Peah 2:2

If a channel of water passes through a field so that one could not reap from one side to the other side, Rabbi Yehuda says this qualifies as a division. Hills that cannot be plowed do not count as a division even though the cattle cannot pass over them with the plow.

Peah 2:3

All the things listed in the previous two mishnayos form a division for seeds but only a fence can make a division for trees. If the branches of the trees intertwine over the fence, then even that does not divide the field and one would give peah for the whole field as one.