Wannon River insights Part 1: What does the ‘big wet’ mean for the river upstream of Brady and Walker Swamps?

It happened so long ago that it has been all but forgotten in recent years, but back in the 1960s the proposed diversion of the upper Wannon River’s headwaters off Mt William in the Grampians was a source of great concern to many people in the local community. We’ll revisit the finer details of that story in a future article, as it makes for a fascinating case study in river and water politics, but that tumultuous time eventually led to the Victorian state government endorsing engineering works to modify the the upper Wannon River’s headwater streams near Mt William. As a result, ever since 1970 these flows from the highest elevation area in the Grampians, with the most reliable rainfall, have been diverted north into the Wimmera catchment, via Fyan’s Creek and Lake Bellfield – the artificial dam constructed in the 1960s near Halls Gap.

The rules of the diversions over the past 50 years have been tilted significantly in favour of Lake Bellfield over the needs of Wannon River, with all flows – up to the capacity of the offtake pipes – diverted over the winter/spring flow period each year (i.e. on average, throughout the wettest six months of our annual rainfall cycle). When flows in those headwater streams temporarily exceed the capacity of the offtake pipes, then some water can still pass downstream, but otherwise all low-moderate flows are captured and diverted away from the Wannon River. The valves on the offtake pipes are typically closed around the end of November each year, meaning that any summer rainfall events at Mt William do find their way down the Wannon, but of course sizeable flows at that time of year are much less common due to the typically dry summer and autumn climate we experience in southern Australia.

That is how things have operated over the past 50 years, but this year, something that doesn’t happen very often has occurred. The current ‘big wet’ has caused Lake Bellfield to reach full capacity for the first time in 25 years, generating flows over the dam’s spillway. There is certainly no denying that it is a spectacular sight when this happens!

The following video by Chris Dunmill shows the appearance of the Lake Bellfield spillway on the 1st of November 2022. To see Chris’ other videos, please follow this link.

On those rare occasions when Lake Bellfield fills to capacity (as you will see here, it is currently sitting at 101%), then the Wannon River gets a temporary reprieve and the artificial diversion offtake valves are closed early. This was confirmed in the October operational update by GWM Water, where it was stated that:

“Lake Bellfield received increased inflows in response to significant rainfall events over the month (of October). The Wannon Diversion was closed early in mid-October to reduce the volume flowing to Lake Bellfield. The majority of inflows were coming from the Fyans Creek catchment upstream of the Bellfield wall. The reservoir reached its full supply level in the final week of October. As of 8 November, the reservoir was still spilling and is estimated to have spilled 6,600 ML to this date.“

Of course when Lake Bellfield fills – like right now – it is typically very wet everywhere in the general region and the Wannon River itself is supplemented by flows from other downstream sources, including artificial drainage catchments like those that contribute additional water to Brady Swamp and Walker Swamp from drained wetlands to the east of the Grampians. As we explored last month, these floodplain wetlands of the Wannon are currently full to capacity.

For an example of a large drained wetland that is currently contributing flows towards the Wannon River via Brady Swamp and Walker Swamp, feel free to zoom around and take a closer look at the image below of Heifer Swamp, with a number of the nearby wetlands we have restored labelled. I took this photo about a month ago, and it shows the drains in Heifer Swamp temporarily overwhelmed and inundation starting to occur.

The combination of these drainage inflows and river flows have filled Brady, Gooseneck ad Walker Swamp to capacity – as shown below.

Of course, an extra six weeks of all Wannon River flows from Mt William (coming down the river from mid-October to late November) is likely to extend the summer inundation of these wetlands, capitalising on what has rapidly become a peak flow season. While it is a positive outcome for the river to have temporary extended access to its full natural catchment, it is noteworthy that the closure of the Wannon diversion to Lake Bellfield this year is, in essence, a dam management rather than an environmental flows decision.

So what have these diversions meant for the river every winter and spring over the past 25 years since Lake Bellfield last filled, and even further back over the past 50 years? What might the cost be to the Wannon River of missing out on the bulk of water from its most reliable headwater streams during below average rainfall years, when almost all of this water is diverted away? More so than in a wet year like the one we are currently experiencing, it is the dry years when the management of such flows really counts.

We started to touch on these issues in this article by Lachie last year, but stay tuned for the next instalment, when we take another closer look at this important topic for the future health of the Wannon River.

NGT is currently investigating the eco-hydrology of the upper Wannon River within the Grampians National Park, thanks to the support of:

Parks Victoria and the Australian Government via the Australian Heritage Grants Program 2020-21.