Thanks so much for your service!
The gemara says:
מתקיף לה רבא היכי ליעביד ליקף קמפיש בהילוכא ליעבר זימנין דמיתווסן מאני מיא ואתי לידי סחיטה
My question is: why is there no problem of מלבן if they go through the river like that (especially if the clothing is dirty)?
Aryeh Goldman, Flushing, NY
That is a very interesting question, Reb Aryeh!
Tosfos in Kesuvos (6a) and Shabbos (111a) DH Hai writes, in fact, that Sechitah is prohibited because it is מלבן, in which case the Gemara actually is saying exactly what you suggested!
Others (cited in Tosfos there) suggest that Sechitah is מפרק. Perhaps they maintain that one is not Chayav for Libun since if something dips into water without intent, it is considered דרך טינוף which is not considered מלבן (as Tosfos writes in Beitzah 18a DH Ma'aremes).
Kollel Iyun Hadaf