11 



Fig. 4. Distinguishing features and examples of habitats in the Riverine System. 



velocity is slow. There is no tidal influence, and some 

 water flows throughout the year. The substrate con- 

 sists 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 concen- 

 tration 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 con- 

 tains nontidal flowing water for only part of the year. 

 When the water is not flowing, it may remain in iso- 

 lated pools or surface water may be absent. 



Classes. Rock Bottom, Unconsolidated Bottom, 

 Aquatic Bed, Streambed, Rocky Shore, Uncon- 

 solidated Shore, and Emergent Wetland (nonper- 

 sistent). 



Lacustrine System 



Definition. The Lacustrine System (Fig. 5) includes 

 wetlands and deepwater habitats with all of the fol- 

 lowing characteristics: (1) situated in a topographic 

 depression or a dammed river channel; (2) lacking 

 trees, shrubs, persistent emergents, emergent mosses 

 or lichens with greater than 30% areal coverage; and 

 (3) total area exceeds 8 ha (20 acres). Similar wetland 

 and deepwater habitats totaling less than 8 ha are also 

 included in the Lacustrine System if an active wave- 

 formed or bedrock shoreline feature makes up all or 

 part of the boundary, or if the water depth in the deep- 

 est part of the basin exceeds 2 m (6.6 feet) at low 

 water. Lacustrine waters may be tidal or nontidal, but 

 ocean-derived salinity is always less than 0.5°/oo. 



Limits. The Lacustrine System is bounded by 

 upland or by wetland dominated by trees, shrubs, per- 



