Fig. 2. Distinguishing features and examples of habitats in the Marine System. EHWS = extreme high water of spring tides; 

 ELWS = extreme low water of spring tides. 



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 border- 

 ing an old river .... Often the floodplain may take the form 

 of a very level plain occupied by the present stream chan- 

 nel, and it may never, or only occasionally, be flooded .... 

 It is this subsurface water [the ground water] that con- 

 trols 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 lilies and Botosa- 

 neau 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 Peren- 



nial Subsystem. The floodplain is typically well developed. 



Lower Perennial.— The gradient is low and water veloc- 

 ity 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 Sub- 

 system, 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. 



