Depending on English dialect, a watershed can mean either all the land area that belongs to the same drainage basin or a boundary that divides drainage basins. The World Conservation Union, also known as the IUCN or International Union for the Conservation of Nature and Natural Resources, maintains an online atlas of watershed maps:
Water Resources eAtlas - Watersheds of the World
Water Resources eAtlas - Global Primary Watersheds Map
Although transient by the timescale of geology, watersheds endure longer than the jurisdictional boundaries of human polities like states, provinces and nations. Persistence and predictability make watershed boundaries more suitable than political boundaries for mapping the Meltwater geochronology.
By their effects on rainfall and agricultural zone location, climate change and sea change are expected to drive rapid, massive migrations and also probably wars of invasion. Surely this turmoil will make political jurisdictions more transient, less predictable and less empirically evident than they’ve been in recent generations.
We, who collaborate to create that fictional future, must learn the way watersheds divide the continents and islands, memorize the shapes of drainage basins and name all the lands after the rivers, lakes and courses where their waters run. Of course we will have to include the desert and coastal basins that the IUCN omits. Every atlas, globe and puzzle we publish can deepen this watershed world view.
Using map colors to distinguish drainage basins instead of nation-states, we can develop legends, methods and conventions that use color mixing and shading within each watershed to show its tributary basins, wetlands and regions covered in summer by ice or snow. We can also inscribe some of our cartographic works with multiple coastlines showing sea-level highstands at different dates in our geochronology, the innermost and highest being dated after the polar ice is gone.More....