THE Dartmouth Dam, the largest storage in the Murray-Darling Basin, is spilling for the first time in 26 years, as floodwaters rise across the nation's biggest river system.

The Victorian alpine dam, which holds up to four million megalitres of water, is the source of about 10 per cent of the Murray River's inflows.

The Murray-Darling Basin Authority said the dam, which has been full since early...