A recent report concluded that when King Hezekiah built the water tunnel connecting the Gihon Spring with the Siloam Pool, he installed a sluice gate to deprive the invading Assyrians of water when they lay siege to Jerusalem. If the researchers are correct, this would be the first example of men utilizing that mechanism to control water flow.

The Siloam Pool was a significant element of Biblical Jerusalem. The pool is filled by the Gihon Spring channeled by a  1,750-foot tunnel built by King Hezekiah in the 8th century B.C.E. when the city was threatened by the approaching Assyrian army, as recorded in the Bible:

Chizkiyahu stopped up the spring of water of Upper Gichon, leading it downward west of the City of David; Chizkiyahu prospered in all that he did. II Chronicles 32:30

According to the Siloam inscription, the tunnel was excavated by two teams, one starting at each end of the tunnel and then meeting in the middle. The Gihon was heavily fortified, assuring Jerusalem of a water source. But scholars conjecture that by diverting the waters of the Gihon, Hezekiah prevented the enemy forces under Sennacherib from having access to water.

When Chizkiyahu saw that Sennacherib had come, intent on making war against Yerushalayim, he consulted with his officers and warriors about stopping the flow of the springs outside the city, and they supported him. A large force was assembled to stop up all the springs and the wadi that flowed through the land, for otherwise, they thought, the king of Assyria would come and find water in abundance. II Chronicles 32:2-4

The discovery addresses a technical glitch in this plan. The redirection of water through the tunnel would make the water level so low in the upper part of the city to render it virtually inaccessible, thereby depriving the city of its water source.

A recent study published in April in the academic journal Archaeological Discovery titled “A Sluice Gate in Hezekiah’s (Iron Age ii) Aqueduct in Jerusalem: Archaeology, Architecture and the Petrochemical Setting of Its Micro and Macro Structures,” written by Aryeh E. Shimron, Vitaly Gutkin and Vladimir Uvarov—researchers from the Geological Survey of Israel and Hebrew University’s Center for Nanoscience and Nanotechnology, suggested that the tunnel’s engineers had devised an ingenious sluice gate to cope with controlling the water level:  a movable blocking wall, referred to as a sluice, to control the water level in the new aqueduct and thereby also the spring.

“Hezekiah’s engineers were well aware that such a diversion of water from the northern spring source south into the Siloam pool would lower the water level not only in the immediate environs of the Gihon spring cave but also from the main reservoirs and water conduits,” the researchers wrote. “It would thereby threaten the water sources supplying the city’s religious and political heart. To deal with the problem, a ‘device’ to control the water level in the new aqueduct and thereby also the spring environ was designed and eventually constructed about 71 m from the tunnel’s southern exit.”

The sluice theory is based on the discovery of four 8-centimeter long iron bolts sunken into the bedrock walls of the tunnel, 71 meters from the tunnel’s exit. The badly corroded bolts are symmetrically placed, two on each side of the tunnel’s walls at a point where the height rose from two meters to almost six. This height would have been necessary to sufficiently house the tall, vertically-sliding gate. The sluice was constructed from wooden panels attached to the stone walls by iron bolts. A cable, probably woven of wool fiber, raised and lowered the gate. The researchers retrieved two of the four bolts that bore cedar wood slivers, now petrified to iron hydroxide.

Near the location of the bolts was a shaft that extends from the tunnel ceiling to an accessible subterranean passage and out to the surface. The researchers believe the sluice gate was operated by a rope system running through this shaft. Inspection of the plaster lining the shaft revealed calcified wool fibers, indicating that their theory was plausible. Blackened mortar bearing traces of smelting ore on the ceiling directly above the location of the sluice frame hints at some metal device for guiding the rope. This system would have allowed for operating the sluice even when the city was under siege.

The sluice allowed selective control over water levels for the upper and lower parts of the city, diverting the water to the section of the city which needed it most at that time. This degree of control would have proved helpful in a siege situation, granting the capability of selectively cutting off easy access to water, depending on the situation.

Based on carbon dating of the plaster used to seal the tunnel and the fine sedimental laminae along the tunnel walls, the researchers dated the sluice gate to sometime between the eighth and fourth centuries BCE. 

The authors noted that not only would this structure have been a significant innovation in its own right, but it would also constitute “to the best of our knowledge, the oldest sluice gate known.”

To the best of our knowledge, no sluice gates have been recorded that predate the Roman ~1-2nd century c.e. Period. The oldest Iron Age structures, referred to as sluice gates, were found in the Judean Desert (Stager, 1976). Constructed to raise water level behind a stationary stone dam, these structures are weirs rather than sluice gates. Consequently, if Hezekiah’s Tunnel sluice ever functioned as a movable blocking wall, it may well be the oldest sluice gate on record.

The effects of the sluice are still visible today as a waterline remains in some sections of the tunnel high above the current knee-deep water level.