Estuaries are coastal ecosystems where the freshwater from streams and rivers meet the marine waters in an estuary and mixing occurs. Freshwater inflow is the term referring to the freshwater that flows from rivers into estuaries. Mixing in estuarine ecosystems occurs spatially and temporally from climatic influences including tidal action, seasonal variability and storms. Mixing is affected by the amount of seawater in the estuarine system. The tides and the volume of the receiving estuary govern the volume of seawater. Tides are defined as the periodic rise and fall of the surface of the sea along the coast that are driven by the gravitational pull of the moon and of the sun (Sumich 1996). Estuaries are influenced by the tides but are often somewhat protected by storms and tidal action by buffers further offshore including barrier islands and peninsulas (Montagna et al. 2013).
Estuarine ecosystems are among the most productive ecosystems on the planet. Estuaries house such species as the blue crab, red fish, flounder, spotted seatrout, and many others. Marine habitats such as those found in estuarine ecosystems are valued at providing an estimated US14$ trillion worth of ecosystem goods and services annually, or 43% of the global total (Costanza et al. 1997). Some economically important estuarine habitats include tidal flats, salt marshes, sea grass beds, oyster reefs, and mangroves.
An estuary cannot properly function without freshwater inflows from the rivers and streams. Freshwater inflows are important to estuaries because they provide low-salinity nurseries, the transportation of nutrients, sediment, and organic material downstream effecting species movement and reproductive timing (Montagna et al. 2002a).
Global changes caused largely by anthropogenic influences are altering the amount of freshwater inflows to estuaries (CAMEO 2008). Humans are diverting water from rivers and streams, decreasing the amount of flows making it to estuarine ecosystems. Scientific evidence in the face of anthropogenic global change is showing many of these marine ecosystems are threatened (IPCC 2001). As the human population grows and the strain on water resources continues, the ability to effectively manage freshwater inflows into estuaries is becoming a priority worldwide (Montagna et al. 2013).
The Environmental Protection Agency has information on estuaries around the world. This can be found online at:
Role of Freshwater Inflows in Estuarine Ecosystems
Freshwater inflow is the freshwater from streams and rivers that flows into estuaries. Freshwater inflows are important to estuaries because they provide low-salinity nurseries, the transportation of nutrients, sediment, and organic material downstream effecting species movement and reproductive timing. Studies done to determine the roles of freshwater inflows found altering the hydrology could cause changes in estuarine systems.
Dewatering, or the removal of freshwater before it reaches the estuary, causes changes in the structure and function of that estuarine ecosystem (Palmer et al. 2011). Previous inflow studies performed to assess the effects of changing freshwater flows used benthic invertebrates and macrofaunal biomass as bioindicators (Palmer et al. 2008). Bioindicators are species that can be used to signify the condition of an ecosystem. The condition can be defined as the state of ecological integrity and the functionality of connections between them (EPA 2008). Therefore, the changes in biomass correlated with changing freshwater inflows indicate secondary production is changing with altered inflows (Kim et al. 2009).Estuarine systems may experience changes in many ecosystem components including hydro climate, water quality, benthic communities, epibenthic communities, fish communities, invasive species, ecosystem services, and other water resources. Estuarine ecosystem changes have resulted in losses of habitat, biodiversity, and productivity (Montagna et al. 2002b).
Global change refers to changes in the environment caused by climate and human systems that may alter the capacity of the Earth to sustain renewable resources. Coastal areas are especially vulnerable to global changes due to the characteristically high human densities that occur (Ronnback et al. 2007). One major component of global change in coastal areas is the removal of freshwater from rivers and streams for anthropogenic use before it reaches the coast. Since the 1960s dewatering, or removal of water from streams and rivers, has doubled and around 60% of the Earth’s runoff is captured. Removal of freshwater is altering three kinds of environmental flows including instream flow, inflow, and outflow.
The profound anthropogenic and climatic influences are illustrated the diagram below. The depiction shows the cascading effects downstream of an upstream human diversion. The impounded water decreases inflows to the estuaries and therefore the nutrients carried by the inflows. Decreased inflows and nutrients cause salinity to increase and loss of productivity of estuarine organisms that have a confined range of salinity tolerances and rely on nutrients for life processes and decreased predators who rely on these organisms for consumption. The arid climate due to climate change increases evaporation rates and temperatures further compounding the negative affects on the estuary.
Call for Action
The occurrence of global changes and new legislation supported by federal and regional governments has led to an increasing interest in managing freshwater inflows in order to conserve and restore estuarine health and functioning of rivers (Acreman, M.C. 2004). New regulations are being implemented to address environmental flows. In Texas, Senate Bill 3 addresses the need for environmental flow provisions including the maintenance of freshwater inflows to estuaries. Management initiatives are now being geared towards preserving or restoring natural flow regime patterns in order to create sustainable estuaries.
In order to meet the growing need of information regarding freshwater inflows, this web site has been created as a decision-support tool. The next section will discuss some current decision-support tools as well as frameworks of existing decision-support tools.