Classification of Wetlands and Deepwater Habitats of the United States
Riverine System
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 System.
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.
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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
developed.
- 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
development.
- 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.
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| Fig. 4. Distinguishing features and examples of habitats in the Riverine System. |
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