Everything here starts with the rock.

Rain is faintly acid. What that acid meets underground decides everything above it. Limestone and marble dissolve in it and release calcium, which leaves the soil sweet, the water hard and the ground full of the fissures and springs that hard water makes. Granite and gneiss barely react at all, so nothing neutralises the acid, and the soil stays sour, thin and hungry over rock that has not moved in a billion years.

That is why a gardener on the Canadian Shield can grow blueberries by accident and fights for cabbages, while a gardener an hour away on Ordovician limestone has the opposite problem. Same climate, same weather, same seed packet — different rock.

The kinds of rock, and how each one forms →

Everywhere on land belongs to exactly one basin, and the boundary between two of them can run down the middle of a field. Rain falling on one side of that line reaches the Gulf of Mexico; rain on the other reaches Hudson Bay. Nothing on the ground marks it, which is why almost nobody knows which one they live in.

It is worth knowing because it is the unit everything in the water actually obeys. What washes off the land upstream arrives downstream; a fish that migrates can only reach what its own basin connects to; and the reason the same species is native in one river and invasive in the next is usually that the two have never been joined.

A discharge figure on its own means nothing — three hundred cubic metres a second is a drought on the Fraser and a flood on a prairie creek. What matters is the ratio: how much water is moving compared with what normally moves on this date, worked out from ten years of record rather than from an average that one wet spring can drag around.

Low water concentrates fish and makes them nervous, and warms fast enough that a hard fight can kill one you meant to release. High water pushes them into the slack edges and makes crossings that are normally nothing genuinely dangerous. Both are worth knowing before you leave the house.