 On the north-western side, the border was marked by the river Arno, a much wider and more important waterway, which flows westward from the Apennine Mountains (its source is not far from Rubicon's source) into the Tyrrhenian Sea.Governors of Roman provinces were appointed promagistrates with "imperium" (roughly, "right to command") in one or more provinces.The governors then served as generals of the Roman army within the territory they ruled.