The preceding section shall not destroy or impair rights acquired by legislative grant prior to July first, eighteen hundred and seventy-eight, or destroy or impair prescriptive rights of drainage or discharge to the extent to which they lawfully existed on that date; nor shall it be applicable to the Merrimack or Connecticut rivers, or to so much of the Concord river as lies within the limits of Lowell.
Mass. Gen. Laws ch. 111, § 168