The גמרא says in the name of שמואל that a river is not מטהר בזוחלין except if you know there isn’t more rainwater that fell in the river than natural spring water. The reason is because if the river became mostly rainwater then the spring water would be בטל in the rainwater and since rainwater requires אשבורן a moving river would not be valid for טבילה. The רא"ש asks an important question: why don’t we apply the principle of קמא קמא בטל to say that the river is still considered מי מעיין? The concept of קמא קמא בטל is found in the גמרא in עבודה זרה דף ע"ג ע"אwhich says that if יין נסך falls into a jug of kosher wine one drop at a time, the אסור wine is בטל and it sounds like it stays בטל even if the jug eventually becomes mostly יין נסך. If so, why don’t we say each drop of rainwater is בטל to the זוחלין even if the river becomes mostly rainwater? He answers that we don’t say קמא קמא בטל by something that is natural to occur and the flow of איסור doesn’t stop. תוספות in ע"ז in ד"ה כי disagrees and says that we don’t say קמא קמא בטל once it isn’t בטל in רוב or שישים. (The חידוש of קמא קמא בטל according to this is that it is בטל by wine even though by wine we יין ביין במשהו when היתר falls into איסור.

In a similar vein, the ריטב"א asks why don’t we apply the principle of השקה? The concept of השקה is used in all modern day מקוואות. It says that if you connect or touch the water of a מקוה of שאובין (drawn water) to a כשר מקווה, then all of the water becomes כשר מקוה water. If so, why don’t we say the rainwater becomes spring water via השקה with the spring water? He answers that השקה only works by a דרבנן and the פּסול of a pool is שאובין which is only דרבנן (many ראשונים disagree with this). The ט"ז in סימן ר"א ס"ק גbrings a תרומת הדשן who answers that השקה just accomplishes "זריעה" which means it makes the disconnected connected, so the שאובין which was separated from the water is thought of as having been reattached. However, this would not help rainwater become spring water.

The גמרא says in the name of רב that if the river gets bigger in the Euphrates then it means it rained a lot in Isarel. The גמרא says this argues on שמואל who says (as explained by the ר"ן) that for every drop of rain that falls two drops of spring water get added. The מרומי שדה has a very simple question: how do we know שמואל argues on רב? Maybe שמואל agrees with רב and would simply say that if you see the Euphrates having grown it must be because it rained in Isarel which caused the spring water to come up as well which is why its bigger in בבל as well? This is actually תוספות’s question in בכורות דף נ"ו. The מרומי שדה answers that the whole point of רב saying “if the water is larger in the Euphrates it means it rained in Israel” was to tell you this נפקא מינה that you can’t be טובל there. If he held likeשמואל then you could be טובל there. This is exactly why the ר"ן spells out this נפקא מינה in ד"ה ר׳ אבין.

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