Salish Sea Basin Boundary Notes

(Further discussion on the Salish Sea Basin Boundary as depicted on the Map of the Salish Sea and Surrounding Basin)


The Map of the Salish Sea and Surrounding Basin depicts a ‘basin boundary’ surrounding the Salish Sea that excludes the entire upper section of the Fraser River (and its associated hydrological catchment area). In part, this was done simply because the upper watershed of the Fraser River is such a large area that to include all of it greatly changed the scale of the map itself. The intent of the map was, first and foremost, to highlight the Salish Sea. As such, excluding the upper Fraser River allowed that Sea to be the primary focus of the map. Moreover, the desire was to capture that area of land that is most associated with, most influenced by, and which has the most influence upon, the marine waters of the Salish Sea. Certainly the Upper Fraser River has a large influence upon Salish Sea, but in many ways that influence is both qualitatively and quantitatively different from those areas that are closer to the Sea. Thus, in defining the Basin Boundary a number of factors were considered:

     - Hydrological catchment areas, i.e, the combined watersheds of the individual rivers draining into the Salish Sea (with the notable exception of the upper reaches of the Fraser River)
     - The area identifying itself as a ‘maritime’ culture
     - The area identified as being a ‘maritime’ climate
     - The extent of the area that was within sight of the Salish Sea (i.e. the uppermost ridge line of the bowl-like land area surrounding the Salish Sea)

For all of these factors the upper Fraser River can be seen as being distinct from the lower Fraser River. Of course, delineating a clear break between the ‘upper’ and ‘lower’ sections of a river is impossible, so a line was chosen that (more or less) fit with the general line of the hydrological catchment areas on either side of the Fraser River, striving to make the basin look ‘right.’



The term 'Watershed' is generally used to describe “the region or area drained by a river, stream, etc.” More specifically, a watershed is the entire area of land draining to a given point (typically the mouth of a river or stream). This is also referred to as a hydrological catchment area or a hydrological basin. Given this definition, the term watershed can be applied to the mouth of a river or to a lake (if one excludes the outfall of the lake). It is not, however, appropriate with regards to a body of marine water. In the example of the Salish Sea, which is open to the Pacific Ocean at the mouth of the Strait of Juan de Fuca as well as at Johnstone Strait, it is impossible to define a true watershed (i.e., the land that drains into the Salish Sea). If one tried, it would have to include Japan and Alaska, etc. as those lands drain into the same water that enters the Salish Sea too... Since the entire ocean is effectively one body of water, the entire world is the watershed for any / every body of marine water (one quickly realizes the difficulty of precisely defining bodies of ocean as distinct areas at all). Since the Salish Sea does not have a ‘low point’ where the water drains out and exits the Salish Sea, it is impossible to define a watershed. Indeed, no marine body of water can be described as having a watershed. The best one can do is define the combined watersheds of the rivers that flow directly into that body of water.

Such a delineation (of the combined watersheds of the rivers that drain directly into the Salish Sea) is indeed what is used on the Map of the Salish Sea and Surround Basin in all cases except for the Fraser River. In the case of the Fraser River, however, the entire upper portion of it’s watershed has been eliminated from the map (for reasons discussed above and below). As such, the use of the term watershed (for the Fraser River and thus for the Map) is not appropriate.

Of course watersheds are often broken down into sub-watersheds, typically for individual creeks or tributaries, with a each sub-watershed starting at the mouth of that creek. This allows the sub-watersheds to be analyzed and discussed individually. But these sub-watershed are still a part of the total watershed of the river. So, for example, if one chose to divide the upper Fraser River catchment area into sub-watersheds one might do so at Lytton, BC, where the the Thompson River joins the Fraser River. One can then talk about and consider the properties of these distinct sub-watersheds, but one cannot treat the remaining lower Fraser River watershed in the same way. When describing the watershed for the lower section of the river one must include both of the upper, sub-watersheds as being a part of the watershed of the Fraser River. If one is talking about a watershed for the Fraser (i.e., from the mouth of the river) one has to include ALL of the sub-watersheds since all of the Thompson River as well as all of the Fraser River drains into the Salish Sea via the Fraser River. So while one treat the up-river sub-watersheds as an individual entity (i.e., the watershed for just the Thompson River), apart from the rest of the Fraser, one cannot consider the Fraser River watershed without including the Thompson River sub-watershed. And again, since the map does not include the entire watershed for the Fraser River, the use of the term watershed boundary (for just the lower portion of the Fraser River) does not apply.



Given that the term watershed is not appropriate for the Map of the Salish Sea, the term 'Basin Boundary' was used instead. Which begs the question, what is a basin? By some definitions (i.e. a ‘hydrological basin’) the term is essentially the same as a watershed. More generally, however, the term basin is less clear cut, a bit more casual, and is a term with alternative definitions and usages. Basin implies a bowl or (geographically) a bowl-like area surrounding a low point or body of water. And again, for most of the area surrounding the Salish Sea the watershed boundaries (of the individual rivers) and our 'basin boundary' are the same. Except for the Fraser River, the Strait of Juan de Fuca and the Johnstone Strait, where the term watershed does not apply, but one can imagine the sides of the bowl extending across these gaps. Thus the boundary is more of a conceptual bowl than an true hydrological one. The term 'basin,' being less well defined, is better suited.

In addition to the definition of a basin as being the hydrological drainage area for an individual river, there are a number of other definitions that are (perhaps) relevant (emphasis added):

     - A hollow or depression in the earth's surface, wholly or partly surrounded by higher land
     - A larger area consisting of a number of watersheds for a group of rivers or streams
          (i.e., a basin is a collection of watersheds)
     - A large, bowl-shaped depression in the surface of the land or ocean floor
     - A partially enclosed, sheltered area along a shore where boats may be moored: i.e, a yacht basin

The term basin can also be used to apply to other aspects of the region besides hydrological, i.e., a "cultural basin" being that area that is populated by people influenced by and influencing the Salish Sea. Likewise, weather patterns within the Salish Sea basin can be thought of as different (in a variety of qualitative ways) from those regions along the Pacific coast, or further north, south or east of the area defined. So one might describe a ‘weather basin’ (or micro-climate).

Note that the term catchment likewise has non-hydrological usage: i.e., the catchment (or service) area for a school or hospital. However, the term catchment is less commonly used than basin, and less descriptive for describing the surrounding geography.

The bottom line is that the boundary defined on the map is Not a watershed boundary. Nor is it a hydrological catchment area or a hydrological basin boundary. None of these terms apply so are not appropriate for use on the map. But the more vague term ‘basin’ boundary can’t be ruled out in quite the same way, and thus became the chosen term for the map, appreciating the fact that it sounds a lot like a watershed but also could be applied as a ‘cultural basin’ etc. Admittedly, most people probably confuse the term basin with the terms watershed or catchment, and that association is appropriate, since most of the basin boundary could also be so defined. At the same time, ‘basin’ allows for those areas where watershed is not appropriate and can include other, non-hydrological definitions. In short, the goal was to be as descriptive without being scientifically incorrect.

Another possible, but ultimately not applicable term, is viewshed (i.e., that area that is visible from a given point). While viewshed doesn't strictly apply (the terrain is too undulating), along most of the basin boundary one could imagine looking out and seeing the salt waters of the Salish Sea from the ridge (i.e., the basin boundary is the defining ridge at the edge of the bowl). Or that the ridgeline of the basin boundary could be seen from the waters of the Salish Sea. Of course, the further one travels away from the Salish Sea the less this would seem to be true (i.e., the Upper Fraser River watershed). So while the basin boundary is definitely not a viewshed, this sense of the area within sight of the salt water was another part of the logic for splitting the Fraser River (and its associated drainage area) as we did.


The area now defined as the Salish Sea has also been variously described as the Puget Sound / Georgia Basin (or just the Georgia Basin). The Encyclopedia of the Puget Sound defines these marine waters as:

“The Georgia Basin is an international water body that encompasses the marine waters of Puget Sound, the Strait of Georgia, and the Strait of Juan de Fuca. The coastal drainage of the Georgia Basin is bounded to the west and south by the Olympic and Vancouver Island mountains and to the north and east by the Cascade and Coast mountains.”


Approximately the same area defined as the Basin Boundary of the Salish Sea was identified as the ‘Ish River Country’ by David McCloskey (of the Cascadia Institute) on his Ish River Country map in 1987. McCloskey describes the Ish River region as a sub-region of the larger Cacadia ecoregion.



See also map of the Combined Watersheds Draining to the Salish Sea which includes the full watershed of the Fraser River. Note that the watershed for the upper Fraser River is larger than the entire area described as the Salish Sea Basin...

Page Updated 10.26.2018