Manhattan's Water Boundaries, Bridges, and Tunnels

Manhattan's geography gave the proposed eruv a physical advantage that most large cities do not have. Water surrounds the island, and long stretches of the waterfront had substantial retaining structures rising from the water to the land above. In hilchos eruvin, however, a body of water on a map is only the beginning of the analysis. The bank must function as a valid boundary, and every bridge, tunnel, roadway, and other break in that boundary has to be considered.

Rav Moshe Feinstein and the Rebbe both addressed these features directly. Their discussions are important precisely because they did not treat the island's geography as either irrelevant or automatically sufficient.

Rav Moshe began with a real mechitza

In Igros Moshe, Orach Chaim I:139, Rav Moshe begins the Manhattan discussion with the wall formed at the shoreline. He writes that the structure rising from the water to the higher ground, more than ten tefachim, which encircled the city was certainly a complete mechitza.

He then considers whether the enclosure had been made for residential use, an issue that can affect a very large enclosed area. Rav Moshe points to the developed waterfront and the use of the area for walking, sitting, and ordinary city life. His starting point was therefore quite favorable: Manhattan possessed a substantial pre-existing boundary that could have halachic force.

The problem shifted to the places where the city passed through that boundary.

A bridge can be an opening in the enclosure

Bridges linking Manhattan to Brooklyn and New Jersey crossed the shoreline wall and carried public traffic beyond it. Rav Moshe analyzes the bridge decks through the laws of an enclosed area that opens into another domain, the effect of public passage through a boundary, and the different requirements that may apply depending on whether the bridge itself is a reshus harabim.

This produces several possible outcomes. Under some approaches, a tzuras hapesach at the opening can repair it. Under stricter approaches, if the crossing is treated as a full reshus harabim, doors may be required, with the further question whether they must actually be closed at night.

Rav Moshe did not assume that all of Manhattan's traffic passed through one bridge. He specifically refers to thirteen bridges and additional underground routes called tunnels. Dividing the traffic among those crossings, he wrote, would mean that six hundred thousand people did not pass over each bridge every day.

That observation did not settle the matter, because Rav Moshe then developed his city-wide understanding of shishim ribo.

Is the bridge part of the city or an outside road?

In anaf 5 of the responsum, Rav Moshe argues that according to the opinion requiring six hundred thousand, a city need not have six hundred thousand people traversing each individual street. He reads the classical sources as referring to the city as a whole. He further suggests that a city can reach the number through residents together with people who enter it each day.

That led to a precise question about Manhattan's bridges. If a bridge is considered part of the city, Rav Moshe writes, Manhattan's city-wide population may supply the shishim ribo relevant to the bridge. If the bridge is considered an external road, the people in the city cannot simply be added to the bridge's own count, and the traffic on that particular route becomes more significant.

Rav Moshe regarded the classification as uncertain. Under the stricter side of his analysis, if the bridges were part of a city containing shishim ribo, doors that close at night could be required. Under other approaches, a tzuras hapesach could suffice. The existence of the water wall therefore moved the halachic question to the bridge openings rather than eliminating it.

The full treatment of the six-hundred-thousand question belongs in the shishim ribo section.

Tunnels raised the same basic problem

Tunnels are physically different from bridges, but for Manhattan they served the same practical function: large numbers of people could pass through the island's perimeter without stopping at the water boundary.

The Rebbe's notes to Rabbi Hertze Eisenstadt on 15 Iyar 5714 mention the two together. Responding to the assertion that the streets were unquestionably a Torah-level reshus harabim, the Rebbe wrote that the point required investigation because the streets eventually terminated at bridges and tunnels which, as he understood them, were closed by a tzuras hapesach, a partition, or barriers used for toll collection.

The Rebbe introduced the matter with tzarich iyun uveirur, requiring examination and clarification. He identified physical facts that could affect the halachic classification and had to be verified. The note does not rule that the bridges and tunnels had already enclosed Manhattan.

The water itself also required investigation

The Rebbe separately addressed the proposal's reliance on the sea and rivers. He discussed the distinction between standing and flowing water, the condition of the bank, and the halachic concern that water may recede or deposit earth in a way that changes the boundary. He concluded that all the parts of Manhattan's perimeter cited in the proposal had to be examined.

That is an important limitation on the phrase “Manhattan is an island.” A continuous water boundary may be a powerful halachic feature. Its legal effect depends on the physical shore, the height and form of its banks or retaining walls, the openings created by crossings, and the condition of each segment of the perimeter.

A favorable perimeter did not decide the case

Manhattan's water boundary was one of the strongest features discussed in its favor. Rav Moshe treated part of it as a complete mechitza. The Rebbe directed careful attention to the same physical questions. The remaining problems concerned what happened where that enclosure was penetrated and how a city of Manhattan's scale and public use should be classified.

Those are the reasons the Manhattan discussion became relevant to other modern cities. Existing walls, embankments, highways, rail structures, rivers, bridges, and tunnels can all matter. They have to form a valid and continuous enclosure under the applicable halachos, with every significant opening treated according to its own status.