The Mishnah (9:4) teaches that one is not allowed to transport mei chatat or eifer chatat (the ashes of the parah adumah) in a boat along the river. Similarly, one may not allow it to float on the water or throw it over the river. The Bartenura explains that this law is a rabbinic decree that was in response to a case where the mei chatat was transported in a boat and a kezayit-sized flesh from a corpse -- tumat ha'met -- was found in the boat.
The Gemara (Chagiga 23a) cites a beraita that records a debate regarding the scope of the gezeira. The first opinion expands the decree to include crossing the river while riding an animal if all its feet lifted off the ground and argues that the gezeira applied to all rivers. R' Chananya argues it was limited to the yarden river only, since that is where the incident occurred.
The Tosfot however finds the motivation for the gezeira difficult. The Gemara (Zevachim 93a) cites a Beraita where R' Elazar explains that if the mei chatat became tameh it can still be used. His proof is that we do not require a niddah to become tahor prior to having the mei chatat sprinkled upon her. If mei chatat that became tameh could still be used, then it seems there is no reason for the gezeira.
The Tosfot provide two answers. In Chagiga they explain that there is a difference regarding how the mei chatat become tameh. While it is true that in general the mei chatat would be fine, our case is different because it became tameh met. The Tosfot in Zevachim however answer that it depends on when it became tameh. If it was after kidush (after the ashes had already been placed on the water) then it can still be used. In our case, it became tameh prior to that point.
The second answer of the Tosfot is at first difficult. Our Mishnah explains that the gezeira did not apply to water that had not yet had kidush. According to the Tosfot's answer, one would have expected the Mishnah to have taught the opposite: the gezeira applies to the spring water but not to mei chatat.
The Chazon Ish (5:11) answers that the Chachamim disagree with R' Elazar. In other words, if the mei chatat became tameh it cannot be used. Our Mishnah therefore is according to the Chachamim. What the Tosfot addresses is how we understand R' Elazar. It is difficult to suggest that R' Elazar maintained that gezeira was never made. We must therefore say that R' Elazar understood the gezeira differently, and that the incident involved the ashes prior to the kiddush, and the gezeira therefore only applied to the ashes.
The Chasdei David (9:9) however provides a novel answer that also answers another question. Note that while the gezeira appears to be limited to match the incident that occurred, the Mishnah teaches that the ashes are also covered by the gezeira. Why? If the case involved the already prepared mei chatat, then why would the gezeira also include the ashes?
The Chasdei David suggests that the incident was where those involved were rushed and boarded the boat with the water and ashes in hand and performed the kiddush on the boat. Consequently, the gezeira applied to mei chatat since it was already in that state when the tumah was discovered. It also applied to the ashes, since it violated the requirement that they only be in a tahor place.
The Chasdei David's explanation of the incident therefore also answers the difficulty with Tosfot's second answer. The water did indeed become tameh prior to kiddush. Consequently, even R' Elazar would agree that it could not be used. Nevertheless, the gezeira did not apply to spring water prior to kiddush since, as we explained, water alone was not brought on board and not what motivated the gezeira.
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