The slough subsystem is a sheltered environment, which is usually a narrow, isolated arm of the estuary with a very limited freshwater flow from uplands. Salinity is influenced by the proximity of the slough to the estuary mouth. Sloughs usually have fine organic sediments and high percentages of intertidal land consisting of flats, eelgrass beds, and marshes. The classification system distinguishes between intertidal and subtidal habitats, since biological communities often differ significantly according to the degree of tidal influence. Special adaptations are required by intertidal species to resist desiccation and tolerate large variations in temperature and salinity associated with tidal exposure.
Subtidal habitats are below extreme low water, and thus have continuously submerged substrates.
Intertidal habitats are exposed and flooded by tides as often as twice daily or as seldom as a few times a year. The upper limit of the intertidal zones is defined for regulatory purposes as the line of nonaquatic vegetation, or as mean higher high water where such a line cannot be determined. Within intertidal areas, a marked zonation of species is often apparent due to variation in the frequency and duration of exposure between lower and upper intertidal elevations. Although modifiers indicating tidal regime may be appropriate to differentiate intertidal habitats, intertidal elevations are not presently mapped for any Oregon estuaries.