Data gathered by Goulburn Murray Water, and shared on its social media, shows January's flood at Seymour and Yea - as well as in 2022 and 2023 - were most likely the result of heavy rainfall downstream of Lake Eildon.
Goulburn-Murray Water figures, contained on its website, show unregulated creeks and rivers downstream of Lake Eildon were the "primary contributors" to peak flows at the two towns, during the three events.
Landholders in the Upper Goulburn area have questioned GMW's management of Lake Eildon storage levels.
GMW Water Resource manager Mark Bailey said no two floods were the same.
"The role water storages are able to play in mitigating flooding therefore varies, with every rain event," Dr Bailey said.
"Different parts of the catchment are impacted by the water stored and released from Lake Eildon in different ways.
"Any decisions relating to releases must subsequently consider conditions across the broader catchment, the time it will take for the water to reach different parts of the catchment, and inflows that are forecast from rivers and creeks downstream of Lake Eildon."
He said GMW had the ability to take Lake Eildon beyond full capacity - surcharging - to minimise releases from Lake Eildon as much as possible.
Information on the GMW website shows in October, 2022, there was widespread rain in the Goulburn River catchment, with inflows peaking at 145,000 megalitres a day.
The figures on the GMW website show outflows peaked at 38,000ML a day.
This was above the moderate flood level, immediately downstream of Lake Eildon, and had an impact on Molesworth, as well as communities and landholders, immediately downstream of the storage.
"Releases from Lake Eildon take approximately 48 hours to reach Seymour," the reports said
"48 hours before the peak flows at Seymour, releases from Lake Eildon were about 12,400ML per day, meaning Lake Eildon releases accounted for less than 10 per cent of the peak flows.
"Most of the flows originated from tributaries downstream of Lake Eildon, such as Sunday and Sugarloaf creeks."
In October last year, inflows peaked at 120,000 ML per day, while outflows reach 17,000 ML, which was above the minor flood level.
But the website said the organisation was able to surcharge the storage ensuring releases weren't higher.
"Downstream creeks and tributaries were still a significant contributor to peak levels downstream of the storage, with the peak flow at Seymour being about 28,000ML per day, with Lake Eildon releases only contributing about 4500ML per day."
This year, most of the rain fell below Lake Eildon, with flooding on the Goulburn driven by tributary flows, downstream of the dam.
The releases from Lake Eildon accounted for about 1,000ML per day of the 66,000ML per day peak at Seymour, the latest report said.
"While inflows to - and releases from - Lake Eildon varied with each rainfall event, in all three instances, peak outflows from Lake Eildon were significantly less than peak inflows, indicating the flood mitigation benefits, provided by the storage," the reports say.
Meanwhile, Melbourne University Professor of Hydrology and Water Resources Rory Nathan said predicting floods was "a bit like a pokies slot machine.
"Instead of pears, apples, and bananas coming up when you pull the handle, we generate combinations of different rainfall patterns, soil moisture conditions, storm locations, and distributions in time," Professor Nathan said.
"This kind of modelling is called "Monte Carlo" sampling precisely because the approach we use to emulate Mother Nature takes account of how all these different random factors might combine to cause a flood."
GMW Storage Services general manager Martina Cusack said under the Victorian Water Act, said the primary role of storages was to harvest and store customers' water entitlements, while providing flood mitigation if possible.
"The framework provided by the Act requires target filling points to be set for the storage between May and October/November," she said.
While those targets were not yet in place, GMW had begun making releases from Lake Eildon to increase airspace and ensure the initial filling target on May 1 could be met without the necessity for significant releases in the days preceding it.
"Lake Eildon cannot prevent these communities from experiencing either flooding or drought but can mitigate the impacts of both," she said.
The Department of Energy, Environment and Climate Action is conducting an assessment of the operating arrangements for Lake Eildon, due to be released in March.
Andrew Miller
I'm a general reporter with Stock & Land, with a special interest in irrigation issues. I completed my cadetship, with the Age, in 1980. Over my career, I've worked for ABC radio news (Mt Isa, Qld) and at provincial and suburban newspapers.
I'm a general reporter with Stock & Land, with a special interest in irrigation issues. I completed my cadetship, with the Age, in 1980. Over my career, I've worked for ABC radio news (Mt Isa, Qld) and at provincial and suburban newspapers.
