Watershed

A precisely defined geographic region through which drains a dendritic (i.e., bifurcating) pattern of waterways (i.e., rivulettes, streams, and rivers). All water falling on the watershed exits at the bottom of a single valley, usually as a full-fledged river, or flows into a lake that has an outlet allowing the river to resume its journey once the lake is full.

Links
  http://www.ctic.purdue.edu/KYW/glossary/whatisaws.html
http://www.epa.gov/surf/