The classical Roman law sources, on the other hand, were rea sonably clear as to the former of these two questions. Let us again turn to Ulpian's commentary on the Praetorian edict: A public river is distinguished from a private one magnitudine . . . aut existima tione circumcolentium, Dig. 43, 12, 1, § 1. Rivers can be perennial or torrential; a perennial river is one which always flows.Id. § 2. Ul pian then proceeds to note that Cassius defined a public river as a perennial one, that Celcus had approved this definition, and that it could thus be viewed as probable. ld. § 3. The primary need for dis tinguishing between public and private rivers becomes apparent id. at § 4: The praetor's interdict (or judicial injunction) runs only to interference with navigation on public rivers, for a private river is not different from any other private premise: nihil enim differt a ceteris locis privatis flumen privatum.