It is permitted on Chol ha'Mo'ed to use a stream to water a field that requires a lot of watering. (1)
Our Mishnah which holds that a newly flowing stream may be used to water a field that requires a lot of watering is in accordance with Rebbi Yehudah.(2)
Rebbi Yehudah says that a person may not divert a stream on Chol ha'Mo'ed for the purpose of watering his garden or Churvah (an area that once had trees and is now a garden).
One who weeds or waters his garden on Shabbos should be warned that he is about to transgress the Melachah of plowing according to Rabah or planting according to Rebbi Yosef.(3)
Rebbi Kahana says that a person who prunes a tree on Shabbos with the intention to use the branches is liable for two Melachos, planting and harvesting.
A person that weeds Kilayim or covers seeds of Kilayim with earth is liable for lashes. Rebbi Akiva says that simply allowing the Kilayim to remain makes one liable for lashes.
A BIT MORE
1. Even a newly flowing stream may be used to water a field that needs a lot of watering. However, a field that does not need a lot of watering may not be watered on Chol ha'Mo'ed even from an already established stream.
2. According to Rebbi Meir one may water any type of field even from a newly flowing stream. Rebbi Eliezer ben Yakov holds that a newly flowing stream may not be used at all even to water a field that needs a lot of watering.
3. According to Rabah, weeding and watering softens the land and is therefore included in the Melachah of plowing. According to Rebbi Yosef it stimulates the crops to grow and therefore is included in the Melachah of planting.
Rashi says that the Torah prohibits work on Chol ha'Mo'ed but does not specify which types of work are forbidden. It was left up to the sages to specify the types of work that are forbidden. The Ran notes that we do not find any Rishonim that included the prohibition of work on Chol ha'Mo'ed as one of the Taryag Mitzvos. The Ran therefore concludes that the prohibition is Rabbinic in nature. The Ramban takes a compromise position. According to him, work on Chol ha'Mo'ed is prohibited from the Torah. However any work done for the needs of the Mo'ed or to avoid a financial loss is permitted. The sages, however, prohibited certain types of work even though they may be included in one of those categories.
Certain types of work are prohibited on Chol ha'Mo'ed and other types are permitted. The Halachic status of the work depends on whether the sages determined that type of work to be necessary (Shulchan Aruch OC 530:1).
The Mishnah Berurah comments: A person shall be very diligent not to do any type of work that is prohibited on Chol ha'Mo'ed because our sages say that one who disgraces the Mo'ed [by doing work on Chol ha'Mo'ed] is tantamount to one who worships Avodah Zarah. A person is also obligated to honor Chol ha'Mo'ed with food and drink and clean clothing and should not treat it like a regular weekday.