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Ecological Site descriptions for landscapes dominated by the influence of water are unique because the drivers of change are dominated by hydrology. Water provides the energy to form and maintain landforms, creates anaerobic conditions needed to perform unique bio-geochemical wetland functions, supports the form and function of stream reaches, and supports unique plant communities dominated by hydrophytic species. Surface runoff and sub-surface flow moves through the landscape from interfluves to watershed outlets in a continuum. Each discrete ecological site along this continuum has its own water budget consisting of inputs, outputs, and storage capacity. The magnitude and direction of these water budget parameters dictates the potential functions performed, and changes in state are either caused by, or respond directly to changes in these parameters. The Hydrogeomorphic (HGM) landscape classification system was developed specifically for the development of functional assessments of wetlands. However, HGM concepts lend themselves to the description of all sites, whether they are "wet" or "dry". The three HGM parameters are landscape position, dominant water source, and hydrodynamics. Hydrodynamics is a description of the water budget inputs, outputs, and site storage. Storage can occur in the soil profile and on the surface. Surface storage occurs either as lentic ponded water or as lotic floodwater. In both ESDs and HGM landscape classes, site boundaries can be drawn around aggregations of soils that have the same water budget parameters, support the same plant communities, and have the same responses to disturbances. Watersheds are aggregations of these individual sites. Ideally, each watershed within a given reference domain should consist of a set of ecological sites which have boundaries that fit seamlessly, and through which water moves in a continuum. Those ESDs which fall under the scope of Riparian include those sites which receive surface and groundwater from a watershed source area that is above a certain flow accumulation threshold. Riparian sites includes headwater reaches that may not necessarily meet the wetland hydrology threshold.