What is a watershed?

And why are they important?

A watershed is an area of land that drains all the streams and rainfall into a common outlet, like a lake, river, or ocean. Water falls as rain or snow, and then flows downhill, coming together as streams and rivers, and eventually downward through the watershed into a body of water. The description of this natural process leads watersheds to be misunderstood as “drainage basins,” but watersheds include more than just water. They also contain the full spectrum of people, animals, and plants that live and grow inside the system.

Ultimately, a watershed is about where the water comes from before it arrives at the destination where you find it – be it the beach or your tap. Given the interconnected nature of water systems, the perceived scale of watersheds can vary wildly. For instance, LA County contains some major local watersheds, like the Santa Monica Bay watershed on the westside of LA County. But at their source, many local watersheds rely on water from far off places outside of political boundaries. Given that watersheds are about where water is coming from, even the Sierra Mountains are a watershed for LA County! The health of that water system – seemingly far off – impacts the availability of water for local residents in LA County.

Why are watersheds important? Because without water, there would be no life. Water is the backbone of any ecosystem. Everything inside watersheds, therefore, is interconnected. What happens upstream, near the source of a watershed’s water, affects what goes on downstream, and all through the basin.

Major Watersheds

The Los Angeles River Watershed is 834 square miles, spilling out from the Santa Monica, Santa Susana, and San Gabriel Mountains north of Los Angeles before funneling down south into the Los Angeles Harbor in Long Beach. The watershed’s land is put to many different uses, with hundreds of miles of forest and open space, hundreds of thousands of homes, streets, freeways, factories, farms and apartment buildings.

This watershed in the eastern part of Los Angeles County is 640 square miles, stretching from the San Gabriel Mountains down into the river, where it then flows south into Alamitos Bay.

This 133-square-mile watershed in the southern part of LA County includes the cities of Inglewood, Hawthorne, Gardena and Torrance. Nearly a million people live within this watershed, which also supports numerous factories and businesses.

The Santa Clara River, one of the only natural river systems in Southern California, originates in the San Gabriel Mountains, near the town of Acton, and flows west, crossing into Ventura County before emptying out into the Pacific Ocean between the cities of Ventura and Oxnard. The 1,030-square-mile watershed lies mostly in LA County with portions stretching into neighboring Ventura and Kern Counties.

This sprawling, 3,226-square-mile watershed, about a third of which is located in LA County (the rest in Kern County), is unique in that it doesn’t outlet into the Pacific Ocean. Rather, the water that originates in numerous streams from the mountains above flow across the valley and pond in dry lakes. Most of this watershed’s land remains undeveloped.

From Malibu in the north to Rancho Palos Verdes in the south, the Santa Monica Bay Watershed stretches across 55 miles of coastline. This 385-square-mile watershed is home to a vibrant combination of undeveloped natural space and urban centers.