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.