Rain that lands on high ground doesn't wander at random — every drop takes the steepest way down. Add up how many uphill cells drain through each spot and the rivers draw themselves: the busiest channels are where the most land funnels its water. Geologists call that number the drainage area.
All the land that feeds one river is a watershed, or drainage basin. The high ridge between two basins is the divide — stand on it and a step either way sends your raindrop to a different sea. North America's Continental Divide decides whether snowmelt reaches the Pacific or the Atlantic.
Divides aren't permanent. Lower one notch of a ridge and a river can be captured — rerouted into its neighbour's basin, stealing all its water. That is a real, dramatic thing rivers do to each other over geologic time.
Try it: dig a valley to grow a trunk river to the coast, raise a ridge to split the land in two, then breach the divide beside the spring and watch its river switch oceans.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.