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Classification of Wetlands and Deepwater Habitats of the United States
Definition. The Riverine System
(Fig. 4) includes all wetlands
and deepwater habitats contained within a channel, with two
exceptions: (1) wetlands dominated by trees, shrubs, persistent
emergents, emergent mosses, or lichens, and (2) habitats with
water containing ocean-derived salts in excess of 0.5 . A channel
is "an open conduit either naturally or artificially created which
periodically or continuously contains moving water, or which forms
a connecting link between two bodies of standing water" (Langbein
and Iseri 1960:5).
Limits. The Riverine System is bounded on the landward side by
upland, by the channel bank (including natural and man-made
levees), or by wetland dominated by trees, shrubs, persistent
emergents, emergent mosses, or lichens. In braided streams, the
system is bounded by the banks forming the outer limits of the
depression within which the braiding occurs.
The Riverine System terminates at the downstream end where the
concentration of ocean-derived salts in the water exceeds 0.5
during the period of annual average low flow, or where the channel
enters a lake. It terminates at the upstream end where tributary
streams originate, or where the channel leaves a lake. Springs
discharging into a channel are considered part of the Riverine
Description. Water is usually, but not always, flowing in the
Riverine System. Upland islands or Palustrine wetlands may occur
in the channel, but they are not included in the Riverine System.
Palustrine Moss-Lichen Wetlands, Emergent Wetlands, Scrub-Shrub
Wetlands, and Forested Wetlands may occur adjacent to the Riverine
System, often on a floodplain. Many biologists have suggested that
all the wetlands occurring on the river floodplain should be a
part of the Riverine System because they consider their presence
to be the result of river flooding. However, we concur with Reid
and Wood (1976:72,84) who stated, "The floodplain is a flat
expanse of land bordering an old river . . . . Often the
floodplain may take the form of a very level plain occupied by the
present stream channel, and it may never, or only occasionally, be
flooded . . . . It is this subsurface water [the ground water] that
controls to a great extent the level of lake surfaces, the flow of
streams, and the extent of swamps and marshes."
Subsystems. The Riverine System is divided into four Subsystems:
the Tidal, the Lower Perennial, the Upper Perennial, and the
Intermittent. Each is defined in terms of water permanence,
gradient, water velocity, substrate, and the extent of floodplain
development. The Subsystems have characteristic flora and fauna
(see Illies and Botosaneau 1963; Hynes 1970; Reid and Wood 1976).
All four Subsystems are not necessarily present in all rivers, and
the order of occurrence may be other than that given below.
Tidal. -- The gradient is low and water velocity fluctuates under
tidal influence. The streambed is mainly mud with occasional
patches of sand. Oxygen deficits may sometimes occur and the fauna
is similar to that in the Lower Perennial Subsystem. The
floodplain is typically well developed.
Lower Perennial. -- The gradient is low and water velocity is
slow. There is no tidal influence, and some water flows throughout
the year. The substrate consists mainly of sand and mud. Oxygen
deficits may sometimes occur, the fauna is composed mostly of
species that reach their maximum abundance in still water, and
true planktonic organisms are common. The gradient is lower than
that of the Upper Perennial Subsystem and the floodplain is well
Upper Perennial. -- The gradient is high and velocity of the
water fast. There is no tidal influence and some water flows
throughout the year. The substrate consists of rock, cobbles, or
gravel with occasional patches of sand. The natural dissolved
oxygen concentration is normally near saturation. The fauna is
characteristic of running water, and there are few or no
planktonic forms. The gradient is high compared with that of the
Lower Perennial Subsystem, and there is very little floodplain
Intermittent. -- In this Subsystem, the channel contains flowing
water for only part of the year. When the water is not flowing, it
may remain in isolated pools or surface water may be absent.
Classes. Rock Bottom, Unconsolidated Bottom, Aquatic Bed,
Streambed, Rocky Shore, Unconsolidated Shore, and Emergent Wetland
Fig. 4. Distinguishing features and examples of habitats in the Riverine System.