Gemara

(Beraisa - R. Shimon ben Elazar): One who saves from a lion, troop, a high tide, overflowing river, or finds something in a big thoroughfare, he keeps it, because the owner despairs.

Bava Metzi'a 22a (Beraisa): If a river swept away Reuven's beams and rocks and deposited them on Shimon's property, he may keep them, for Reuven despaired.

Inference: If Reuven did not despair, Shimon could not keep them! (This supports Abaye, who says that Ye'ush she'Lo mi'Da'as is not despair.)

Rejection: The case is, Reuven could save them.

Question: The Seifa says that if Reuven was chasing after them, Shimon must return them. If he could save them, in any case Shimon must return them!

Answer: Reuven could save them with difficulty. If he chases them, he shows that he does not despair. If not, we may assume that he despairs.

27a - Question (R. Yochanan citing R. Yishmael ben Yehotzedek): What is the source that one may keep something swept away by a river?

Answer (R. Yochanan citing R. Yishmael): "And so you will do to his donkey... that will be lost from him, and you found it (you must return it)" applies only when it is lost from him, but available to others. It excludes what is hopeless for anyone to retrieve it.

24a (Beraisa - R. Shimon ben Elazar): One may keep what he saves from a lion, bear, leopard, polecat, or Shelulis Shel Nahar (this will be explained), and what he finds in a big encampment or thoroughfare, or any place where many people pass;

This is because the owner despairs.

24b - Question (Rava): If one found a wallet in the market of shoemakers (some say, Chachamim), and Levi came and gave Simanim, what is the law?

Answer (Rav Nachman): He may keep it.

Question (Rava): Can you say so even if Levi is screaming?!

Answer (Rav Nachman): He is like one screaming over his house that burned or his ship that sank.

93a (Beraisa): If a shepherd was grazing the flock, and he left it and went to the city, and a wolf or lion came and killed some of the flock, we estimate. If he could have saved, he is liable. If not, he is exempt.

Rishonim

The Rif and Rosh (Bava Metzi'a 2:2, 2:6): bring the teachings of R. Yochanan and R. Shimon ben Elazar.

Ba'al ha'Ma'or: Shelulis Shel Nahar is the middle of the river. What is there is totally permitted. What was swept away by a river that overflowed its banks is not lost to everyone. Heavy things (beams and rocks) can be deposited. Also, perhaps the river will subside, and deposit what it took.

Milchamos Hash-m: Also, if there are obstacles in the river, what was taken can be detained there. If the owner pursues, one must return to him even though there is no Siman, for he did not despair.

Hagahos Ashri: If a river swept away Reuven's beams, wood and rocks and deposited them on Shimon's property, he may keep them if Reuven was there are could have saved them if he chased them, but he did not bother to chase. Even though he still could save with difficulty, since he did not bother to chase when he could saved easily, presumably he despaired. If Reuven was not there when they were swept away, it is not despair, for he could save with difficulty. It came to Shimon's property b'Isur (he was not allowed to keep it). If Reuven could not save even with difficulty, this is like Zuto Shel Yam, and it is permitted.

Rosh (ibid.): R. Shimon ben Elazar said that one may keep what he finds in places where many pass because the owner despairs. The other cases (what he saves from a lion... or from Zuto Shel Yam or Shelulis Shel Nahar) are permitted even if the owner stands and screams (that he did not despair). He is like one who screams that his house fell (it does not help).

Question (Piplulei Charifta 8): In Siman 7, the Rosh says that if one found something in a place where many pass, and the owner stands and screams, he is like one who screams that his house fell! The Gemara explicitly says that if something was found in a public place, we ignore the owner's screams. Why did the Tur omit this? The Rambam makes no mention of the owner screaming. He explains that Rava asked 'if the owner is screaming that he lost such an object, we should infer that he did not despair!' Rav Nachman answered that he is like one screaming over his house that burned. I.e. this is no proof that he did not despair, for people scream about things that cannot be fixed. Therefore, in Perek 11 the Rambam wrote that surely he despaired. Regarding one who saved from a lion, the Tur mentions even if he is screaming, i.e. like the Rambam explains. However, the Rosh is difficult. Presumably, the Tur follows the Rosh. This requires investigation.

Rambam (Hilchos Gezeilah 6:1): If a river swept away Reuven's beams and rocks, if Reuven despaired, they are permitted, and whoever saves them keeps them. If Reuven did not despair, he must return them, and all the more so if Reuven was chasing after them.

Magid Mishneh: The Rambam does not distinguish this from something lost in a river, which is permitted. He permits Zuto Shel Yam and Shelulis Shel Nahar only when Reuven despaired, like the simple reading of the Beraisa. Letter of the law, if Reuven cannot save, even if he did not despair, the finder keeps it. Despair is needed only when Reuven could save with difficulty, and all the more so if he could save easily. If he could save only through a miracle, despair is not needed. Perhaps he holds that in such a case, surely he despaired.

Pnei Yehoshua (22a DH Emnam): It seems that the Hava Amina was that the owner cannot save. We ask because above we totally permitted Zuto Shel Yam, and here we permit only if he despaired. Also the Mishnah in Bava Kama (114a) permits only if he despaired. The Gemara did not ask from the Mishnah, for it knew that we could say that it is when he can save. Alternatively, Zuto Shel Yam refers to a river that never ceases. Therefore, one can save only through a great exertion; the Torah permitted this. Here we discuss a river that overflowed its banks. Later it will subside and deposit what it took, and it will be easy to get it, like actually happened (it deposited it in another's field). The one who asked assumed that the owner despairs, and it is as it is impossible to save. The owner will not exert himself for a Safek; perhaps the river will carry the beams so far that it is not worth going to retrieve them. Therefore, 'if he despaired' means 'if he knew that the river took them.' We answer that he can save through obstacles in the river, and 'if he despaired' simply means explicit despair.

Rambam (2): Therefore, if one saved from a river, high tide, troops, fire, a lion... if he is sure that the owner despaired, he keeps it. If he does not know, he returns it.

Rebuttal (Ra'avad): This is astounding.

Kesef Mishneh: We learn (that if he does not know if there was despair, he returns it) from the argument of Abaye and Rava.

Mishneh l'Melech: Mahariko (3) says that even if it is not certain that the property will be lost, if it probably will be lost, we can say that there was despair. R. Shimon ben Elazar permits keeping what he saves from a lion... Another Beraisa obligates a shepherd if he could have saved from a lion, but did not. The difference is, R. Shimon discusses saving from a lion that is here; in the other Beraisa, had the shepherd not left, the lion would not have damaged.

Rambam (11:10): If one finds something in Zuto Shel Yam or Shelulis of a river that never ceases, even if it has a Siman, he may keep it. It says 'that will be lost from him, and you found it." You must return what it is lost from him, but available to others. It excludes what is hopeless for anyone to retrieve, for the owner despairs.

Poskim

Shulchan Aruch (CM 259:7): One may keep what he saves from a lion... Zuto Shel Yam (something swept away by a high tide) or and Shelulis Shel Nahar (it overflowed its banks), even if the owner stands and screams.

SMA (16): He keeps it even if he took it before despair, and even if the owner screams that he did not despair.

Gra (16): The Gemara says so about something found in a place where many are found, and all the more so in this case!

Rema: In any case it is proper to return it, like we said in Sa'if 5.

Beis Yosef (DH Kosvu, citing Tosfos 22a DH Shotaf): We must say that Reuven could have saved his beams easily had he chased them when they were washed away. If not, this is like Zuto Shel Yam, which is lost to everyone. In such a case, we would not say 'Stam (if we did not hear of despair) it is forbidden.' This is even if there is no Siman, for obstacles in the river will detain them, and the owner can take them. Even if others take them first, they will return them. Since Reuven pursued immediately, it is evident that he owns them. The Gemara thought that if he does not pursue, he cannot save at all. Others will take them and not return them, for he has no Siman. 'If the owner despaired' means that he was there when it was washed away, and did not bother to chase. Even though if he says that he does not despair, this does not help. Surely he despairs in his heart! 'Stam' is when Reuven was not there when they were swept away. He does not despair until he finds out, and then he cannot save, for he has no Siman. It came to the finder's hand b'Isur, for Ye'ush she'Lo mi'Da'as is not despair. The Gemara answered that he can save with difficulty even after it was washed away, i.e. he has a Siman. We asked, if so, if he does not pursue, why do we say that he despaired? We answered that he could save with difficulty. They will get deposited very far away. If he does not pursue, surely he despaired.