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Estuarine (System)

Unique Identifier: 2

System Estuarine
Subsystem Estuarine Coastal
Estuarine Open Water
Estuarine Tidal Riverine Coastal
Estuarine Tidal Riverine Open Water

Definition The Estuarine System is defined by salinity and geomorphology. This System includes tidally influenced waters that (a) have an open-surface connection to the sea, (b) are regularly diluted by freshwater runoff from land, and (c) exhibit some degree of land enclosure.

The Estuarine System extends upstream to the head of tide and seaward to the mouth of the estuary. Head of tide is identified in accordance with the Metadata Profile for Shoreline Data, FGDC-STD-001.2-2001 (FGDC 2001) as the inland or upstream limit of water affected by a tide of at least 0.2 foot (0.06 meter) amplitude. The mouth of the estuary is defined by an imaginary line connecting the seaward-most points of land that enclose the estuarine water mass at MLLW. Islands are included as headlands if they contribute significantly to the enclosure.

Estuaries occur on continents or on islands and include waters of any depth. In CMECS they are defined as waters bounded by significant enclosure by land, having a direct connection to the sea and receiving measurable freshwater input to some part of the enclosed system during an average year. Salinity, a dimensionless conductivity ratio as measured on the practical salinity scale (PSS), was established by the IAPSO (International Association for the Physical Sciences of the Oceans) in 1978 (UNESCO 1981), and is of prime importance in distinguishing freshwater from saline estuarine environments and differentiating among estuarine and marine environments of differing salinity. The range of salinity considered in the CMECS classification extends from zero to hyperhaline (>40). Oceanic salinities normally encountered throughout the world range from 30-40 on the PSS scale. Highly saline negative estuaries such as Laguna Madre and Florida Bay may experience salinity as high as 70-80. Extreme environments like the Dead Sea have salinity near 300.

The tidally influenced part of the estuary may occur in a fresh reach where salinity is <0.5. According to FGDC-STD-004, this area would be classified within the Riverine System. However in CMECS, the Tidal Riverine area is considered to be an integral part of the ecology of the estuarine ecosystem, so it is classified within the Estuarine System instead.

The Estuarine System has four subsystems: Coastal, Open Water, Tidal Riverine Coastal, and Tidal Riverine Open Water.