Watersheds

Every drop of rain lands on a slope that tips it toward one stream system or another. The line separating two such systems is the drainage divide, and it runs along ridgelines and hilltops, the high points where water is forced to flow one way or the other. Trace that line all the way around a river and you have mapped its watershed: every square meter of land whose runoff eventually funnels into that one channel. Two raindrops falling two meters apart can land on opposite sides of a ridge and end up in entirely different rivers, even different oceans.

Once rain hits the ground, three things decide how fast it reaches the stream: slope, vegetation, and soil. Steep slopes send water downhill quickly with little time to soak in. Vegetation slows water down, its roots and leaf litter holding soil in place and opening channels for infiltration. Loose, porous soil lets water sink in rather than run off; compacted or saturated soil forces more water to travel overland. Runoff volume itself is simple arithmetic: watershed area times rainfall depth times the fraction that runs off rather than infiltrates, giving the total volume of water pouring into the stream network.

This is why watersheds matter beyond mapping. Change what covers the land and you change the hydrograph, the graph of how a stream's flow rises and falls after a storm. Pave over forest or fields and infiltration drops sharply, so a bigger share of every storm becomes fast surface runoff. That produces higher, earlier flood peaks downstream, tying watershed land use directly to flooding, erosion, and water quality problems covered elsewhere in this unit.