whatRain falls on a smooth, rolling landscape and cuts it into country: valleys deepen, tributaries find trunks, ridgelines sharpen between them, until the map is a branching tree of streams. The land is drawn as hachures, the old cartographers' downhill strokes, and the streams as lines that thicken as they gather. Click to drop a cloudburst and gully one hillside.

whyNo one plans a river network, yet every continent is covered by the same branching form. It appears because water that happens to gather cuts deeper, and deeper cuts gather more water. The piece shows that loop sharpening smooth chance into structure you could navigate by.

howThe ground is a field of heights made of smooth seeded noise. Raindrops land at random, roll downhill with a little momentum, pick up ground while they move fast and have capacity to spare, and drop it where they slow. That is all. Once a second the piece traces where the water would gather, every cell passing its flow to its steepest neighbour, and draws the cells that collect enough as streams, thin at the tips and bold at the trunks; the count of junctions is the branching. The hachure strokes lie along the downhill direction, longer and darker where the slope is steep, so ridges and valley walls emerge by themselves. The sparkline is the total stream length growing as the network cuts in. One integer seeds the terrain and the rain.