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A watershed or drainage basin is a region that drains into a particular body of water, such as a river, pond, lake, or ocean. Within the City of Los Angeles area, there are 4 primary watersheds — the Los Angeles River, the Santa Monica Bay (Upper and Lower), Ballona Creek and the Dominguez Channel.

The size of a watershed, and the speed and direction of flow of its rivers, is determined by landforms. High ground, such as mountain ranges and hilltops, directs water one way or another. Within each large watershed, there are many, many smaller watersheds.
For example, a small creek that flows into the Santa Monica Canyon has its own watershed, but it is also part of the much larger Santa Monica Bay watershed.

Along the edges of the major watersheds, the precise location where a single drop of rain lands could make a huge difference in its journey downstream and to the sea. Rain falling on the north side of the Hollywood Hills flows into the Los Angeles River, and travels down eventually emptying into the San Pedro Bay.

However, if that same drop of rain were blown by a breeze to the south part of the Hollywood Hills (Griffith Park), it might land just within the watershed boundaries of the Ballona Creek, which flows west and out to the Santa Monica Bay.

Click on map for a larger view.
Related Links

Aerial maps of the LA River

A Vision for the LA River