Definition: The Class Streambed includes all wetlands 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, Intermittently Flooded, and Regularly Flooded-Tidal Fresh. Not all Water Regimes apply to all subclasses. 


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 “pioneer” 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: 


  • Bedrock - This Subclass is characterized by a bedrock substrate covering 75 percent 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, combined, cover 75 percent or more of the channel; however, bedrock alone covers less than 75 percent. 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 flowing water. 


  • Cobble-Gravel -  In this Subclass at least 25 percent 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 pioneer plants that, unlike Emergent Wetland plants or Scrub-Shrub Wetland plants, are usually killed by rising water levels. Many of the pioneer species are weedy mesophytes or xerophytes. At least 30 percent cover of pioneer plants is required. Common panic grass (Panicum capillare) is a typical Dominance Type in the Riverine System. 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)

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