Perth looks back to the day when Mundaring Weir last overflowed

Thirty years ago, Jens Jorgensen, the owner of the Mundaring Weir Hotel, noticed the water levels in the dam were rising every day.

Over almost a month, he went down and recorded how far the water was from the top, a record he saved on a board in the bar of the hotel, which he still owns and runs.

"I measured it from the top of the wall. I wasn't supposed to do that, but I dropped a tape down and measured it up," Mr Jorgensen recalled.

When he first measured, it was 1,800mm from the top, and day by day, amid strong rainfall, the level grew higher.

On Sunday, September 1, 1996, it was just 30mm from spilling over the top, and anticipation was building in Perth that the dam was going to overflow for the first time in years.

It took another two days to reach those last 3 centimetres, but when it did, on September 3, the event was a sensation. 

'A terribly exciting time'

Thousands of people flocked to the hills to see the spectacle.

"It was turmoil … people just started to drive up, and I know the police had quite a few issues with parking," Mr Jorgensen said.

"It was something completely different, and you know, I found it very exciting."

The water overflowed and spilled down into the Helena riverbed for two weeks before the level fell.

Almost 30,000 people came to see it, but what those thousands of people did not realise was that it was the last time the dam would ever overflow.

First constructed to dam the Helena River and supply water to be piped to the Goldfields in 1903, the Mundaring Weir regularly overflowed after it was built, so much so that work began to raise the wall in the 1940s, and was completed in 1951.

Overflows continued in the 1950s and 1960s, as rain fell steadily each winter. Rainfall volume began to drop in the mid 1970s. Prior to 1996, the last overflow had been in 1974.

In 1996, Diana Frylinck had just started working for the Water Corporation on-site, at what was then the CY O'Connor Museum (now named the No 1 Pump Station) and vividly remembers the spectacle of the overflow.

The museum, where she still works today as a senior educational officer for the National Trust, is close to the base of the dam wall.

"It came over gently and quietly, and it just looked like lace when it was coming over,"

she said.

"That's thanks to [engineer] CY O'Connor, one of my heroes, because he designed the wall such that it has a curve, and the idea to dissipate the energy in an overflow, so it just came over calmly and gently."

It was, she recalled, a "terribly exciting" time and her role gave her special access.

"We had thousands of visitors coming, and we had to open — from memory it was 8 o'clock in the morning till 6 o'clock at night — to accommodate all these people beating down our door, which was fabulous.

"[Sadly] the walkway across the wall had to be closed, because there were all these thousands of people, and we weren't sure if it would actually hold. Only very special people were allowed to go out and have a look."

'We thought it would last forever'

For two weeks the water continued to flow and people "parked all over the place", Ms Frylink said.

"One of my memories is that one of the visitors who came was actually Alan Bond," she said.

"That week, he was due to go to court and be sentenced and sent to jail, and I thought, isn't that interesting? The last thing that you would want to do is actually come and see the dam overflowing, so that was quite an exciting visitor to have."

Although she knew it was a rare sight, Ms Frylinck and the other visitors had no idea that it would be the last time it would happen.

"I think we were thinking that it would last forever. The next year we all got training in how to handle interviews, because we thought it would happen again.

"We were just thinking, this is wonderful, there's going to be so much rain, and sadly, that has not happened."

Rainfall declines steeply

Thirty years later, winter rainfall in the Perth catchment is lower than it has ever been.

The Mundaring Dam is just 42 per cent full.

Waterwise programs manager Damien Postma said over the past 10 years, an average of 75 billion litres of streamflow has come into Perth water dams each year.

This year, just over 12 billion litres has been collected, with total storage at the lowest level since 2017.

It is a far cry from the average 420 billion litres that flowed annually up until the mid-1970s.

According to Michael Wiltshire, Water Corporation manager of water treatment operations from Perth to Harvey, it would take a cyclone to fill the dam to overflowing now, and even then, it probably would not be allowed to overflow because it would be a waste of a precious resource.

"Since the dam overflowed in 96, I think there have been upgrades to the integrated water supply system," he said.

"That supply system allows us to move water around through the Perth network to better store our water and ensure that we don't waste water, which is what an overflow event would be."

As the person who oversees the management of everything at the weir, from picnic areas to the pump stations and all the water treatment, he said the thought of thousands of people rushing to see an overflow event also sounded to him like an "operational nightmare".

"So many people, so many cars. I've heard all the stories. From a public perception, it would have been fantastic," he said.

Every drop is precious

Now, on the 30^th anniversary of the overflow, and coming into spring after a terrible winter for water collection, the Water Corporation is firmly focused on preserving as much water as possible.

While the water in Mundaring Dam does not supply Perth, it's sent into the Wheatbelt and Goldfields via CY O'Connor's famous pipeline, collection is down across the network.

"We've had one of the driest winters in over a decade," Mr Postma said.

Rainfall now provides just 15 per cent of Perth's drinking water; the rest is supplied from desalination plants and groundwater.

Mr Postma implored people to do everything they could to reduce water use and stressed that while spring had commenced, there was no need to start using garden sprinklers or irrigation until the end of September.

"This will leave billions of litres of water in the dams for summer when we need it the most,"

he said.

"We have climate-independent [water] sources planned in many areas around the state, but the best thing we can do is use the water we have already wisely."

Like his colleagues, Mr Postma did not expect to ever see the dam overflow again and would be happy if people could focus on things like taking short showers and leaving the tap off when they brush their teeth.

"Those things seem small, but they add up," he said.

"Watching a dam overflow is always a draw card for people to come and it's a shame we won't see that again, but that is the reality of climate change here in the southern parts of Western Australia."