Classification of Wetlands and Deepwater Habitats of the United States
Definition. The Class Streambed includes all wetland contained
within the Intermittent Subsystem of the Riverine System and all
channels of the Estuarine System or of the Tidal Subsystem of the
Riverine System that are completely dewatered at low tide. Water
regimes are restricted to irregularly exposed, regularly flooded,
irregularly flooded, seasonally flooded, temporarily flooded, and
Description. Streambeds vary greatly in substrate and form
depending on the gradient of the channel, the velocity of the
water, and the sediment load. The substrate material frequently
changes abruptly between riffles and pools, and complex patterns
of bars may form on the convex side of single channels or be
included as islands within the bed of braided streams (Crickmay
1974). In mountainous areas the entire channel may be cut through
bedrock. In most cases streambeds are not vegetated because of the
scouring effect of moving water, but, like Unconsolidated Shores,
they may be colonized by "pioneering" annuals or perennials during
periods of low flow or they may have perennial emergents and shrubs
that are too scattered to qualify the area for classification as
Emergent Wetland or Scrub-Shrub Wetland.
Subclasses and Dominance Types.
Dominance Types for Streambeds in the Estuarine System were
taken primarily from Smith (1964), Abbott (1968), and Ricketts and
Calvin (1968) and those for streambeds in the Riverine System from
Krecker and Lancaster (1933), Stehr and Branson (1938), van der
Schalie (1948), Kenk (1949), Cummins et al. (1964), Clarke (1973),
and Ward (1975).
Bedrock. -- This Subclass is characterized by a bedrock
substrate covering 75% or more of the stream channel. It occurs
most commonly in the Riverine System in high mountain areas or in
glaciated areas where bedrock is exposed. Examples of Dominance
Types are the mollusk Ancylus, the oligochaete worm Limnodrilus,
the snail Physa, the fingernail clam Pisidium, and the mayflies
Caenis and Ephemerella.
Rubble. -- This Subclass is characterized by stones, boulders, and
bedrock that in combination cover more than 75% of the channel.
Like Bedrock Streambeds, Rubble Streambeds are most common in
mountainous areas and the dominant organisms are similar to those
of Bedrock and are often forms capable of attachment to rocks in
Cobble-Gravel. -- In this Subclass at least 25% of the substrate
is covered by unconsolidated particles smaller than stones;
cobbles or gravel predominate. The Subclass occurs in riffle areas
or in the channels of braided streams. Examples of Dominance Types
in the Intermittent Subsystem of the Riverine System are the snail
Physa, the oligochaete worm Limnodrilus, the mayfly Caenis, the
midge Chironomus, and the mosquito Anopheles. Examples of
Dominance Types in the Estuarine System or Tidal Subsystem of the
Riverine System are the mussels Modiolus and Mytilus.
Sand. -- In this Subclass, sand-sized particles predominate among
the particles smaller than stones. Sand Streambed often contains
bars and beaches interspersed with Mud Streambed or it may be
interspersed with Cobble-Gravel Streambed in areas of fast flow or
heavy sediment load. Examples of Dominance Types in the Riverine
System are the scud Gammarus, the snails Physa and Lymnaea, and
the midge Chironomus; in the Estuarine System the ghost shrimp
Callianassa is a common Dominance Type.
Mud. -- In this Subclass, the particles smaller than stones are
chiefly silt or clay. Mud Streambeds are common in arid areas
where intermittent flow is characteristic of streams of low
gradient. Such species as tamarisk (Tamarix gallica) may occur,
but are not dense enough to qualify the area for classification as
Scrub-Shrub Wetland. Mud Streambeds are also common in the
Estuarine System and the Tidal Subsystem of the Riverine System.
Examples of Dominance Types for Mud Streambeds include the
crayfish Procambarus, the pouch snail Aplexa, the fly Tabanus, the
snail Lymnaea, the fingernail clam Sphaerium, and (in the
Estuarine System) the mud snail Nassarius.
Organic. -- This Subclass is characterized by channels formed in
peat or muck. Organic Streambeds are common in the small creeks
draining Estuarine Emergent Wetlands with organic soils. Examples
of Dominance Types are the mussel Modiolus in the Estuarine System
and the oligochaete worm Limnodrilus in the Riverine System.
Vegetated. -- These streambeds are exposed long enough to be
colonized by herbaceous annuals or seedling herbaceous perennials
(pioneer plants). This vegetation, unlike that of Emergent
Wetlands, is usually killed by rising water levels or sudden
flooding. A typical Dominance Type is Panicum capillare.
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