Definition of Filum aquae
A thread of water; a line of water; the middle line of a stream of water, supposed to divide it into two equal parts, and constituting in many cases the boundary between the riparian proprietors on each side. Medium filum is sometimes used with no additional meaning. Cf. Thalweg.
That's the definition of Filum aquae in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.