Natural Community Systems -- Photo Guide

Brackish tidal riverbank marsh system

brackish tidal riverbank marsh system along Garvin Brook (photo by Bill Nichols for the NH Natural Heritage Bureau)
Brackish tidal riverbank marsh system along Garvin Brook (photo by Bill Nichols)


Description Brackish tidal riverbanks are flooded by tidal seawater that is diluted by freshwater flowing in from the watershed above.  They consist of low and high brackish tidal riverbank communities.  Brackish marsh, another type of estuarine marsh occurring in oligo- to meso-haline soil water settings, may occur intermittently along the upper edge of the high brackish riverbank marshLow brackish riverbank marshes typically occur in zones between mean sea level and mean high tide along moderate to steep brackish tidal river- and stream-banks.  The hydroperiod (duration and frequency of tidal flooding) in low brackish riverbank marshes roughly corresponds to that found in the low salt marsh, whereas soil water salinity is more equivalent to brackish marshes (0.518 ppt).  Fresh water can form a lens on top of the seawater, causing salinity to fluctuate widely with the tides.  High brackish tidal riverbank marshes typically occur as narrow zones along moderate to steep brackish tidal river- and stream-banks flooded less than daily (e.g., between the mean high water mark and the upper reaches of spring tides).  The hydroperiod of high brackish riverbank marshes corresponds to that found in the high salt marsh.  Where slopes are gentler, the low and high marshes may cover broader areas.

Much of the high and low marsh soil along stream and river mouths entering the Great Bay complex and the narrow margins around the bay consists of organic materials 16 to 50" thick overlying silty materials.  Some stretches of riverbank consist of marine silt or clay, and gravelly or cobbly material is found along upper sections of large streams corresponding to this system.
Numerous rare plants that occur in brackish tidal riverbank marsh systems but not in salt marsh systems are diagnostic (listed below).  Spartina alterniflora (smooth cord-grass) typically dominates the physically stressful low marsh.  As salinity decreases, Scirpus robustus (stout bulrush) and Typha angustifolia (narrow-leaved cattail) become more prominent and may dominate the low marsh in some examples.  A variable mix of graminoids and forbs characterize the high marsh zone.

Diagnostic natural communities:

      • Low brackish riverbank marsh (S1S2)

      • High brackish riverbank marsh (S1S2)

Peripheral or occasional natural communities:

      • Brackish marsh (S2S3)

Landscape settings: tidal riverbanks and near mouths of low-gradient coastal rivers feeding estuaries

Soils: marine peat and silt and clay; organic materials 16 to 50" thick overlying silty materials; oligo- to meso-haline (0.518 ppt); strongly minerotrophic

Spatial pattern: large patch (extensive narrow-linear) along riverbanks (several to 50 m wide by 502,000+ m long); linear zonation parallel to riverbank

Physiognomy: herbaceous, sparsely vegetated

Distribution: restricted to tidal sections of primarily Great Bay coastal rivers and large streams below the lowest dams

Characteristic species:

Dominant to locally abundant species: 
   Spartina alterniflora
(smooth cord-grass)
   Scirpus robustus (stout bulrush)
   Typha angustifolia (narrow-leaved cattail)

Other common species:
   Aster novi-belgii (New York aster)
   Spartina patens (salt-meadow cord-grass)
   Juncus gerardii (salt marsh rush)
   Typha angustifolia (narrow-leaved cattail)
   Spartina pectinata (fresh-water cord-grass, slough-grass)
   Agrostis stolonifera (marsh creeping bent-grass)
   Carex paleacea (chaffy salt sedge)
   Solidago sempervirens (seaside goldenrod)
   Scirpus pungens (three-square rush)

Characteristic rare plants:
   Limosella australis (Atlantic mudwort)
   Lilaeopsis chinensis (eastern lilaeopsis)
   Tillaea aquatica (pygmy weed)
   Samolus parviflorus (false water pimpernel)

Associated natural community systems:  This system may grade into sparsely vegetated intertidal systems and subtidal systems toward the channel and upland forest or freshwater stream borders landward or upstream.

brackish riverbank natural communities

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