Almost there: The first flows over the spillway at Dartmouth Dam in 26 years are imminent.
Dartmouth Dam, the largest water storage in the Murray-Darling Basin, is set to spill over its wall for the first time in 26 years.
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After hovering around 99 per cent capacity for a month, water is expected to start flowing over the spillway on Thursday, September 22, thanks to inflows from last weekend’s rain.
Murray-Darling Basin Authority senior director of river management Joe Davis said water would begin flowing over the spillway at low rates and contribute minor flows to the Mitta Mitta River downstream.
Water will also continue to be released through the valves at the base of the dam.
“There has been plenty of local banter about the possibility of the spill, and I know people have been taking bets on when it might happen,” Dr Davis said.
“To begin with, water is expected to trickle over the spillway before the flows build up.
“Given it’s been such a long time since the last spill, we expect locals and tourists will be keen to visit Dartmouth, particularly over the school holidays. Please be courteous of others and take great care on the road up to the site and in the limited parking area.”
Rare occurrence: Dartmouth Dam, which has been considered effectively full since early August, has only spilled in four of the 43 years since it was completed in 1979.
There have been four years the dam has spilled since it was completed in 1979, each in the 1990s, the last of them 26 years ago in 1996.
“Dartmouth Dam has been considered effectively full since early August when we started pre-releasing water from the dam to manage air space,” Dr Davis said.
“The flow of water over the spillway at this stage is not expected to impact on the height of the Mitta Mitta River downstream, with flows expected to remain within the river channel at Tallandoon.”
During the spill in 1996, water flowed over the dam wall from September 30 to November 24, with a peak flow of 19,600 megalitres per day on October 4 that year.
River operators work in close collaboration with the Bureau of Meteorology to ensure dam management is guided by up-to-date rainfall and inflow forecasts.
Jointly managed by the MDBA and Goulburn-Murray Water, Dartmouth Dam is the most upstream storage in the River Murray System and collects almost 10 per cent of the system’s inflow. The Mitta Mitta River flows from Dartmouth Dam into the Hume storage, which is currently at almost 97 per cent capacity.
Like Hume Dam, the primary purpose of Dartmouth Dam is to conserve water during wetter periods as insurance against future drier times. Its management is governed by the rules of the Murray-Darling Basin Agreement.
The Bureau of Meteorology has forecast a wetter than average spring and summer, and with storages such as Dartmouth, Hume Dam and Lake Eildon at capacity, communities near rivers, or on a floodplain, are reminded to be prepared for flooding.
G-MW began releasing water from Lake Eildon in the Goulburn River system when it reached 93 per cent on August 26.
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