Watershed
A watershed is the area of land that drains all its precipitation and runoff to a common outlet such as a river, lake, or bay.
Definition
A watershed is all the land where rain flows downhill to the same river, lake, or ocean, bounded by ridges called drainage divides, and watersheds nest inside one another, with small creek watersheds joining to form river watersheds that in turn join larger basins draining to the sea. As the fundamental unit of hydrology, a watershed's water balance can be closed at its outlet, precipitation equaling evapotranspiration plus discharge plus any change in storage, and its size, shape, slope, soil, and land use, captured in morphometric measures such as drainage density and stream order, determine how quickly water and pollutants reach that outlet and how prone the watershed is to flooding.
Example
Rain falling on most of Charlotte flows into creeks that drain to the Catawba River, so the city sits in the Catawba watershed, and North Carolina has $17$ major river basins split by the Eastern Continental Divide along the Blue Ridge, with rain on the east side reaching the Atlantic via rivers like the Catawba and Yadkin and rain on the west side reaching the Gulf of Mexico via the French Broad, Tennessee, and Mississippi. The Mississippi watershed alone drains about $3.2$ million $\text{km}^2$, $41\%$ of the contiguous United States, and the Catawba-Wateree basin covers about $12{,}000\ \text{km}^2$, supplying drinking water to over $2$ million people through a chain of $11$ reservoirs.
Key Insight
Anything dumped on the ground in a watershed, such as oil in a driveway, can end up in that watershed's river, and watersheds ignore political borders entirely, since the Catawba begins in the North Carolina mountains and ends in South Carolina, meaning what one state does upstream affects the other downstream. Over geologic time even the boundaries themselves can shift, as headward erosion by an aggressive stream breaches a divide and steals the upper reaches of a neighboring watershed, reorganizing entire drainage networks.