Archaeologists in San Gabriel have uncovered an important piece of Southern California’s history: the foundation to an ancient water distribution system that has laid buried a few feet beneath the surface of the old Union Pacific Railroad tracks for more than century.

Alameda Corridor-East Construction Authority’s lead archaeologist John Dietler said the feature, a series of stone-lined ditches and reservoirs, is the missing piece of an important story behind the success of the San Gabriel Mission and ultimately the springboard to the growth of Los Angeles.

“What this represents in the bigger picture are the very roots of Los Angeles,” Dietler said. “Figuring out how to control water, bring it to people and to provide enough water to support a large population is really the key to the dense European settlement that ultimately flourished here in the Los Angeles Basin.”

Archaeologists believe what they’ve found is a precursor to Chapman’s Millrace, which was discovered during earlier phases of construction a few feet from the same site, located across the street from today’s mission building.

“Some of the histories you read make it seem like Chapman showed up out of thin air and invented this idea, and basically he took something that was already here and just made it better — literally by stealing rocks from it and building right on top of it,” Dietler said.

In order to lay the groundwork for the ACE project, archaeologists have spent the past few years excavating a major trench through one of the most important archaeological sites in the entire country.

Throughout the excavation process, archaeologists have discovered hundreds of thousands of artifacts and several features, including adobe brick foundations and water distribution systems.

With their findings, archaeologists are in the process of recreating a map to show how the community was laid out on the mission property, aside from the buildings that are still standing today. They have a list of more than 150 buildings, but the oldest map created 20 years after the mission closed shows only the location of three of them.

Although the artifacts will be documented and preserved, the features themselves will ultimately be destroyed.

“These earlier things are unmortared cobbles. It’s big and impressive, but there’s nothing but dirt in between them,” Dietler said. “It would be virtually impossible to preserve them.”