Erosion
The land starts as a heap of noise: lumpy, aimless, obviously not a place. Turn the rain on and the water finds its own way down, and within a minute it has cut ridgelines, valley networks and river deltas that nobody drew.
Press and hold, or hold B, and the rain is undone for as long as you hold it: the same heap of noise with a lake in every hollow and nothing joined up. Split the picture and the left half stays as it began while the right half goes on eroding.
The land you start with
Nothing in here knows what a river is. It is layers of noise added together, each one half as tall and twice as fine as the one before, and the number of layers stops climbing once the finest of them would be smaller than a grid square.
What the water does
A drop lands somewhere, rolls downhill, and carries as much loose ground as its speed and its remaining water allow. Where it speeds up it takes more; where it slows down or dries out it puts the difference back. That is the whole of it, and valleys are what it leaves behind. Below the sea, or in any hollow the water has filled, a drop stops cutting and lets go of what it carries, which is what builds a delta at a river mouth and slowly fills a lake in.
Left to itself the rain would grind the whole map flat: it takes from the ridges and hands the material to the floors, and after a few million drops there is nothing left to cut. So the land is held up. Whatever the rivers carry off is pushed back in the broad pattern the map began with, ranges up and basins down, at the rate it is being lost, and the relief stays where it started while the rivers keep cutting into it. That is what a living mountain range does: the Alps rise a millimetre or so a year and lose about the same to the Rhine and the Po.
Water alone leaves ridges like knife edges, because nothing pulls a slope down once it is too steep to stand. Slumping does that job: anything steeper than the angle below sheds material onto the slope beneath it, which is what puts screes under the crags and softens the divides.
Light, water and rivers
The shading is the slope and the way it faces against one sun, how much sky each square can see, and the shadow the ridges throw when the sun is low. Standing water is found by flooding the map from the sea, or from its lowest point if there is no sea, so every hollow fills to the height it would spill over at. Rivers are drawn from how much of the map drains through each square, and are wide where a lot does.