Why wait to get water in the Z'chut of Miriam?

Question

After they left Egypt the Children of Israel crossed the Reed Sea and needed water. They got to Marah after 3 days (at which there were bitter waters) and complained as mentioned in Sh'mot 15:23. Moshe puts wood in the water and the water turns sweet.

In 15:27, they travel to Eilim at which there is plenty of water. They leave Eilim (perek 16) and complain about food so the mon descends. By 17:1 they get to Rephidim and there is mention of a lack of water. Moshe is instructed to strike the rock and water comes from it.

The gemara states that the people were accompanied by a well during their wanderings ("He elaborates: The well was given to the Jewish people in the merit of Miriam;") and Rashi connects it to the actual rock that Moshe struck. This means that before the events at Rephidim, there was no water accompanying the people. They had no water unless they happened upon naturally occurring sources.

I'm not sure why Miriam, specifically, is connected to water (though the description of bitter waters as מרים seems like a natural bridge) nor do I understand why, if it was in the merit of Miriam, Moshe had to strike the rock at all the first time, but my question is if it was in Miriam's merit, why did it start at this particular point and not earlier, when the people also needed it?

Answer 1

The Midrash explains that until that time, they had lived on water taken from amongst the rocks of the Yam Suf. Rabbeinu Bachaye quotes Rabbeinu Chananel who explains that though the Torah writes that the Jews walked in the desert for three days, they didn't actually go without water for three days, but rather they walked three day's journey in one day. (Similar to food, which they ran out on the 15th of Iyar, and the manna fell the next day.)

Rabbeinu Bachaye further explains, (excerpt from Sefaria's translation):

וילכו שלשת ימים במדבר, “they walked for three days through the desert.” I have already mentioned at the beginning of my commentary on this portion that every occurrence that took place in the desert was in the nature of a trial. There has never been a reported march of a nation numbering three million souls plus through the desert without their being supplied with a drop of water especially at a hot time of year. According to our sages (see our comment on 13,17) when the Israelites arrived at Marah the waters they found there had first been sweet, water fit to drink. These waters turned bitter, bracken, to intensify the trial they were undergoing. This is best reflected in the Torah itself writing “there He subjected them to a test” (verse 25). When Moses prayed and G-d showed him how to sweeten the water by throwing some bitter piece of wood into it, this was all part of the same trial. The very irrationality of bitter wood turning bitter water into sweet water was designed to underline that the whole experience was designed to test the Israelites’ dependence on G-d’s goodwill.