A watershed or drainage basin refers to a system controlled by topography  defining how water will flow. You refer to a watershed by the largest body of water that creeks, rivers or streams feed into. For example, all creeks that flow in the San Francisco Bay are part of the San Francisco Watershed. However, there are many smaller watersheds within this area depending on flow patterns. Alameda Creek Watershed would be an area that drains into Alameda Creek.

When it rains, water will flow into its appropriate watershed. A ridge or a topographical higher areas that connects two water areas is called a divide. In the United States there is an area in the Rocky Mountain called the "continental divide." This refers to water on the east of the divide flows into the Atlantic. The water on the west side flows into the Pacific.

Continental Divide