1) THE DEFINITION OF "NACHAL EISAN"
OPINIONS: The Gemara cites several verses in order to define the term "Eisan" in the context of "Nachal Eisan," the place where the procedure of the Eglah Arufah is performed. What exactly does the word "Nachal" mean in this context, and how does "Eisan" describe it?
(a) The RAMBAM (Hilchos Rotze'ach 9:2) writes that Nachal Eisan is "a river that flows swiftly." The ME'IRI here (46b) points out that this is consistent with the Gemara's statement in Pesachim (53a) that a Nachal is a place where reeds grow. Reeds grow along rivers, as is clear from empirical observation as well as from a number of statements in the Gemara and verses in the Torah (such as Shemos 2:3, "And she placed him among the reeds at the edge of the river"; see also Shemos 2:5 and Melachim I 14:15).
According to the Rambam, the reason why the Gemara here cites verses that use the word "Eisan" with reference to rocks is in order to prove that the water of the Nachal must be strong like a rock in the force of its flow, rather than great in quantity.
The word "Eisan" is used with reference to a river in other verses, such as, "... the sea returned to its power (l'Eisano)" (Shemos 14:27). Similarly, strong and swift rivers are described as "Naharos Eisan" (Tehilim 74:15) and "k'Nachal Eisan" (Amos 5:24).
Another support for the Rambam's explanation is the verse that requires that the elders "wash their hands in the Nachal" (Devarim 21:6) after they have performed the procedure of the Eglah Arufah, which implies that the Nachal is a body of water.
If, however, the Nachal Eisan is a river as the Rambam says, why does the Torah prohibit the Nachal Eisan from being tilled or planted? Obviously, the Torah would not prohibit cultivating the Nachal Eisan if it was a river, as a river cannot be planted.
The ME'IRI and the CHAFETZ CHAIM (in LIKUTEI HALACHOS, Sotah 46b) explain that the Eglah Arufah is not killed inside the river but on the banks of the river. The earth on the river banks is arable and can be planted. This might be the intention of the opinion cited by RABEINU BACHYE that Nachal Eisan is a very productive and fertile land. The Torah's prohibition against using it after the Eglah Arufah has been killed there provides incentive for cities to appoint guards along the highways in order to prevent murders (see also RADAK, Sefer ha'Sharoshim, Alef Yud Tav).
(b) Most Rishonim, however, interpret "Nachal Eisan" differently. They explain that a Nachal Eisan is a hard, unplowed valley. Among those who explain this way are RASHI, RAMBAN, and RABEINU BACHYE (Devarim 21:4; see MAHARIK #158), the RASH and ROSH (Pe'ah 2:1), and the RASHBAM (Bava Basra 55a; see TOSFOS REBBI AKIVA EIGER, Pe'ah ibid.).
Similarly, RASHI here in Sotah (46b, DH Eisan) implies that Nachal Eisan refers to dry land which is as hard as rock; a "Nachal" is a valley, and "Eisan" means "hard." This is supported by the Gemara here which cites the verse, "Your residence is strong (Eisan), and your nest is set in a rock (ba'Sela)" (Bamidbar 24:21), as a source for the definition of "Eisan." The association of "Eisan" with "rock" implies that "Eisan" describes the ground, and not a river.
This is also the opinion of the TARGUM ONKELUS (Devarim 21:4), who translates "Eisan" as "Bayar," or untilled, barren land.
The SHE'EILAS YA'AVETZ (#25) cites further support for this explanation from the Gemara in Nidah (8b) which says that the term "Karka Besulah" refers to land that was never worked and that meets the requirements of Nachal Eisan with regard to the Eglah Arufah.
However, if Nachal Eisan refers to hard land, why is it necessary for the verse to prohibit the land from being tilled and planted? Land as dry and hard as a rock cannot be planted! The RITVA in Makos (22a) answers that the prohibition is necessary because hard land can be planted with much effort.
Nevertheless, this explanation is difficult to resolve with the Gemara here which defines Nachal Eisan as a place where reeds are abundant. (TOSFOS REBBI AKIVA EIGER, Pe'ah ibid.)
The CHACHAM TZVI (#32) cites RASHI in Yeshayah (19:6) whose words offer a possible answer to this question. Rashi explains that when a river or wadi dries up, its reeds desiccate and crack, and the waterless valley becomes filled with broken, parched reeds. Perhaps both the Rambam and the other Rishonim understand that Nachal refers to a riverbed. According to the Rambam, it is a riverbed full of water, and according to the others it is a dry riverbed which can be recognized by the parched reeds in it.
Indeed, some suggest a compromise between the two opinions and reconcile the apparently contradictory sources for the definition of "Eisan." They explain that Nachal Eisan refers to a wadi, the bed of a river that flows only in the rainy season. When the corpse is found in the winter, during the rainy season, there is a strong current of water in the river. When the corpse is found in the summer, there is no water in the river, but rather hard-packed earth of the riverbed. This is consistent with the fact that strong rivers that flow during both winter and summer are relatively rare in Eretz Yisrael, while wadis are found near almost every city. A source for this definition of a Nachal may be found in Iyov (6:15-17; see METZUDAS DAVID there). (See TORAH TEMIMAH to Devarim 21:4; CHIDUSHIM U'VI'URIM to Sotah 46b; see also Insights to Pesachim 53:2.)
2) HALACHAH: THE MITZVAH TO ESCORT A GUEST
OPINIONS: The Gemara teaches the importance of escorting a guest on his way when he departs. If the host escorts the guest even four Amos within the city, the guest will be protected from all harm.
The obligation of escorting ("Leviyah"), however, requires that one escort his guest more than just four Amos. A Rav must escort his student until the last house in the city. One escorts a friend until the Techum Shabbos (2000 Amos outside of the city). A student must escort his Rav one Parsah (4 kilometers) outside of the city, and if the Rav is his Rav Muvhak, he must escort him three Parsa'os (12 kilometers).
The RAMBAM (Hilchos Avel 14:3) cites these Halachos of the Gemara. However, the TUR and SHULCHAN ARUCH make no mention of the obligations of Leviyah, and the various measures of Leviyah prescribed by the Gemara are not practiced today. Why are these laws not practiced today?
(a) The BE'ER SHEVA explains that nowadays travel on an inter-city road poses more danger than in the times of the Gemara. A person is not obligated to put himself in danger in order to escort his friend.
(b) The DARCHEI MOSHE (end of Choshen Mishpat) cites the MAHARASH who explains that nowadays most guests forgo their entitlement to Leviyah, and thus the host may assume that his guest also forgoes his entitlement. Nevertheless, the host still should escort his guest to the gates of the city or at least four Amos. (This is because the Gemara mentions that it is improper for the guest to forgo the escort entirely, for doing so is akin to an act of "Shofech Damim"; see AHAVAS CHESED 3:2).
The CHAFETZ CHAIM in LIKUTEI HALACHOS (Ein Mishpat #300) adds that if the roads are confusing and the guest might easily become lost, it stands to reason that the host has not fulfilled his obligation b'Yedei Shamayim until he actually escorts his guest and leads him on the proper road.
The EMEK BERACHAH (p. 135) writes that the four Amos which one escorts his guest must be outside of his house and not inside (since, otherwise, he has not escorted his guest "on his way").
(c) HE'OROS B'MASECHES SOTAH cites the SHEVUS YAKOV who writes that a host is not required to escort a guest nowadays. Since the purpose of escorting a guest is to direct him upon the proper way (see MAHARSHA), the host does not need to escort his guest nowadays because road signs (in most civilized countries) provide that service.
(d) The CHAZON ISH (cited by the Emek Berachah ibid.) writes that the purpose of Leviyah is to provide protection. Therefore, it applies only when a guest departs by himself. Today, when a number of people usually travel together, each traveler provides escort for his fellow traveler.
(The Emek Berachah adds that if the host is a Talmid Chacham, he should be exempt from the obligation of Leviyah, since it is a Mitzvah that can be fulfilled by someone else. A Talmid Chacham is not obligated to be Mevatel Torah to fulfill such a Mitzvah. See Moed Katan 9a.)