What is Aqua Currit Et Debet Currere, Ut Currere Solebat?

Legal Definition
Water runs, and ought to run, as it has used to run. 3 Bnlst. 339; 3 Kent, Comm. 439. A running stream should be left to flow in its natural channel, without alteration or diversion. A fundamental maxim in the law of water-courses.
-- Black's Law Dictionary