What do the official numbers reveal about last month’s footage of the Wannon River Diversions?

Last month I shared a video that showed the operation of the Wannon Diversions on a single day back in August, along with a range of other locations downstream of the diversions within the Grampians National Park, in both the Wannon River and Fyans Creek catchments.

You can take another look at that story and the video by clicking this link.

This month, we’re going to revisit a few key locations highlighted in the video, with the benefit of now being able to refer to the official recorded flow data which is now available for that specific day, the 14th August 2026, to see how the vision corresponds to the precise volumes of water measured (over 24 hours) on that date.

So, let’s take a closer look to see what the data reveals…

1. Diversion Volume on the 14th August 2026

Firstly, the total volume of water diverted from the Wannon River to Lake Bellfield that day, which – as we could see in the video (and the still images below), comprised 100% of the flow coming down the First and Second Wannon Creeks off the Major Mitchell Plateau – was 29.37 megalitres (ML).

Please click on any image to view an enlarged version.

Over that full 24 hour period, this means that an average of 1.2 megalitres (ML) per hour – i.e. 1.2 million litres per hour – was being diverted away from the Upper Wannon River on the 14th August.

I’ve taken a still image from the video, to show you what that total amount of water at the outlet pipe from the three diversions looked like:

2. Fyans Creek Volume on the 14th August 2026

Secondly, the total volume of water recorded going over the nearest downstream weir on Fyans Creek, en route to Lake Bellfield, over the same 24 hour period was 62.68 ML, or an average of 2.6 million litres per hour. Of that total and as highlighted above, 1.2 million litres per hour of this recorded volume was diverted Wannon River water.

So at this location, over this whole day, almost half (47%) of the volume of water measured flowing down Fyans Creek was coming from the water being diverted from the Wannon River. Once again, I’ve taken a still image from the video to show you what that amount of water looks like:

3. Upper Wannon River Volume on the 14th August 2026

Next up, the total volume of water recorded going over the nearest weir on the Wannon River, downstream of the diversions at the Jimmy Creek Campground (but above the confluence with Jimmy Creek), over the same 24 hour period was 25.98 ML, or an average of 1.1 million litres per hour.

So on the day of our visit, just over half (53%) of the 2.3 million litres per hour that should have been flowing past this location (under natural conditions) was being diverted away from the Wannon River. Here is what this location on the Wannon River looked like that day:

4. Jimmy Creek Volume on the 14th August 2026

For comparison, the volume of water recorded coming down Jimmy Creek that day, just before it reaches its confluence with the Wannon River, was 7.4 ML, or an average of approx. 300,000 litres per hour. Here is what that volume of water looks like:

5. Wannon River Volume at Dunkeld on the 14th August 2026

Finally, on the day of our visit, the Wannon River at Dunkeld recorded a total 24 hour period flow volume of 14.25 ML, or an average of approx. 600,000 litres per hour.

Here is the footage of a very gently flowing Wannon River at this location:

In case you are wondering why the recorded flows at this location are significantly less than the Wannon River at the Jimmy Creek Campground, despite being further downstream – which at first seems counterintuitive, because rivers normally get bigger and gain flows as they travel downstream – I can assure you that there is a simple explanation, which is revealed in the data!

Below is a table showing 3 weeks (21 days) of official rainfall and flow data in August, covering a period either side of the date of our visit on the 14th August (which is highlighted yellow).

[Note: Mount William, which is the location of the nearest BOM rainfall gauge, and nearby areas of similar elevation on the Major Mitchell Plateau within the upper Wannon River catchment, receive some of the highest average annual rainfall in south-west Victoria (>900 mm of mean annual rainfall). For example, on the 3 dates highlighted below in blue, of 41mm, 52 and 25 mm rainfall events, nearby towns at lower elevations such as Hamilton (2mm, 11mm, 8mm), Willaura (2mm, 6mm, 4mm) and Ararat (21mm, 21mm, 13mm) recorded far less rainfall. Ararat’s falls also indicate that these particular weather systems generated more rainfall in the northern Grampians.]

Three of the more significant rainfall events on Mt William are also highlighted (in blue), so that you can see the influence of heavy rainfall at higher elevations in the Grampians on upper catchment flows. As you can see, these events are immediately evident – on the very same day – for measured volumes at both Fyans Creek and on the Wannon River at Jimmy Creek Campground. This is because the catchments of the First and Second Wannon Creeks have steep slopes and consist of large areas of exposed rock, meaning that a much larger percentage of this rainfall rapidly runs off and flows down the creek lines than areas of the catchment in the National Park further downstream. These characteristics, of high average rainfall and reliable runoff generation, clearly illustrate the importance of the upper Wannon catchment to the health of the river downstream.

After a closer look at the data in the table above, it appears that the 40mm rainfall event on the 10th August may have been more isolated than the 52mm and 25mm rainfall events on the 19th and 25th August respectively, because notice how Jimmy Creek flows only responded more dramatically to the later rainfall events?

Also take a look at how much time it takes for these rainfall events to begin to register on Wannon River flows recorded at Dunkeld, in the column on the right.

Because this location on the Wannon River is downstream of both the alluvial fan and instream wetlands of the river, like Gooseneck and Brady Swamp (which occur in a slow moving, naturally sluggish reach of the river, with lots of instream capacity to store and slowly release flows), there is a significant delay in water reaching Dunkeld after heavy rainfall events. This effect can be exacerbated significantly if the alluvial fan is dry, and/or if Gooseneck and Brady Swamp are not yet full. However, at the time of our visit, Gooseneck and Brady Swamp were near full and had begun passing flows to the Wannon River downstream.

Hence, the data illustrate (as we explored previously in this article by NGT’s Lachlan Farrington) how flashy flows higher up in the Wannon River catchment take much more time to pass through the lower, flatter part of the National Park, with that water eventually finding its way into and through the alluvial fan and instream wetlands.

As this occurs, a gradual, steady increase in flows in the Wannon River at Dunkeld can be seen developing in the table above. This reflects the river’s natural ‘flow attenuation’ character at this location (i.e. its ability to use the storage of instream wetlands to reduce the peak volume of water being discharged after heavy rainfall, but then extend the duration of flow, offering a valuable form of ‘flood buffering’ for the river downstream and sustaining critical wetland habitat, all at the same time. This natural attenuation effect has been enhanced within this reach of the Wannon River over the past 13 years as a result of NGT’s past restoration work to reverse the impact of artificial drainage on both Brady Swamp and Gooseneck Swamp.

To help visually illustrate this effect, the annotated image below from early September 2026, shows how the Wannon River disperses across the alluvial fan (blue arrows), before entering Brady Swamp and Gooseneck Swamp. The yellow line shows how the river reforms again into a single flowpath downstream, as it exits Brady Swamp en route to Dunkeld.

Also note how dry the surrounding landscape is this year, beyond these two instream wetlands that are directly fed by the river.

For example, given the lack of runoff generating rainfall this year in the southern Grampians at lower elevations, NGT’s adjacent Walker Swamp Restoration Reserve (to the right of Gooseneck Swamp in the image above) is virtually dry, except for a few puddles and a small area of shallow water in the deepest part of Walker Swamp. This illustrates just how critical upper catchment river flows are to sustaining aquatic species in the landscape during patchy rainfall years like the one we are experiencing this year, where the wider landscape (in this area at least) is not contributing any flow to the upper Wannon.

Finally, I thought I’d also share the 3 weeks of data from the table above in another way, to more clearly illustrate the impact of the diversions…

The chart above shows just how much difference the diversions made to the total upper catchment flows of the Wannon River and Fyans Creek over this 3 week period in August 2026.

As you can see, if there were no diversions in place, the Wannon River at the Jimmy Creek campground would have seen a flow volume of of 1,797 ML over this 3 week period, which would have actually exceeded (by 167 ML) the total Fyans Creek’s natural flow of 1,630 ML. However, with the diversions in place, the measured flow in Fyans Creek was 2,425 ML, dwarfing the artificially modified Wannon River flows, which were instead reduced to 1002 ML.

In summary, instead of both waterways carrying similar natural flow volumes over this 3 week period, the diversions meant that Fyans Creek received almost two and a half times the volume of water that flowed down the Wannon – a substantial difference that was clear to see on the day of our visit, as shown in the images throughout this story.

Presented in this way, the impact of the diversions on the Wannon River during the critical winter and spring flow period is stark and – in my view – impossible to ignore.

Despite how wonderful Brady Swamp and Gooseneck Swamp appear in the image I shared above, it is worth remembering that looks can be deceiving… after all, with significantly less water currently passing through these instream wetlands than should be the case, the health of the Wannon River and its associated floodplain wetlands, both upstream and downstream of this location, are being silently impacted each and every year that the current diversion rules remain in place.

To learn more about the upper Wannon River, you can view our previous stories here.