When Padre Juan Crespi first sighted the Sacramento-San Joaquin Delta in 1772, he thought he would be able to walk around it.

The Spanish missionary and his party of 15 soldiers had been dispatched to find a land route from Monterey to Point Reyes, where Spain hoped to build a port. But 10 days into their journey, in the heart of Alta California, Crespi and his men encountered a maze of water, mud and swamp, instead of solid ground. It was the confluence of the Sacramento and San Joaquin rivers, the largest estuary on the West Coast of the Americas.

Crespi expected the estuary to function like others he had seen, fragmenting into dozens of small braided channels fanning out toward the sea. Upstream, he figured, they would find a single channel to cross.

But this estuary did the opposite. As Crespi traveled upstream, the water spread out, and his hopes thinned. On foot and horseback for three days in March, he and his companions searched fruitlessly for a way through the tangle of channels.

"Crossing these rivers by boat or canoe would be apt," a chastened Crespi wrote in his diary. "Because if you do not, it's (necessary) to climb the mountains to the southeast and seek the path of the large river. To climb such a high pass certainly requires a greater number of soldiers and more provisions, which is why I withdrew."

Crespi was the first European to glimpse this odd California landscape, and the first of many to be confounded by it.

Sixteen rivers and hundreds of creeks converge from all over California on the Delta's vast central plain – all mud, tules and marsh – finally forming one mighty river that drains the state's whole churning belly. It's called an "inverted" estuary because its waterways unite before reaching the sea. The only place comparable is the Okavango Delta in Botswana.

When Crespi encountered the estuary, its floodplain extended 100 miles north and south, filling the Central Valley with a wealth of snowmelt, all of it destined to squeeze through the land gap later called Golden Gate. Within a century of Crespi's expedition, European settlers were trying to engineer their own logic into the place, trenching new channels and building levees to create some of the world's richest farmland. Today, the Delta is crossed by three state highways and hundreds of miles of railroad tracks and county roads. There are 1,100 miles of navigable channels, and 72 islands ringed by levees. Modern charts detail where to anchor, where to catch the best striped bass, where to find the most convenient bridges and ferries.

But the levees may be vulnerable to earthquakes. If they fail, the water supply would be compromised by a flood of salty water from San Francisco Bay. And rising sea levels could taint the water supply permanently.

The Delta, which still covers an area the size of Rhode Island, provides half of all the freshwater consumed by a thirsty state, serving 3 million acres of farmland and 25 million Californians from Silicon Valley to San Diego. Gov. Jerry Brown hopes to better serve them by spending $15 billion on a new water-diversion system. If approved this year, it would shunt a portion of the Sacramento River out of the estuary into two giant tunnels, 30 miles long and 150 feet underground. The intent is to divert freshwater in a way less harmful to imperiled native fish species, while protecting those diversions from floods, earthquakes and a rising sea. The tunnels would serve existing state and federal canal systems that begin in the south Delta, near Tracy, and divert water to cities and farms, mostly in Southern California and the San Joaquin Valley. Another $10 billion would go to wildlife habitat improvements, in part to breach levees and restore tidal action to some islands. The state believes the proposed tunnel intakes would be far enough upstream to protect the water supply from disaster, at least under present climate-change scenarios. The intakes would also include modern fish screens, potentially preventing the extinction of the native Delta smelt, spring-run chinook salmon and other species that are being killed by the current water export system.

But after seven years of study, state officials acknowledge that removing so much freshwater upstream may cause "unquantifiable" water-quality changes. Meanwhile, critics say taking so much freshwater from the estuary could harm Delta farms and perhaps concentrate pollutants in a way that hurts the same fish that state officials hope to restore. The Delta continues to confound.

Forty-five years after Crespi turned back, Padre Narciso Durán came through with two small boats on an expedition led by Lt. Don Luis Arguello. They left the Spanish presidio, or fort, at San Franciso, which was established three years after Crespi's visit. Their trip through the watery maze began on May 13, 1817, and lasted two weeks. Durán, who kept a journal, came along to baptize Indians.

His party had a hard time from the start: They set out in a storm, and the boats became separated at the confluence. The storm blew Arguello onto the shore of the San Joaquin River, near its mouth. Durán and a second padre, in the other boat, took refuge for the night on a soggy mound of tules in the middle of the Sacramento.

When the storm finally quit and the boats were reunited, another challenge arose. It was snowmelt season, and the downstream current in the Sacramento River was so strong that it nearly halted their progress. Without wind, days of brutal rowing followed, with little upstream progress to show for it.

On top of that, the men experienced a condition that plagues Delta visitors to this day: They became disoriented.

Seeking to remain on the Sacramento River, the party soon encountered a variety of branching side-channels. They could not be sure which one was the river itself. Because the Delta was in flood, the true riverbanks and many of the natural islands were submerged. A gap in the trees that looked like a river channel might turn out to be a flooded island where a boat would quickly run aground. "The thick leafiness makes the whole river like a tree-lined promenade," Durán remarked.

The next day, May 16, they traveled only four leagues upriver. They also took a wrong turn and left the Sacramento on a side channel – a serious mistake, as any detour meant more labor for the rowers. Eventually, though, they got lucky and recovered their course.

Familiarity with this labyrinth benefitted the locals, who fled on rafts as soon as they spotted the expedition boats. The Europeans found two villages vacated, either because of the spring flood or because word had spread that the Indians might be conscripted as laborers in the Spanish missions.

Occupants of a third village "fled at the noise of the launches, leaving only two old women, more than 60 years old."

Durán felt obliged to baptize both women, "because it seemed to us that they could die before Divine Providence could arrange another convenient time when we could baptize them in one or another of the missions."

Durán, who was no naturalist, made no effort to identify important land features or tree species, and does not mention sighting any animals. But the Delta was teeming with wildlife in a way that is difficult to imagine today: Vast herds of elk and pronghorn antelope roamed here, hunted by wolf and grizzly bear. Giant tidal marshes, packed with tules and cattails, hosted millions of waterfowl. The maze of curving sloughs was a nursery for one of the world's most productive fisheries.

The Delta remains the most important salmon fishery on the West Coast, producing most of the wild-caught king salmon in the Lower 48 states. Yet it may not survive. There are 57 endangered species here, including steelhead trout and two runs of salmon.

Modern-day Californians are as oblivious to the region's natural wealth as Durán seemed to be. A January 2012 survey found that 78 percent of California residents don't know where the Delta is, or even what it is.

The day after baptizing the two women, Durán and his party reached their turnaround point. They hoped to find a place to erect a cross, "and there to end our quest and retreat downriver." After rowing upriver three more leagues, they pulled ashore to rest, where, by chance, they spotted some rafts in the tules and a village of Natives, "who came out at them armed with their customary fierce clamor."

Arguello mustered his soldiers to confront the Indians, who "calmed down, to everyone's relief, and said they had armed themselves believing we were hostile people." The travelers were invited to visit a larger village one league upriver, where they were promised fish.

But Durán and his cohorts, possibly disoriented, never found the second Indian village, and never got the promised fish. Exhausted and frustrated, they were ready to turn back. Amid the flood, they could find no solid ground to erect a cross. So they carved one on an oak tree.

The exact location of that cross is unknown today. But according to Durán's diary, they carved it about 80 miles upstream from the confluence of the Sacramento and San Joaquin rivers, or approximately near today's state capital, Sacramento, where Gov. Brown weighs the fate of the Delta today.

Matt Weiser covers environmental issues for The Sacramento Bee and has written about the Delta and California water for 15 years.

The contemporary translation of Crespi and Durán's journals is by Alexa Mergen.