Gemara

2a (Mishnah - R. Elazar ben Azaryah): We may not make a new irrigation channel on Chol ha'Mo'ed or in Shemitah;

Chachamim permit.

We may fix a ruined irrigation channel on Chol ha'Mo'ed and fix the water supply in Reshus ha'Rabim and unclog it.

4b - Question: What is considered ruined?

Answer (R. Aba): If it is now only one Tefach deep, one may restore it to its initial depth of six Tefachim.

Clearly, if it was initially three Tefachim and now only half a Tefach (deep), one may not fix it. Since water did not flow properly before, and it will not flow properly after it is fixed, this is unnecessary exertion.

Clearly, if it was initially 12 Tefachim and now it is only two Tefachim, one may not restore it to 12. (Since six Tefachim suffice,) this is unnecessary exertion.

Question: If it was initially seven Tefachim and now it is two, may one restore it to seven?

Just like one may clear out five clogged Tefachim of a six Tefach channel, it is also permitted here;

Or, perhaps it is forbidden, for the seventh Tefach is unnecessary exertion?

This question is not resolved.

Abaye permitted people of Bar Hamrach to be Shochef a river. R. Yirmeyah permitted people of Sukosah to open the source of a stopped up river.

Rav Ashi permitted people of Masa Machsiya to dig out the river Bornitz. Since people drink from it, this is a public need, and the Mishnah permits all public needs.

(Mishnah): We may fix the water supply in Reshus ha'Rabim (and unclog it).

Inference: We may unclog it, but we may not dig it anew.

(R. Yochanan): This is only if it is not needed for the Rabim

Question (Beraisa #1): We may unclog an individual's pits, all the more so the Rabim's pits. We may not dig anew pits for the Rabim, all the more so for an individual.

Suggestion: This is even if the Rabim need them!

Answer: No, it is if the Rabim do not need them.

Question: Surely, the case of an individual's pit is similar. If it is not needed, why may we unclog it?

(Beraisa #2): We may gather water into an individual's pit, but we may not unclog it or plaster over cracks.

This is permitted for pits of the Rabim.

Counter-question: If the Rabim need the pits, one may dig them anew for the Rabim!

(Beraisa #3): We may gather water into an individual's pit and unclog it. We may not plaster over cracks or clear out (the cracks; alternatively, deepen the pit) or plaster the pit. We may dig anew pits for the Rabim and plaster them.

Question: Beraisa #3 contradicts Beraisa #1!

Answer: We must explain Beraisa #1 as follows:

We may unclog an individual's pits if he needs them. All the more it is permitted for the Rabim's pits if they need them; one may even dig them anew! We may not dig new pits for the Rabim if they are not needed, all the more so for an individual; in this case, he may not even unclog them.

Support (Rav Ashi): The Mishnah permits 'all public needs'. This includes digging pits!

Rishonim

The Rif and Rosh (1:6) bring the Mishnah, what the Amora'im permitted regarding rivers, and the Heter to restore an irrigation channel from one Tefach to six Tefachim.

Nimukei Yosef (DH Gemara): If the channel is now less than a Tefach deep, it is considered sealed and one may not restore it. One may not restore a channel to seven Tefachim, for six is enough. Also, it is a great exertion to remove dirt that is so deep.

Nimukei Yosef (ibid.): Abaye permitted to be Sochef a river, i.e. to make it steeper so the water would flow faster. Rav Ashi permitted digging out the source of the river to make a nicer flow.

Rashi (4b DH Lishchufei and DH Le'akduchei): Abaye allowed uprooting trees growing in the river. Rav Ashi allowed clearing out debris deposited in the river.

Rif and Rosh (ibid.): We may unclog water supplies even if the Rabim do not need them. We may dig anew only if the Rabim need this. If an individual needs his pit we may unclog it, but we may not dig anew. If he does not need it, he may not unclog it or plaster it. He may gather water into it.

Rambam (Hilchos Yom Tov 7:10): On Chol ha'Mo'ed we may fix water problems in Reshus ha'Rabim. We may dig water pits and unclog rivers for the Rabim to drink. We may gather the water into public pits and fix their cracks.

Ra'avad: We may unclog rivers (and dig pits) only if the Rabim needs them. We may unclog an individual's pit if he needs it, but we may not dig anew.

R. Mano'ach: Perhaps the Ra'avad means that even pits needed for the Rabim may be dug only if they are of the Rabim.

Kesef Mishneh: The Rambam mentions 'for the Rabim to drink' regarding digging and unclogging rivers, but not regarding fixing water supplies. This suggests that he agrees with the Ra'avad, who allows Ma'aseh Uman only if it is needed for the Mo'ed.

Rambam (8:4): We may divert a river and open a clogged river. If an individual needs his pit we may unclog it and plaster the cracks, but we may not dig anew. We may gather water into it even if it is not needed.

Magid Mishnah: The Ramban says that surely, if an individual has nothing to drink he may dig anew, as long as he does not scheme to dig a big pit for many days. This is like one who has nothing to eat; he may harvest and thresh. The Gemara discusses one who could toil to drink from the river, just he prefers to drink from his pit. He is permitted to unclog, but not to dig anew. If his normal water source is intact, even unclogging is forbidden. One should be stringent because water is available and cheap. Perhaps digging is forbidden even if it is very difficult to obtain water. It seems that the Rambam holds like this, for he did not distinguish.

Poskim

Shulchan Aruch (OC 544:2): We may fix pits of the Rabim into which dirt and stones fell, even if they are not needed now, since it is not Ma'aseh Uman. To dig pits is Ma'aseh Uman, so it is permitted only if the Rabim need them now. This is even if it was left to be done on Chol ha'Mo'ed and it is done overtly and it is it is a great exertion.

Mishnah Berurah (13): If they are not needed, it is forbidden even Ma'aseh Hedyot and covertly.

Shulchan Aruch (ibid.): One may not dig pits for an individual even if he needs them now. He may fix existing pits only if he needs them now. He may direct water into them even if he does not need it now.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): This is permitted because it is not a great exertion.

Mishnah Berurah (16) and Sha'ar ha'Tziyon (16): He may use Kelim to take water from his roof to the pit, but he may not make a channel. The Ritva forbids Kelim. He permits only to direct the water to the pit through existing channels.