Moed Katan, Chapter 1, Mishnah 2. The Mishnah opens with the words of Rabbi Elazar ben Azaryah: "Ein osin amas hamayim batechilah bamoed uvashvi'is" - one may not dig a water channel from scratch on Chol Hamoed or in the Sabbatical year. An amah is a trench through which water flows, and it is called by this name because it is a cubit (amah) deep and a cubit wide.
The dispute between Rabbi Elazar ben Azaryah and the Sages:
Rabbi Elazar ben Azaryah: One may not dig a new channel on Chol Hamoed or in the Sabbatical year, because this task involves great bother and considerable exertion, which we do not want performed at these times, and because it looks like hoeing the ground, a labor forbidden in the Sabbatical year.
The Sages: "Osin amah batechilah bashvi'is" - one may dig a new water channel in the Sabbatical year, since in their view it does not look like hoeing. And in any case, the earth is not left in the field but is carried out of it, whereas one who hoes leaves the earth in place.
Public needs that are attended to on Chol Hamoed:
"Umesaknin es kilkulei hamayim" - water channels that need repair, or that have become blocked with earth, are repaired on Chol Hamoed so that they will be fit for use.
"Sheb'reshus harabim" - even problems with water conveyance in the public domain, such as pipes in a public area that have broken down, may be repaired on Chol Hamoed, since this is done for the use of the public.
"V'chotzvin tinaros" - the cisterns are cleaned out of the dirt, the stones and whatever else has accumulated in them.
"Umesaknin es hadrachim v'es harechovos" - the paths and the streets are repaired.
"V'es mikvaos hamayim" - the mikvaos are repaired and cleaned of everything that collected in them over the course of the winter and the rainy season.
"V'osin kol tzorchei harabim":
The Gemara views this statement as an addition that would seem to have been unnecessary, since several public needs attended to on Chol Hamoed have already been listed. The Gemara therefore explains that it means that one may open the water reservoirs that filled during the winter months and make them available for public use. The Rosh adds that this statement comes to include the fact that a court may deal with many matters, all of which are public needs: from the redemption of items that were consecrated, to giving a sotah the sotah waters to drink, to the burning of the parah adumah, and many other matters.
Furthermore, the Mishnah says that the graves are also marked: the practice was to place paint or a kind of lime on the graves in order to indicate that there is tumah there, and this work is done on Chol Hamoed because the markings wore away over the course of the winter.
Likewise, agents were sent out by the court to deal with matters of kilayim, where people had grown forbidden mixtures in their fields. They would warn them before the festival to remove the kilayim and uproot the mixtures, and if they did not attend to it, the agents were sent to make sure the matter was taken care of.
The workers engaged in these tasks are paid from the Temple treasury, from consecrated funds, and therefore care is taken with them. Since the people are not busy with their own work on Chol Hamoed, it is easier to find inexpensive labor on those days. And although work is generally forbidden on Chol Hamoed, since this is work required by the court, and this is a time when wages are cheaper, the court may exercise this authority and hire the workers specifically on Chol Hamoed.
In summary: In this Mishnah we learned the dispute between Rabbi Elazar ben Azaryah and the Sages concerning digging a new water channel on Chol Hamoed and in the Sabbatical year, and the list of public needs attended to on Chol Hamoed: repairing damaged waterways, including those in the public domain, clearing out cisterns, repairing the roads and streets and the mikvaos. We examined the principle "v'osin kol tzorchei harabim" and the explanations of the Gemara and the Rosh regarding it, the marking of graves, the agents of the court who went out to deal with matters of kilayim, and the permission to hire the workers from the Temple treasury specifically at this time.