Watersheds and Drainage

Your supervisor asks the question that gives this mission its name: 'If a single raindrop lands anywhere in this basin, can you tell me exactly where it ends up?' It sounds impossible until she shows you that the DEM already contains the answer, you just need to trace it.

A watershed (or drainage basin, meaning all the land that drains to the same outlet) is all the land where rainfall eventually flows to the same point, usually a river mouth or lake. Every drop of rain, given enough time, moves downhill toward its watershed's outlet.

Delineating a watershed (mapping out its exact boundary) means using the DEM to trace, for every cell, which direction water would flow, always toward the lowest neighboring cell, until you can group cells into the areas that drain to the same outlet.

A watershed works like the funnel of a bathtub drain, but stretched across an entire landscape. No matter where you pour water onto that funnel, gravity walks it toward the same drain. The ridge lines are the rim of the funnel; cross over one and your water heads toward a completely different drain instead.

Click anywhere on the basin to trace the flow path a raindrop would take to the coast.

Watershed boundaries define almost every environmental policy decision in the real world, from water rights to pollution regulation, because contamination or land use anywhere inside a watershed can affect the water quality at its outlet, even miles away. This is exactly why Bluewater Basin's agricultural zone (upstream) matters for Millhaven's water quality (downstream at the outlet).

Explain why two farms on opposite sides of a ridge, only 200 meters apart, might drain into completely different rivers.