Some Observations on the "Siloam Dam"

A massive dam wall was uncovered in excavations at Birket al-Ḥamra in Jerusalem and dated to ca. 800 BCE. Let's take a closer look.

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The amount of major archaeological news coming out of Jerusalem over the last few months has been difficult to keep up with. One example is the dam wall that was uncovered in excavations taking place at Birket al-Ḥamra in the southern part of the Jerusalem basin. This is the first major discovery from the excavation that has been widely announced.

It was published in a short open-access article: “Radiocarbon Dating of Jerusalem’s Siloam Dam Links Climate Data and Major Waterworks” by Johanna Regev, Nahshon Szanton, Filip Vukosavović, Itamar Berko, Yiftah Shalev, Joe Uziel, Eugenia Mintz, Lior Regev, and Elisabetta Boaretto (accompanied by a 3-page supplement of supporting material). The article presents the dam and its dating. It also discusses this find in the context of Jerusalem’s entire water system and provides climate data that may explain the rationale for its construction. News about the discovery was covered very widely in popular media, and a few detailed analyses have also appeared online.

In this post I will attempt to do three things: summarize what is currently known about the dam, provide some preliminary reactions to claims in the article, and highlight other relevant issues. Keep in mind that the information that is currently available about the excavation and the dam is fairly limited, and what you read below is based mainly on the short article.

The Excavation Context

The current excavation is unfolding at a place known locally in Arabic as Birket al-Ḥamra (“The Red Pool”). The pool sits at the southern outlet of the Central Valley. This area is quite low in elevation relative to the rest of Jerusalem and was an ideal spot for a dam that collected rain runoff as water followed the course of gravity down the valley floor. The pool is also located below the southern edge of the Southeastern Hill of Jerusalem. A vertical cliff overhangs the pool and serves as its northeastern boundary. Two channels were built to carry water southward toward the pool from Jerusalem’s spring. In the scholarly literature, these are known as Channel II and the Siloam Tunnel (aka Hezekiah’s Tunnel).

The Ordnance Survey Map of Jerusalem from 1865, showing the area of the excavation. Birket al-Ḥamra is circled in light blue (labeled “Old Pool”), and the red line shows the position of the dam wall. The location of the spring is circled in purple. The green star sits over the crest of the Southeastern Hill, which at this time was broken up into plots for annual and perennial agriculture.

Several prior excavations have taken place near Birket al-Ḥamra. Perhaps the most well-known of them revealed a series of steps that were widely believed to be part of the ancient Pool of Siloam mentioned in the Gospel of John and the Mishnah (Reich, Shukron, and Lernau 2007; Reich 2011:225-236). However, new data has recently cast doubt on that identification. I wrote another post explaining how findings in the current excavation have made things a little complicated for locating the Roman Period Pool of Siloam. The new announcement about the Siloam Dam relates to a much earlier stage of the pool’s history, dating hundreds of years prior to the time of the Judahite kings (Iron Age II).

The current excavation is also establishing new realities on the modern landscape of Jerusalem (see this paragraph for links to relevant info). Birket al-Ḥamra was property of the Greek Orthodox Church and through a series of circumstances has become subsumed into the City of David National Park. This trend is progressively reshaping the Wadi Hilweh/Silwan neighborhood toward an antiquity-centered place of tourism which is not infrequently leveraged in national and diplomatic efforts. This background was not well-covered in initial media reports about the pool’s excavation and seems likely to be neglected as more material emerges.

What Was Uncovered?

The major find recently announced at Birket al-Ḥamra is a dam wall that is “at least 19 m long, 11 m high, and 10 m wide” (pg. 1). This wall forms the southeastern boundary of the pool area. Its existence was already known, but the current project has revealed its inner face and provided new dates for its construction. According to the article, the Siloam Dam consists of two different elements: a vertical exterior wall (W001) with a sloping interior wall (W114) abutting it (pg. 2). The sloping wall faced northwest and would have taken the brunt of rain runoff when it arrived at the outlet of the Central Valley.

A view of the dam wall during the excavation, looking south. The outer face of the sloping wall (W114) is prominently visible. The vertical section (W001) can be partially seen above and behind it. I am grateful to Shelley Jones for this photo.

Four samples of organic material were extracted from the dam wall that successfully underwent radiocarbon analysis in order to date the structure (pg. 1):

• two separate pieces of uncharred straw in the mortar between outer stones in the vertical wall (W001)

• a few charred twigs in the mortar between outer stones in the vertical wall (W001)

• one uncharred straw sample in the core of the sloping wall (W114)

The radiocarbon date of this material was all very similar, enabling an average date of around 800 BCE (pg. 1). In biblical and historical terms, this would put the dam’s construction near the end of Jehoash/Joash’s reign or at the beginning of Amaziah’s.

The samples of uncharred straw extracted from both sections of the dam provide important evidence. The straw must have been gathered and incorporated into the dam’s mortar relatively quickly after the end of its life and was probably not stored for long-term use (pg. 2). Therefore, as best as I can tell, the dating of the dam’s construction presented in the article seems to be convincing.

An Ottoman Period road on top of the dam wall captured in a photo from ca. 1868, looking southeast. The Kidron Valley floor can be seen in the center of the photo, with the rise of the southern Mount of Olives on the left beyond. Photo by William E. James and digitized by the Getty Museum.

Interpretations of the Dam’s History and Function

The excavators make two major claims relating to the Siloam Dam. First, they set its construction within the wider context of Jerusalem’s entire water system and argue (based on other data) that it was built together with two other components (pg. 2). These include the Spring Tower and Channel II. Let’s briefly say something about these features.

The Spring Tower is a large fortification built over Jerusalem’s perennial spring, which is usually identified as the Gihon of the Bible (II Ki. 1:33). The tower is constructed of very large stones and seems to have originally been connected to two parallel upslope walls nearby that are also constructed of the same large masonry. Together, these features enclosed and fortified the spring. Both the tower and upslope walls were dated by the original excavators to the Middle Bronze Age II, ca. 18th century BCE(?) (Reich et al in Geva (ed) 2019:32-44).

However, a later excavation under the Spring Tower challenged this understanding. In 2014, organic material was extracted from below the tower. The latest of these samples surprisingly yielded a radiocarbon date similar to the Siloam Dam in the late 9th century BCE (Regev et al 2017). It was proposed then that this new date for the tower either reflects the original time of its construction or to a rebuilding/renovation, with the original construction of the tower being much earlier. The authors who published the Siloam Dam see a connection between building activity at the Spring Tower and the dam due to the similar radiocarbon dates from each monument.

The Spring Tower, looking west. Blue stars mark the corners of the tower on the north (right) and south (left). The red arrow at the top points to the upslope walls that were likely built together with the tower but are no longer physically connected to it.

The other item we need to describe is Channel II, sometimes called the “Siloam Channel,” “dry tunnel,” or “Canaanite tunnel.” This is a rock-cut channel that begins below the Spring Tower. It directed spring water southward and had its outlet in front of the dam. In other words, the Spring Tower and Siloam Dam are connected by Channel II.

Channel II reflects two different styles that are usually understood as two different stages of construction (Reich and Shukron 2021:300-301). Its northern half was cut through soft limestone as an open water channel and capped with large stones. This section has been dated by its most recent excavators to the Middle Bronze Age. It seems to have been built together with the Spring Tower because the tower’s southern wall was constructed over the beginning of Channel II, though in theory Channel II could be older (Reich and Shukron 2021:335-336). When the northern section of Channel II was originally hewn, it may have directed water to a pool in the Kidron Valley, but this has not yet been archaeologically demonstrated (Grossberg 2013; Reich and Shukron 2021:359).

The southern half of Channel II looks different. It was cut not as an open channel but instead as a rock-cut tunnel through harder limestone (Reich and Shukron 2021:333-335). This section is usually thought to date later than the northern part, to sometime during the reign of the Judahite kings (Iron Age II). After its completion, the spring water flowed southward to the area in front of the Siloam Dam, where a pool would have formed.

The article under discussion here argues that the Siloam Dam, Spring Tower, and the entirety of Channel II were all constructed as one collective project that was completed around 800 BCE (pg. 2). It also mentions a pit (L145), which may have been an early cistern (pgs. 1-2). However, because the pit has not yet been published and seems to have been disconnected from all other features of the water system, its relevance to this discussion is not immediately clear to me.

Part of the northern half of Channel II, looking south.

The second major claim in this article has to do with the environmental rationale for construction of this water system. Using diverse climate data, the excavators argue that the dam’s construction was occasioned by a period of drought during the mid-late 9th century BCE, punctuated by occasional heavy rain and flash floods (pg. 2). To those ruling in Jerusalem, this was “perceived as unpredictable weather conditions” and led them to construct the dam to capture and utilize rainwater (pg. 2).

The excavators therefore argue that the dam served two purposes from the time of its construction: (1) to collect spring water arriving from Channel II in an artificial pool and (2) to capture rain runoff that flowed southward through the Central Valley (pg. 2). Channel II was initially the means by which water arrived at the dam, but the Siloam Tunnel later replaced it (pg. 2).

Critical Questions

While the dating of the dam seems to be convincing, I come away with two major questions about the interpretive framework surrounding it. The more significant one concerns the idea that all three components of the water system were built as a coordinated effort. The authors seem to assume that it is possible to date the construction of Channel II on the basis of radiocarbon samples taken from monuments at its beginning and end. While the northern half of Channel II seems to have been constructed together with the Spring Tower, the relationship between the dam and southern half of Channel II is less clear to me.

It makes sense logically that the southern extension of Channel II would not have preceded the building of the dam, but theoretically the dam could have existed well before the southern part of Channel II. Perhaps the dam was constructed as a standalone project to collect rain runoff in the Central Valley while the northern half of Channel II still carried spring water into the Kidron Valley (to a preexisting pool?). These components would have been linked by the southern extension of Channel II at a later stage of urban development in Jerusalem, perhaps as Hezekiah was preparing for the arrival of the Assyrian army and sought to conceal the water source (Grossberg 2013). As far as I know, pottery found in Channel II suggests only that it went out of use in the 8th or 7th centuries BCE after its function was replaced by the Siloam Tunnel (Reich and Shukron 2021:336). Perhaps future data from Channel II will help clarify the date of its southern extension in the future.

The interpretation in the article also relies on radiocarbon dates from material below the Spring Tower that suggest it may have been constructed or renovated at the end of the 9th century BCE. For several reasons, and because the significance of these dates has been disputed by the original excavators of the tower (Reich 2018), I still favor a Middle Bronze Age date for the original construction of the Spring Tower and the northern part of Channel II. A possible renovation of the tower in the late 9th century BCE could have been undertaken due to issues related to the tower itself and thus would not necessarily have a bearing on construction of the dam or southern extension of Channel II. To my mind, these considerations weaken the possibility of an original link between the Siloam Dam and the Spring Tower via Channel II, though eventually they were indeed all connected.

My second question is related to the environmental rationale for the dam’s construction. The collation of climate data from diverse sources is impressive and may indeed provide the relevant background. However, I am not sure that special circumstances are needed to account for the building of a dam wall in the Central Valley. Jerusalem’s terrain was probably always susceptible to floods at the beginning of the rainy season (Oct-Nov), after the ca. six-month dry season when the sun had thoroughly dehydrated the soil. This created hard-packed earth that could not quickly absorb heavy rain, encouraging the pooling of water and development of torrents that followed the course of gravity to the bottom of the valleys. Flash floods were also likely to occur if a heavy storm hit the area at the end of the rainy season (April or May) after several months of less consistent rain and intense sun.

I assume that if we removed all the topsoil and debris that has accumulated in Jerusalem’s valleys over the millennia, it would create an environment with regular potential for flash floods, particularly during heavy storms. The dam may therefore have simply exploited the annual floods in order to assist Jerusalem’s urban growth and its associated agricultural activity at the end of the 9th century BCE. Here I should mention that the end of the article notes “the growing size of [Jerusalem] and pressure from surrounding empires” as part of the rationale for the building of the dam, in addition to the climate considerations they propose (pg. 2).

Jerusalem’s Southeastern Hill looking north. The filling in of the southern Central Valley (marked roughly with red arrows) is so extreme that it is nearly impossible to see at ground level today. The slopes of the Kidron Valley (blue arrows) remain distinct in the modern landscape, although it has also been filled in significantly.

Announcement of the Siloam Dam has added new and important data to our understanding of ancient Jerusalem. While I personally do not come away from the article convinced of the historical reconstruction it presents, perhaps my reservations are not as well founded as I think or they will be addressed in future publications. In any case, I assume that we will hear more news from the area of the pool soon.

Additional Unorganized Thoughts

A wall built over the dam?

• The dam’s monumental structure would have served as an awfully convenient foundation for a wall and would match the route of Jerusalem’s 8th-century BCE fortifications according to the traditional maximalist view. I wonder if any remains of the vertical wall (W001) look like they could relate to an Iron Age city wall. Perhaps this possibility will be discussed in future publications.

A water source for the Western Hill?

• I have seen the Siloam Dam discussed in relationship to Jerusalem’s growth onto the Western Hill, as it is usually assumed that a pool in the area of Birket al-Ḥamra must have served residents living in the new settlement. The implication is that if the dam dates to around 800 BCE, settlement of the Western Hill must have already started by that time. While I think a good argument can be made that settlement on the Western Hill had begun by the early 8th century BCE and perhaps even earlier, I am not convinced that a pool in the area of the dam was created into order to serve residents of the Western Hill. This is in part because the movement of spring water to the south only directed this resource even further away from the plateaus of the Western Hill that were already distant from the spring. Therefore, in my view the construction of the Siloam Dam has no direct bearing on the date of the new settlement, which will be best established with material from the Western Hill itself. Perhaps I will write about this issue in the future.

Dams as an architectural genre in Jerusalem

• It is interesting that other examples of dam walls were also built in Jerusalem’s valleys, such as at the Sultan’s Pools in the Hinnom Valley. Another apparent dam dating to the Iron Age was found in an upper tributary of the Hinnom Valley (Amit in in Geva (ed) 2019b:145-152). Some scholars have also argued that the Pool of Bethesda may have also originally been constructed as a valley dam. Considering the new evidence from Birket al-Ḥamra, it seems that this method of capturing water came into use as early as the Iron Age and continued in Jerusalem during other periods.

The traditional site of Isaiah’s martyrdom on the dam wall

• A large mulberry tree once stood at the southern end of the dam wall, in which Isaiah was thought to have been hiding when Manasseh had him sawn in two:

“...[the pool] now has its bottom overgrown with trees. The Arabs call it Birket el-Ḥamra, or the red pond. To the S. of this point stands an old mulberry-tree, enclosed by stones for its protection, and mentioned for the first time in the 16th cent., where the prophet Isaiah is said to have been sawn asunder in presence of King Manasseh. The tradition of this martyrdom is alluded to by some of the fathers of the church.” (Baedeker 1876:227)

The mulberry tree with stones set around its base marking the spot of Isaiah’s martyrdom, perhaps in the 1850s. Photo by Rev. George Wilson Bridges; digitized by the Getty Museum.

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