The department shall have power to investigate and hold hearings to determine existing water rights in riparian owners and the holders of grants and easements for the diversion and use of waters of the south branch of the Raritan river and the Raritan river and may make findings of fact in respect thereto. Nothing in this act shall be construed to diminish or impair the common law rights of riparian owners. L.1958, c. 34, p. 104, s. 11.