That's the oldest of the suite of statutes that's being revised. It's from 1955, and it was created to allow for a government decision about constructing any dam or diversion that would affect the flow of an international river. It sets out a process for regulating, permitting, and essentially constructing things like dams and diversions that would have a significant effect on the water flow.
On March 26th, 2009. See this statement in context.