A large water system in the City of David has been precisely dated to nearly 2,800 years ago, a discovery that sheds new light on the engineering prowess of Iron Age Jerusalem. The discovery, published in the Proceedings of the National Academy of Sciences (PNAS), shows that the extensive Siloam Dam was built between 805 and 795 BCE, during the reign of either the biblical king Joash or Amaziah of Judah.

Excavation in the City of David. Opposite – the dam wall. Credit: Emil Aladjem, IAA

Archaeologists from the Israel Antiquities Authority, working with scientists at the Weizmann Insтιтute of Science, found that the dam was constructed in direct response to changing climate conditions that were marked by harsh droughts punctuated by violent flash floods. By using advanced radiocarbon dating techniques on fragile organic material such as straw and twigs that had been embedded in the mortar of the dam, researchers were able to pinpoint its construction within an unusually specific 10-year time frame.

“This is the largest dam discovered in Israel so far and the earliest one discovered in Jerusalem,” said the excavation directors Dr. Nahshon Szanton, Dr. Filip Vukosavović, and Itamar Berko. “Its dimensions are remarkable: about 12 meters high, over 8 meters wide, and the uncovered length reaches 21 meters, continuing beyond the limits of the current excavation.”

The straw that broke the dam: microscopic straw samples embedded in the dam’s mortar during construction led to an exceptional dating achievement. Credit: Dr. Johanna Regev / Scientific Archaeology Unit, Weizmann Insтιтute of Science

The dam, which deflected waters from the Gihon Spring to the Siloam Pool and simultaneously captured seasonal floodwaters from the Tyropoeon Valley and Kidron Stream, was a double solution to both scarcity and flood. The project formed part of a comprehensive urban water system, also including a fortified tower at the spring and channels leading into the pool.

The mᴀssive dam wall uncovered in the Israel Antiquities Authority excavations at the Pool of Siloam in the City of David. Credit: Eliyahu Yanai, City of David

Dr. Johanna Regev and Prof. Elisabetta Boaretto from the Weizmann Insтιтute of Science, who directed the scientific dating, explained: “Short-lived twigs and branches embedded in the construction mortar of the dam provided a clear date at the end of the 9th century BCE, with exceptional resolution of only about 10 years – a rare achievement when dating ancient finds.”

To put the discovery into perspective, the scientists compared their results with ᴅᴇᴀᴅ Sea drill core climate records, Soreq Cave stalagmite records, and records of solar activity. Combined, the evidence suggests that Jerusalem’s rulers deemed it necessary to enact a vigorous response to unpredictable rainfall patterns. “Our research points to sweeping urban planning for managing Jerusalem’s water system as early as the 9th century BCE – evidence of the city’s power and strength,” the researchers concluded.

The dam wall from the days of Joash or Amaziah, kings of Judah. Credit: Emil Aladjem, IAA

The dam’s scale shows that the kings of Judah had vast resources and organizational ability many years earlier than the reign of King Hezekiah, who is better known for later waterworks, including the famous tunnel built around 700 BCE.

Dr. Szanton noted that this royal construction project had lasting consequences for Jerusalem’s development. “This mᴀssive project influenced the southern and western parts of the city – including Mount Zion – which relied on the waters of the Siloam Pool.”

The study not only sheds light on Jerusalem’s engineering expertise in ancient times but also provides an ancient example of how societies responded to environmental challenges.

More information: Regev, J., Szanton, N., Vukosavović, F., Berko, I., Shalev, Y., Uziel, J., … Boaretto, E. (2025). Radiocarbon dating of Jerusalem’s Siloam Dam links climate data and major waterworks. Proceedings of the National Academy of Sciences of the United States of America, 122(35). doi:10.1073/pnas.2510396122