Sheviis, Chapter 3, Mishnah 8. This mishnah deals with the building of terraces alongside streambeds, in places where water flows during the rainy season, which were generally used for irrigating the fields.
Building the terraces on the eve of the Sheviis year:
The mishnah opens: "Ein bonin madregos al pi hage'ayos erev sheviis mishepasku hageshamim, mipnei shehu metaknan lisheviis" - one may not build terraces at the mouths of the streambeds, in the areas where the rainwater is destined to flow like a river. This prohibition applies on the eve of the Sheviis year after the rains have stopped, because at that point it looks as though he is preparing the terraces for the approaching Shemittah year, intending to irrigate his fields with them during Shemittah - which is forbidden.
Building the terraces during the Sheviis year:
By contrast, the mishnah rules: "Aval boneh hu bisheviis mishepasku hageshamim, mipnei shehu metaknan l'motzaei sheviis" - it is permitted to build the terraces during the Shemittah year itself once the rains have stopped, since it is then clear that the building is being done for the year following Sheviis. At that stage he no longer has any need for the terraces to irrigate during Sheviis, since that year's irrigation season has already ended, so of necessity he is doing this only for the years after Shemittah.
"V'lo yismoch be'afar aval oseh chayitz":
Even when the building is done in the permitted manner - during the Shemittah year after the rains have stopped - he still may not support the terraces with earth; rather, he makes a chayitz. What is a chayitz? A pile of stones or a stone wall built without any mortar at all: stones placed one on top of the other, with nothing binding them together.
Which stones may be taken for building the terraces?
On this the mishnah rules: "Kol even sheyachol lifshot es yado v'litlah - harei zu tinatel" - any stone that the builder can take by stretching out his hand in the course of the building may be taken. Taking stones involves an additional concern beyond the concern of building terraces for irrigation: perhaps his intention is to clear his field and make it fit for working. For this reason, precisely when he takes only the stones that are right nearby is it clear that his aim is not to improve the field, but to build the terraces for irrigation after Shemittah.
In summary: This mishnah taught that building terraces at the mouths of streambeds is forbidden on the eve of the Sheviis year after the rains have stopped, because it looks as though he is preparing them for irrigation during Shemittah, and it is permitted during Sheviis itself after the rains have stopped, when it is evident that he is preparing them for the year following Sheviis. Even in permitted building, one does not support them with earth but makes a chayitz of stones without mortar, and one takes only the stones that are within reach, so that he will not appear to be clearing his field.