at the end of teh sugya we say that the chayich was due to the fact that R' Chiyah said his din despite kal hu shehikilu ..... Does this answer up that R' Chiya made a mistake, or could he have been challenging that very point....
By reshuyos, the legal fiction of gud achis can account for a valid mechiza, however, by techumin, even if the mechiza is theoretically good, that does not account for the metzius that the water goes back and forth over the line and kal hu would not help, which -seems like a valid point.
moshe rubin, bklyn, ny
Thanks for the very interesting question. First I must say that it is difficult for me to accept your suggestion. In the case of Reshuyos the water is still moving back and forth so why should a theoretical Mechitzah be valid? The fact is that all Poskim reject R' Chiya's view in this case.
Regarding your question, what was the opinion of Rabbi Chiya, I want to suggest that R' Chiya quoted the Tana only to make a point that in this case a Mechitzah is necessary . The type of Mechitzah was not the main issue. R' Chiya however, quoted the Tana verbatim, including the requirement of an iron Mechitzah (which was not accepted l'Halachah).
Yehuda Landy