The gemara doesn't address snow in the context of a mechitza, but it does address snow in the context of reducing the window that connects between 2 houses in the context of transferring tu'mah. The Tosefta says that snow doesn't reduce the size of the window to prevent the spread of tu'mah. Rashi and Tosafos both say that the concern is that they will melt. Both Rashi and Tosafos seem to be holding that since they are going to melt, even though it is freezing outside and they haven't yet melted, they don't serve as a separation. It does not sound like some sort of gezeira d'rabonon, rather on a d'oraysa level it doesn't prevent the spread of tu'mah even prior to actually melting.
Can we learn the halacha of mechitza from here? The tosefta says that a bird that rests in the window that you have no intention of removing can serve as a separation to prevent the spread of tu'mah. Tosafos asks, how does this fit with r' meir who holds that a live animal may not be used as a wall for succah or shabbos? Tosafos says that the rules for mechitza or more severe than for stopping the spread of tu'mah. The rule by tu'mah is that if you are mevatel it there, it prevents the spread of tu'mah but for shabbos it would not qualify as a mechitza. In truth, a live animal does qualify as a mechitzah for shabbos, it is just that the rabonon were concerned that it may escape or die and no longer be a valid mechitzah. But the rule that we can derive from tosafos is that the rules to be a mechitza on shabbos are different than the rules for blocking tu'mah. Tu'mah is dependent on bitul, whereas a mechitzah is not dependent on bitul. The idea of bitul is to establish a permanence - making the object part of the wall. A mechitza doesn't require this level of permanence. Therefore, snow which cannot serve to block tu'mah because it will melt, is missing the permanence needed to block tu'mah. But, this may not be an issue in the context of using snow as a wall. To serve a mechitzah the fact that it will melt doesn't invalidate it as a mechitza so long as at the moment it is a 10 tefach wall. Surely, if it is melting and likely will reduce to less than 10 tefachim it cannot serve as a wall, but so long as it retains a height of 10 tefachim and it is below freezing so that it will be maintained, it can be used as a wall.
Although it seems to me that in the dead of winter when it is freezing outside, snow can be used as a wall, i have to research to find out how the poskim deal with a snow mechitzah.
I Just found the Aruch Hashulchan (362:15) - this is what he says:
ויש להסתפק אם מועיל בימי החורף מחיצה של שלג וכפור וקרח, ונראה לכאורה דלא מהני מדאמרינן בב"ב כ ע"א דאינן ממעטין בחלון לענין פותח טפח המביא את הטומאה משום דכיון דנמוקים מאליהם לא ממעטים את הפותח טפח וכ"ש דאין שם מחיצה על זה וכמים בעלמא הם. אך, אין ראיה מזה דבסתימת פותח טפח צריך ביטול עולמית וכו' אבל מחיצות הא דיו לשעתו וכו' ולשעתו הוי השלג דבר של קיימא בחורף וכו' וקצת ראיה יש להביא דהוה מחיצה ממש"כ המרדכי בפרק כיסוי הדם בשם גאון דמכסין בשלג דכתיב "כי לשלג יאמר הוה ארץ" הרי דמדמי שלג לארץ וכו
B'kitzur the Aruch Hashulchan also leans to the side that it would qualify as a mechitza even though it can't be used to block tu'mah. But see Rabbeinu Gershom 20b where the gemara discusses the types of salt that can be used for a mechitzah on shabbos. Rabbeinu Gershom mentions the idea of bitul even in the context of mechitzah on shabbos implying that if it is not permanent (such as snow which will melt), it can't be used as a mechitza!