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Why has it been so hard to predict the Murray River's flood peak in Echuca and Moama?

After a week spent evacuating homes, sandbagging and helping build levees, residents of Echuca, Moama and surrounding communities are close to experiencing a flood peak.

It is now expected to reach its highest point on Sunday or Monday after passing recent records on Saturday.

Over the past week, the forecast peak has been revised and updated a number of times, in some cases removing or adding whole days to the timeline.

Residents of Kerang, along the Loddon River, have experienced similar changing forecast peaks.

So why is the peak so hard to predict?

Peaks are measured using what's known as the Australian Height Datum

First, it's important to briefly explain that water levels across Australia are measured using the Australian Height Datum (AHD).

AHD is a geographical term that refers to the height of the water above sea level, but taking into account the change in tides.

As ABC Weather explains, the water level at Echuca goes up and down but this year it mostly hovered between 86m and 88m AHD until late August.

What is a dangerous flood level depends less on the AHD height and more on the specific body of water and the infrastructure built to contain it.

The Murray River is at the heart of a very complex system

Echuca and its surrounds already experienced one flood a week ago, when the Campaspe River burst its banks. 

The challenge now is the Murray River, which on Saturday morning had hit 94.77m AHD — the measured peak of the devastating 1993 floods.

"The key thing with a major river system like the Murray River is the complexities of many river systems that flow into it," SES chief operations officer Tim Wiebusch said on Saturday.

The Murray River is Australia's second-longest river and is part of the nation's biggest and most significant river system, the Murray-Darling Basin.

The river is fed by some of the country's other biggest rivers, many of which have experienced significant rainfall in the current wet weather.

In the case of Echuca and surrounds in these floods, the height of the Murray is affected by the Mitta Mitta, Keiwa, Goulburn and Campaspe rivers.

"Which really makes those hydrological predictions on the Murray all the more challenging," Mr Wiebusch said.

He said authorities in Victoria and New South Wales were in "constant contact" with the Murray-Darling Basin Authority about how water was being released out of the Hume Dam, which could affect water levels.

"The models are designed around that," Mr Wiebusch said.

"But it's also designed around the inflows from New South Wales as well, from some of their river systems, and as it goes further down to Swan Hill, where the mighty Murrumbidgee also comes in."

Thunderstorms make predictions harder

While the Murray River is still rising as a result of rain that fell on its tributaries more than a week ago, new rain is also falling this weekend.

Before the storm event started on Friday, Bureau of Meteorology forecaster Jonathan How said the patchy nature of the weather system was another factor.

"With the upcoming rainfall it is difficult to predict where exactly these thunderstorms will form because the next system coming through will be patchy, hit-and-miss thunderstorms," he said on Friday.

"So it does present a challenge when forecasting the peak."

Thankfully, Mr Wiebusch on Saturday said the Murray River was not expected to see a noticeable change due to storms this weekend.

But the Campaspe River which flows to Echuca was expected to rise to moderate levels in the early parts of the week, he said.

Kerang, a small community north-west of Echuca, is expected to be cut off from the rest of the state when the Loddon River reaches its peak there.

Residents were on Friday told it was too late to leave, but then on Saturday, when the forecast peak was revised, were told to evacuate immediately if possible.

No matter when the precise peak occurs, this is still a disaster

Communities across Victoria are reeling from the damage already caused by widespread heavy rain which hit the state late last week.

The clean-up is expected to be expensive and lengthy, with many people unsure about whether they will be able to return to their homes.

Even before the peak, dozens of people living on the "wrong side" of a new levee saw their homes inundated in Echuca, and many more have had to evacuate.

When the waters do peak in communities along the Murray River, authorities have warned it will take days or even weeks for them to fully recede.

"We still have a major flood emergency here in Victoria," Emergency Management Commissioner Andrew Crisp said on Saturday.

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