with the freshwatersaltwater balance created by freshwater delivery to the 

southeastern coast of the United States (Childress, 2012). As 

salinity increases during periods of coastal drought, blue crabs 

move farther upstream, decreasing predation by alligators but 

increasing crab exposure to lethal parasites. Other examples of 

animal movement in response to salinity changes in estuaries 

are common. In the Suwannee River, an increase in freshwater 

flow increased critical habitat for juvenile Acipenser oxyrinchus

(Gulf sturgeon) (Randall and Sulak, 2007), whereas a decrease 

in freshwater due to drought reduced oyster production in 

Apalachicola Bay, Florida (Havens and others, 2013).

Coastal Vulnerabilities

Human infrastructure along the coasts, such as 

infrastructure related to freshwater supply and protection for 

human needs, storm protection, navigation, and tourism and 

recreation, is also vulnerable to salinity changes.

Freshwater Supply and Protection

Many municipal water-supply systems along the 

southeastern coast of the United States are vulnerable to 

saltwater intrusion and climate change (Furlow and others, 

2002). Often, municipal water intakes are near the freshwatersaltwater interface of tidal rivers, so salinity intrusion events 

can threaten freshwater sources for humans. To effectively 

manage water supply, water-resource managers need estimates 

of potential changes in the future frequency, duration, and 

magnitude of salinity intrusion events near water-supply intakes, 

especially in view of potential sea-level rise induced by climate 

change or increase in upstream freshwater extraction. Critical 

sources of drinking water from groundwater can also be affected 

(Prinos and others, 2014).

Alteration of salinity regimes in coastal water bodies can 

influence the health of humans and estuarine species in subtle 

ways. Changes to the water and sediment quality can alter 

species of algae, particularly if the environment is becoming 

more saline. While many types of algae have no influence on 

human or animal well-being, some types of algae produce 

toxins. During toxic algal blooms, toxins reach hazardous 

levels with prolonged and negative effects on estuaries, creating 
Modified from Odum and others, 1984

Extreme drought

Estuary zone and U.S. Geological 

 Survey (USGS) site identifier

 Intermediate (USGS 073802375)

 Brackish (USGS 07380335)

 Saline (USGS 07380251) 

EXPLANATION

073802375

Back-to-back hurricanes

 (3–4 weeks apart)

USGS surface-water-quality

 site and identifier

Salinity, in

practical salinity units (psu)

0

10

20

30

40

2000 2001 2002 2003 2004

Date

2005 2006 2007 2008

Barataria Bay Basin, Louisiana

07380251

07380335

073802375

Limit of

tidal influence

Tidal freshwater

Nontidal freshwater

Intermediate

Brackish

Saline

Ocean

5.0 psu

18.0 psu

30.0 psu

0.5 psu

GULF OF MEXICO

LOUISIANA

07380335 07380251

073802375

07380335 07380251

Figure 3. Monthly salinity concentrations in intermediate, brackish, and saline waters in Barataria Bay Basin, Louisiana, 2000–2008, 

and estuarine response to normal conditions and extreme weather events. Vegetation responds by moving landward or seaward as 

related to the salinity tolerance of species.

conditions of low oxygen, fish kills, and shellfish poisoning 

in humans. Toxic algal blooms also reduce recreational 

opportunities. Similarly, harmful metals such as mercury, 

arsenic, and lead increase in availability when salinity levels are 

low. Harmful metals in clay soils can accumulate along widened, 

slow-moving channels or where salinity increases slightly. In 

some cases, however, as salinity increases, metals detach from 

clayey sediments or transform into more toxic species (for 

example, methylmercury). Such metals can enter the food chain 

and accumulate in fish and other organisms in a process called 

bioaccumulation. This contamination of the food chain can be 

dangerous if contaminated fish are consumed by humans. An 

understanding of the routes of entry of heavy metals into the 

food chain is important to managers for developing management 

strategies to reduce heavy metal contamination of ecosystems.

Salinity intrusion can cause many public safety and national 

security issues, especially near critical infrastructure such as 

municipal and industrial water intakes and military facilities. 

In such cases, freshwater supply for coastal communities can 

dwindle quickly and become a public safety issue if alternative 

freshwater resources are not available. Many industrial intakes 

in tidal freshwater zones have a very low threshold for salinity 

because of the corrosive potential of seawater on industrial 

machinery, particularly boilers. The operations of many facilities 

related to military bases and nuclear power can be disrupted by 

salinity intrusion for the same reason.

Figure 4. Ghost forests 

caused by rising salinity levels 

on the U.S. Gulf Coast. 
Coastal Protection From Flooding and Storms

Coastal vegetation forms a natural defense against flooding 

for coastal communities during storms by reducing destructive 

wave energy. In the United States, wetlands provide about 

$23 billion annually in storm-surge protection services (Costanza 

and others, 2008). If vegetation is damaged, its ability to protect 

coasts from storms can be impaired (Costanza and others, 2008).

Navigation

Modifications to port facilities in estuaries can have a 

substantial effect on the location of the freshwater-saltwater 

interface. In particular, the deepening of shipping channels to 

accommodate deeper draft cargo ships can shift the interface 

farther upstream. These modifications can expose water 

intakes to salinity and exacerbate other problems related to any 

increases in the frequency, magnitude, and duration of salinity 

intrusion events.

Tourism and Recreation

Recreation is an important part of the U.S. economy 

(Colgan, 2004). Estuaries are important areas for activities 

such as hiking, fishing, bird watching, canoeing, and hunting. 

Although estuarine regions encompass only 12.6 percent of the 

continental United States, these regions compose 49 percent of 

the economic output of the United States (Pendleton, 2011). As 

an example of the importance of recreation to the economy, in 

2011, more than 46 million recreational fishermen generated 

over $48 billion in retail sales in the United States (Southwick 

Associates, 2013). Rises in tourism and recreation illustrate the 

importance of maintaining estuarine health for the prosperity of 

coastal communities.

USGS Capabilities

The U.S. Geological Survey (USGS) provides a wide 

variety of products designed to aid decision makers in 

their understanding of salinity dynamics and other factors 

that affect critical coastal habitats. These products include 

hydrologic monitoring data, biological data, reports of scientific 

investigations, data releases, scientific support, and data 

visualization and analysis tools for coastal estuaries and lagoons.

For more information, visit www.usgs.gov.

References

Childress, M., 2012, Status of blue crabs in South Carolina. State of 

Basin Rep. 2: Clemson. S.C., Clemson University, Newsletter.

Colgan, C.S., 2004, Employment and wages for the U.S. ocean and 

coastal economy: Monthly Labor Review, v. 127, no. 11, p. 24–30.

Conrads, P.A., Roehl, E.A., Jr., Daamen, R.C., and Cook, J.B., 2013, 

Simulation of salinity intrusion along the Georgia and South Carolina 

coasts using climate-change scenarios: U.S. Geological Survey 

Scientific Investigations Report 2013–5036, 92 p. and 5 apps.,

http://pubs.usgs.gov/sir/2013/5036/.

Costanza, R., Perez-Maqueo, O., Martinez, M.L., Sutton, 

P., Anderson, S., and Mulder, K., 2008, The value of 

coastal wetlands for hurricane protection: AMBIO, v. 37, 

p. 241–248.

Furlow, John, Scheraga, J.D., Freed, Randal, and Rock, Ken, 

2002, The vulnerability of public water systems to sea level 

rise, in Lesnik, J.R., ed., Proceedings of the Coastal Water 

Resource Conference: Middleburg, Va., American Water 

Resources Association, TPS–02–1, p. 31–36. 

Havens, K., Allen, M., Camp, E., Irani, T., Lindsey, A., 

Morris, J.G., Kane, A., Kimbro, D., Otwell, S., Pine, B., and 

Walters, C., 2013, Apalachicola Bay oyster situation report: 

Gainesville, Fla., University of Florida Sea Grant, Technical 

Publication 200, 32 p., accessed August 8, 2017, at http://

www.flseagrant.org/wp-content/uploads/tp200_apalachicola_

oyster_situation_report.pdf

Knutson, T.R., McBride, J.L., Chan, J., Emanuel, K., Holland, 

G., Landsea, C., Held, I., Kossin, J.P., Srivastava, A.K., and 

Sugi, Masato, 2010, Tropical cyclones and climate change: 

Nature Geoscience, v. 3, p. 157–163. 

Odum, H.T., 1984, Energy analysis evaluation of coastal 

alternatives: Water Science Technology, v. 16, p. 717–734.

Pendleton, L.H., 2011, The economic and market value of 

coasts and estuaries: What’s at stake?: Arlington, Va., 

Restore America’s Estuaries, 175 p.

Prinos, S.T., Wacker, M.A., Cunningham, K.J., 

and Fitterman, D.V., 2014, Origins and 

delineation of saltwater intrusion in the 

Biscayne aquifer and changes in the 

distribution of saltwater in MiamiDade County, Florida: U.S. Geological 

Survey Scientific Investigations Report 

2014–5025, 101 p.

Randall, M.T., and Sulak K.J., 2007, 

Relationship between recruitment of Gulf 

sturgeon and water flow in the Suwannee 

River, Florida: American Fisheries Society 

Symposium, v. 56, p. 69–83.

Southwick Associates, 2013, Sportfishing in America: An 

economic force for conservation: American Sportfishing 

Association, 12 p.

U.S. Geological Survey, 2017, National Water Information 

System: Mapper: Accessed November 29, 2017, at https://

maps.waterdata.usgs.gov/mapper/index.html.

By Paul A. Conrads (deceased), Kirk D. Rodgers, 

Davina L. Passeri, Scott T. Prinos, Christopher Smith, 

Christopher M. Swarzenski, and Beth A. Middleton

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20183022