The St. Lawrence crosses at Cape Vincent already carrying — miles of stream off — square kilometres of Great Lakes basin, none of it drawn on this map. Everything the basin below adds is — miles. So the trunk is not measured here, it is flagged: every reach with Lake Ontario above it is drawn at the top of the scale, where it belongs. The lake at the western edge is dimmed for the same reason — it is water arriving, not water this basin collects.
Every stream is drawn as wide as the water above it. NHD ships a stream order that is null across most of this basin, so the network was rebuilt by snapping — reaches end to end and accumulating length downhill.
The American side is USGS’s NHD High Resolution, three subregions of it. Canada is Canada’s own National Hydro Network, — Water Survey work units of it, said in NHD’s vocabulary so that a reach in Quebec and a reach in New York go through the same code and come out drawn the same way. Both surveys will happily draw the other’s country, and letting them would draw a thousand kilometres of border twice in two different hands. So the seam is the border itself, which is at least a real line on the ground.
In the east the divide stops at Pointe-des-Monts, where the estuary becomes the Gulf. Past there the Romaine, the Natashquan and the Lower North Shore drain to a sea rather than to a river, and they are a different map. In the west it stops at Lake Ontario, on purpose: the Great Lakes are a third of a continent draining in, and they already have a sheet of their own. Everything between those two lines is here, both banks, all the way out.
A river that the border runs down is listed once, not twice: the Richelieu is Rivière Richelieu above Saint-Jean and Richelieu River below Rouses Point, and the two are folded together here. A name has to carry ten miles of channel of its own to make the list at all, which is what keeps a two-mile brook out of it when GNIS hangs its name on a reach of the river it runs into.