perhaps the 645 was to save the dam wall from letting go. Without being an engineer my understanding is that if the dam level rises above the top of the spillway the rammed eath construction at the side of the dam wall could erode away then dam faliure which doesnt bear thinking about! There fore once today was confirmed as rain easing to showers the immediate danger has passed allowing for more control release of flood waters. BTW I am lead to believe the above scenario was a distinct possibility as good be seen on Anna's face last night.
