11. It is forbidden to draw water on Shabbos from a stream that flows through one's courtyard (1).
(1) This prohibition applies only to a stream that is at least ten
"tefachim" deep (approx.95cm) and at least four "tefachim" wide (approx.
37cm), because a stream of those dimensions is considered a separate
domain, and is defined as a "carmelis" (See HY 81:4); therefore, one may
not transfer water from it to the courtyard, which is a private domain. If,
however, the stream was either less than ten "tefachim" deep or less than
four "tefachim" wide, one may draw water from it, because it is considered
part of the courtyard, and not a separate entity. Furthermore, if the
stream is not flowing in from the outside, but is a self-contained pool
within the courtyard, it is considered part of the courtyard, even if it is
over ten "tefachim" deep, and more than four "tefachim" wide.
The solution to this problem is to build a partition ("Mechitzah") at least
ten "tefachim" high at both the place where the stream enters the
courtyard, and the place where it exits; the partitions serve to
halachically separate between the part of the stream which exists outside
the courtyard walls, and the part within the walls; as a result, the
section of the stream within the walls is considered part of the courtyard,
and not a separate entity; one may then draw water from it on Shabbos. The
actual walls of the courtyard itself, are not sufficient to perform this
separation, even if they extend down into the water, because it has to be
clear that the partitions were built specifically for separating between
the different sections of the stream.