1.3 Estuary Definitions and Background
There are numerous definitions of an estuary, of which perhaps the simplest is "a semi-enclosed body of sea water where salinity is measurably diluted by freshwater." Estuaries are often characterized as having low salinities, shallow depths, high turbidities, excess nutrients, high productivity and low species diversity.
The two main characteristics of estuaries in British Columbia are that they are geologically young, as sea levels did not reach their present state until 5000 years before present, and they are ephemeral, meaning they are depositional environments with life spans of hundreds to thousands of years (Dawes 1998). The primary abiotic processes that influence estuarine ecology include seasonal variation in temperature, wave energy, type and rates of sedimentation, and the nature and amount of freshwater inputs. While low species diversity is a characteristic of all estuaries, these regions provide important spawning, nursery, rearing, staging, and holding areas for numerous species. Estuaries are unique environments, where factors which may be limiting elsewhere in the marine environment (e.g. nutrients) are not limiting in the estuarine realm.
Estuaries have historically been classified using abiotic factors such as geomorphic type, salinity stratification, and tidal currents. Geomorphic types include drowned river valleys, bars across the mouth of a river, fjords and different deltaic types. Classifications based on salinity stratification include well-mixed estuaries that have abundant oxygen for benthic areas versus weakly stratified estuaries with an undiluted layer of sea water near the bottom. Strongly stratified estuaries exhibit a landward transport of sea water and the seaward transport of freshwater. Classification of estuaries by tidal currents yields estuaries with standing wave tides, where tidal amplitude is similar throughout the basin. Progressive wave tides are characterized by a delay as the tide travels up the basin. Between the standing wave tides and progressive wave tides are estuaries with mixed tidal waves, which show characteristics of both types (Dawes, 1998; Day et al. 1989).
Many definitions include a combination of geomorphic, hydrologic, and biological elements. A definition incorporating these considerations, and the one used for this standard is "an intertidal, brackish shore-zone linked through ecological processes to surrounding environs (marine, terrestrial and aquatic), generally low energy and characterized by brackish water biotic communities. It is an area of river, tidal and wave process interaction."
The definition of an estuary for this mapping standard includes the area from the marine limit to -20 metres in the nearshore subtidal relative to the "zero tide" or chart datum.
For practical purposes, indicators of the marine limit include the storm log line or the upper extent of salt tolerant vegetation (e.g. the tree line). River channels below the tree line are included in this system, whereas channels above the tree line are excluded. The lateral boundary of the subtidal portion of the estuary from the low water line to the 20 metre depth is often difficult to establish, particularly if there is insufficient subtidal information related to geomorphology. The lateral boundary of estuaries that occur on open, linear coastlines may be defined by extending a line seaward from where the shoreline changes from estuary to non-estuary morphology (refer to Figure 1). In narrow or constricted inlets, the lateral boundary of the subtidal portion of the estuary may be defined by extending the -20 metre shoreline directly to the shoreline as outlined in Figure 2. These guidelines are presented to assist mappers in establishing estuary boundaries. Prior to establishing these boundaries, each estuary should be assessed using the best information including field observations.
Note that the higher high water and lower low water have slightly different definitions for Canada and the United States. Canadian marine charts show the chart datum at the "lowest normal tide," where the US defines chart datum at "mean lower low water". As a result, the US chart datum is slightly higher in elevation than the Canadian chart datum (Thomson, 1981).
Figure 1 - Planimetric diagram of an estuary on an open coastline.
Figure 2 - Planimetric diagram of an estuary in narrow, constricted inlets.
