Along with the San Francisco Bay, it forms the West Coast's largest estuary and is the bottom of a watershed that drains more than 40 percent of California.
No longer rich in game, no longer timbered and no longer a swamp interlaced with tidal sloughs, the Delta's physical structure has radically changed since the Gold Rush.
Once a giant marsh, it has been transformed over 150 years into a maze of channels hemmed in by more than 1,100 miles of levees. Those levees were first built in the late 1800s to reclaim the marsh for farmland. Over time, the levees got higher, the dry land subsided and there are now nearly 60 sunken "islands" in the Delta.
The Delta has been so fundamentally changed that restoration to pre-Gold Rush conditions is all but impossible.
Today, it is a curious pocket in 21st-century California, just a short drive from the suburbs of the East Bay and the state capital. Boat wakes lap against pilings and banks. The pace is more leisurely. It's scruffier.
In places, it looks more like Appalachia or the South. Around its perimeter, it is, in places, developing rapidly.
Like unpopulated versions of New Orleans, these islands are like bowls up to 20 feet below the surface of surrounding water.
For all its laid-back calm, however, the Delta is critical to California.
It is the heart of a vast, aqueous circulation system that moves water from Northern California -- where 75 percent of the available water is -- to the San Joaquin Valley and Southern California, where 80 percent of the demand is.
Delta water irrigates 7 million acres of the most productive agricultural land on Earth in the San Joaquin Valley and is essential to the state's $27 billion farm industry.
The Delta is the state's most important fishery habitat and supports 80 percent of the state's commercial salmon fishery.
It supplies drinking water for 23 million Californians, and it is the sole source of tap water for more than 450,000 people in Contra Costa County.
Statewide, it makes up about one-fourth of the water supply for California.
Still, for millions of Californians who depend on it, the Delta is out of sight and out of mind.
"The Delta is not an area that most Californians really resonate with, the way they do with the coast or the Sierra," said Mary Nichols, former Resources Agency secretary who now heads the Institute of the Environment at UCLA. "We have a history of not paying attention until it's too late and then trying to restore things that have vanished." Jay Sorensen, 69, has been watching the decline for decades.
Motoring in a bass boat on the glassy green water, the fisherman -- who knows the Delta's moods and rhythms as well as anyone -- recalled better days.
As he spoke, low clouds allowed hints of the orange sunrise to linger on the horizon into the late morning. Mount Diablo's silhouette perched in the distance, a familiar piece of geography in the otherwise disorienting tangle of channels that make up the modern Delta.
A great blue heron slid by on the wing.
"We used to go out in May just before sunset. You could look east and west, up and down the San Joaquin River. The water, on the surface, would just be boiling with huge striped bass," Sorensen recalled.
It was the 1960s. The innumerable, thrashing fish were spawning. You could smell it, he said.
One evening in the spring of 1972, Sorensen returned to the familiar spawning grounds.
There were no fish. He went out again the next evening. Again, no fish.
They were gone. Sorensen and his buddies still fish for striped bass, but the fish are smaller and fewer.
The uproarious eruption of spawning stripers on the San Joaquin is now just a memory residing in the minds of a few longtime anglers.
For Sorensen and many others, the culprit is obvious.
Massive pumps that deliver water to the San Joaquin Valley and Southern California were cranking up about that time the stripers were falling off.
"I always thought this was my Sistine Chapel out here," said Sorensen. "To see what the human race has done, it's brought tears to my eyes to see what's happened to our Delta."