What is the Delta? Explaining California's Massive Watering Hole

About two-thirds of Californians drink, bathe, brush their teeth, and flush their toilets with water that comes from the Sacramento-San Joaquin Delta. That's roughly 25 million people who get at least some portion of their hydration from one big triangular watering hole.

But ask most folks what the Delta is, and you're guaranteed to get a lot of blank stares. According to one recent poll,  roughly four of five Californians are pretty clueless about it.

It's pretty easy to take for granted that water magically pours out of the tap when you turn your faucet on. But chances are, that H20 has gone through a pretty serious journey to reach you - and it's probably worth knowing where it comes from, and how safe the supply is.

Here's one of many things that's kind of funky about California: the majority of the population lives in the southern part of the state, but most of the available fresh water supply is in the northern half. So, to hydrate the millions of people living and farming in the state's drier regions, California has had to, ahem, tap the crap out of its available resources, pumping water hundreds of miles over large mountains to reach the populations that rely on it.

So what is the Delta?

Just south of Sacramento on the western edge of the Central Valley is where California's two major rivers meet - the San Joaquin from the south and the Sacramento from the north. Between them, they drain almost half of California's freshwater supply, collecting and concentrating rainfall and snowmelt from the Sierra Nevada and funneling it towards the San Francisco Bay.

And this is what makes the Delta: a 700-square-mile inland estuary at the confluence of these two rivers, where fresh water from the mountains mixes with salt water from the Pacific.

Think of it as California's big drain.

How it used to be ...

Before we started messing with it, the Delta was a vast brackish marsh filled with wetland plants (tules) and winding tidal channels. Much of it was submerged; all the fresh water from mountain snowmelt mixed with salt water from the ocean. In fact, when Spanish explorers first viewed the Delta from the top of Mt. Diablo in the late 1700s, they thought they'd discovered an inland sea. The area teemed with birds and game animals, including elk, antelope, and grizzly bears. Its few human inhabitants – small settlements of Miwok Indians – fished and hunted there only during summer months on small areas of dry land.

And how it is now ...

As you may have guessed, things are a bit different today. Over the last 150 years, we've dramatically and irreversibly changed the environment and landscape of the Delta, an epic effort to meet the steep demands of California’s increasingly crowded, thirsty population. Today's Delta is dotted with more than 1,000 miles of earthen walls - called levees. Much of the area has been "reclaimed" for agricultural use - land that's been drained and cordoned off by the levees. Huge pumps now deliver millions of gallons of fresh water to cities in the East Bay, South Bay, and throughout the lower half of the state, as well as farms in the arid San Joaquin Valley to the south.

A fragile system

But the transformation has greatly upset the fragile environmental balance of the region, threatening many of the native fish and plant species that can't survive without steady streams of fresh water. The debate over how much water should be diverted from the Delta has long been a hot button political issue, instigating ongoing battles between northern and southern cities. It's also pitted environmentalists and ocean salmon fisherman against heavy water users, like farmers, who demand large consistent deliveries of fresh water diverted from the Delta.

The modern development of the Delta has also created an equally fragile water delivery system that millions of Californians depend on. Most of the levees are old and crumbling. If hit by a big enough earthquake, many would likely be destroyed. And because much of the reclaimed land in the region is below sea level (because of ongoing land subsidence), the area would be flooded, along with the nearly 400,000 people who live there.

Perhaps even more catastrophic: because the levees separate fresh water from salt water, if they collapse, the two types of water mix, leaving much of the state high and dry without a major source of fresh water.

Pretty serious business.

State leaders have been arguing for years about how to deal with this threat. A multibillion dollar water infrastructure ballot measure has repeatedly stalled, and now Governor Jerry Brown is floating the very costly prospect of building peripheral tunnels that would capture and divert fresh water before it even enters the Delta.

Learn and explore more about the Delta

• Check out KQED QUEST's multimedia series on the Delta, including an extensive set of interactive historical maps.
• Find out where your drinking water comes from (from the Water Education Foundation).