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A guide to Goulburn Rivers water levels and management

EMAIL RECEIVED:

I much appreciate your regular fishing reports and the effort you put into it.  However, for virtual fly fishing beginners such as myself, I believe that if you were to publish a background paper on Goulburn River water flows that beginners would get more from the reports.  For example, the Goulburn River level @ 130 megalitres a day.  What does this mean?  (I assume that is the volume released via Eildon per day).  Is there a statutory requirement to release a minimum flow? If there is what is it? What impact does the raising or lowering of water levels in the Goulburn have on hatches, likelihood of fish to take flies, differences in approach that fly fishers should take, and the like?  I don't believe that such background info need be provided on an ongoing basis but perhaps as a one-off intro to your reports for the new season.  Please keep up the good work.  I look forward to the resumption of your reports.

Regards and thanks
Bruce

Thanks for the email Bruce. I will attempt to answer your questions and shed some light on the management of the state's most popular trout river. I hope this helps you to better understand how this river works.

Cheers
Antony

Goulburn River Management Overview

The Goulburn River is used to transport irrigation water from Lake Eildon downstream to the Goulburn Valley fruit bowl in the Shepparton region and beyond. While used for town drinking water and recreation along its entire length, its primary purpose is the delivery of water for agriculture during the drier months between October and April of each year.

In a nutshell, rain water is captured in Lake Eildon in the wetter months and then delivered downstream when it is most needed during the long, dry summer. This results in an unnatural situation of low/warmer flows in winter/spring and high/cooler flows in summer. This is the exact opposite flow regime to un dammed streams nearby that run high and cold in winter and low and warm in summer. It's what makes the Goulburn a terrific habitat for trout.

The management of water releases from Lake Eildon to the Goulburn River is overseen by Goulburn Murray Water (GMW).

Lake Eildon

Lake Eildon has over 500 kilometres of shoreline and when full, holds seven times the volume of water as Sydney Harbor. The dam wall located at the town ship of Eildon blocks of the flows of several rivers including the Jamieson, Big, Howqua, Delatite and Upper Goulburn rivers. The largest of these rivers are the Delatite and Upper Goulburn, with both having significantly larger catchments than the others.

Water Releases - Lake to Pondage

Water is taken from Lake Eildon via the power station as opposed to the spillway itself. While the spillway is used at times of higher water levels (not since the late 1990’s), these days water is released from the out take tower and via the power station where electricity is generated off this drop in height from the lake to the pondage.

The Pondage

This water coming out of the power station doesn't go directly into the river from the lake, but rather is held in two connected pondages (lakes) immediately beneath Lake Eildon and adjacent to the town itself. The pondages hold approximately 5,200 Megalitres. The reason that the water doesn't go directly from the lake to river is that constant low-mid water releases to the river cannot be managed via the out take tower.  As such water is dropped into the pondage (and electricity is generated) and then the water is released at a set level into the river via the pondage spillway where smaller flow releases can be maintained.

The Pondage is drained and filled depending on demand for irrigation and power. The level is often changing as a result. While the levels may seem hard to predict to the casual observer, there is a method to the madness. If the river is at high summer levels i.e. 5,000 + MLD, then the pondage will be filled and drained daily as it only holds 5,200 Megalitres. On the other hand, if you arrive to a full pondage when the river is flowing at winter/spring levels i.e. 150 MLD day, then the pondage level is going to remain quite steady.

Water is released from the Pondage into the Goulburn River via the pondage spillway and at times the small Power Station at this lower end of the pondage. This is not to be confused with the large power station that sits at the head of the pondage between Lake Eildon and the pondage. From this point the water can be regulated at set levels even down to the very lowest flow rates.

The height of the pondage is a bit of a contentious issue. Many locals prefer to see it at full capacity as this is the most visually appealing situation for the casual tourist. Others like it below 50% as this tends to draw the fisherman from far and wide. The power companies prefer a low level be maintained as often as possible as this gives them plenty of 'airspace' that they can use to maximise profits when the price spikes occur. That is, the emptier the pondage is, the more electricity can be generated as they can drop more water into it before it fills.

Statutory Requirements - Minimum Water Levels Goulburn River

As with all flow regulated rivers, there are a host of requirements that water managers must conform to. The most obvious and most relevant to us as anglers is the minimum riparian flow or environmental flow as it is often referred to. Currently this is set to 130 ML/D. No matter what happens, this flow rate must be maintained in order to meet domestic and stock water requirements down stream.

Until I started writing this and gave someone at GMW a copy to proof read, I was under the illusion that the environmental flow was to keep the river in a reasonable state for all organisms including fish and macro invertebrates. However this is not the case. It is about delivering basic water supplies to towns and farms downstream.

This low level of 130 MLD is set for most of May through to late September/October. This level restricts the success of trout spawning in the river with 1,200 MLD being close to the ideal level.

Water Release Measurement

There has been a lot of talk of MLD or megalitres per day. As a unit of measurement a megalitre is a million litres or to give a visual representation, the amount of water contained in an Olympic sized swimming pool. As such 130 MLD is 130 million litres of water being released daily i.e. in a given 24 hour period 130 million litres will pass by any point on the river. While this sounds like a lot, it is the minimum flow allowed by law.

Summer releases of 4000-6000 MLD are the norm and in years when the lake is full, releases of 8,000-12,000 MLD were common. The largest releases we have witnessed were around 40,000 MLD when Lake Eildon was full and there was nowhere to hold the excess rain water.

* For those more astute with their numbers a Megalitre is 220,000 gallons or 1,000 kilolitres.

Irrigation Season

While we know that water is held back in the lake during the wetter months of the year with a river level of 130MLD, what happens at other times of year and how is the flow rate calculated at these times?

To begin to answer this we must understand that there is a defined irrigation season. It starts on August 15 and end on May 15 each year. Water can be purchased by farmers down river for irrigation purposes. How much they can purchase is set by two things. Their licence and the annual allocation determined by the regulators (GMW).

For example a farmer may have a 3MLD licence. This entitles him to phone GMW and ask for this to be released/purchased in coming day/days. This explains why you will see the river rise in level in the days preceding a hot, dry spell. Farmers are always watching the weather forecast and judging how much water they will need. Same goes if there is rain forecast you will often see the river drop as the farmers know they are going to get the water they need for free. Simple really. Rain often equals dropping water level. A dry, hot spell equals higher water levels.

Then comes the second determining factor in regards to licence's and allocation. Those same farmers may not be able to get the full amount of water that they are ‘entitled’ to depending on the rainfall for that year and the predictions of what is to come. As the situation changes in the water storage levels, managers/regulators determine how much water there is and how much can be released. For example in a dry year the regulators may decide that everyone is only able to have half of their licence fulfilled. Therefore the 3MLD becomes a 1.5MLD entitlement. This is being rather kind for the sake of nice round numbers to work with as I think the latest data suggests that they will get about 15% of their entitlement this year!

In my experience in recent years I have found that this system has worked well, as the sheer nature of the drought has forced those in charge of managing our water supplies to err on the frugal side. They are much better at what they do as a result and the future looks rosier than it did ten years ago when they were quite wasteful. This is not necessarily a criticism of GMW as water appeared a plentiful and almost inexhaustible resource at the time. Suffice to say that water management is the highest priority for all involved.

Power Generation

As mentioned above, irrigation demand is not the sole reason for water releases from the lake into the river. Power generation plays a huge part in the overall scheme of things and is equally important in determining how much water is released.

Just like the farmers, the power companies also have an entitlement that they can use. While they can often take advantage of water already being released for irrigation purposes, there are times when they will need more water than is being requested by the farmers. At these times they will use some of their own water entitlement.

This often happens at odd times of year including late winter but particularly in the middle of summer heat waves. The relationship between power generation, irrigation supply and water releases is inextricably entwined. Both power generation and irrigation have a major effect on how much water is released.

The Fishing

Undoubtedly the water levels play a major role in the quality of the fishing, but they are secondary in my opinion. Water temperature and dissolved oxygen levels are a much greater indication of the fishing quality to be expected. Nonetheless an explanation of the water levels is necessary.

Low levels (May-October)

These are flows of 1,000 MLD or less and usually 150-500MLD. This allows unlimited wading access to the entire river bed and means that there is a lot less water between fish. These lower levels are the favoured by the vast majority of anglers because it is the easiest time to fish the Goulburn, yet the fish can be spooky.

Mid-Levels (October-November and April)  

Typified by flows between 1000 and 2500 MLD. The traditional river structures of glide, riffle, pool etc; begin to disappear the higher the water level and therefore it becomes intimidating to fisherman. Water levels in this bracket are among the best for all round fly fishing as well as insect hatches.

High Levels (November – late March/April)

As we get over 3000 MLD we start to see a total disappearance of those stream structures mentioned earlier. This is frightening to many as the water appears to be surging past and void of any fish activity. While this is true to some extent, in that the fish will not be holding in the middle of the river and the hatches will be restricted, feeding fish will be found around the edges and in very close to the bank.

Overview

The fishing in the Goulburn starts on opening weekend and improves slowly but consistently with each passing day. In the very first few days of October we get some huge hatches of duns and caddis and the next six weeks are alive with insect/fish activity due to an intersection of favourable water temperatures, dissolved oxygen levels and water levels/river height.

By mid to late November increased water flows has an effect on some of the aquatic insects in that they will now mostly hatch on evening as opposed to daylight hours. This lack of hatching aquatic insects is more than offset by the appearance of terrestrials with beetles, termites and willow grubs appearing en masse. The fish are in great condition by this time of the season and rise strongly with plenty of protein rich food and perfect water temperatures in the offing. December is a great month as the grasshoppers get bigger and the willow grubs peak.

By January we are getting huge flows down the river. In recent years hot spells in January have affected the fishing a lot more than the water releases. Before this drought we had the best fishing of the season in January and February when the river was full and cold with releases of 4000-8000MLD. Having said that, January, barring any prolonged 40 degree stretches, is a wonderful time for stalking the edges with large terrestrials.

February in the Goulburn is a write off at this point in time. The levels are high but this is not the  problem. It’s a low Lake Eildon that is sending 19-20 degree water into the river as opposed to 10-11 degree water when the lake is 40% or higher in summer.

March doesn't start to kick in again until at least half way through the month. April sees a falling river level but more importantly a dropping temperature. Water temps drop back to the mid teens due to the decreasing hours of daylight and the first of the overnight frosts. Temperatures actually fall away very quickly.

Once again this coincides with lower flows so people mistakenly attribute the better fishing with the flow rate only. The quality of the fishing is much more dependent on water temperature.

As such, April though May fishes very well albeit a gradually decreasing quality as opposed to spring when everything is improving.

The other side to all of this, is that as the Goulburn slows down in high summer due to increasing temperatures, many of the un dammed mountain streams are peaking. Part of the upside down flow regime mentioned earlier and one that makes our small valley one of the best fly fishing destinations in the country.

Rapidly Changing Levels

The most contentious of issues in regards to tailrace fisheries the world over and as a result there are strict guidelines in regards to how much they can raise or drop the level in any given point of time. Unfortunately we fall well behind most other nations when it comes to this side of things. Nevertheless a brief explanation is necessary.

A sudden dramatic shift in river height usually has a negative effect on the fishing. This is the general rule but there are many exceptions. I will run through some of the most important in a moment.

Firstly the problem is that fish get comfortable in routine. They are territorial and are very familiar with their immediate environment. As such over time, a fish finds shelter/cover nearby, finds feeding lies (places in the open where they sit when there is a lot of food in the drift) and maybe a prime lie (place where a feeding lie and shelter of some sort intersect). Then the river is raised or dropped dramatically and the fish must relocate and start the process again.

This leads us to which is more challenging? A quick rise or a quick drop? Well here’s a politicians answer. It all depends.

A quick rise at least has the benefit of not forcing the fish to fall back further or deeper into the river bed. While the hydrological aspects of the trout’s underwater world may be clearly altered, it is not forced to move or be left high and dry. They are going to have to move to find the best of the lies at the new river height, but it’s nothing more serious than that. Pretty obvious stuff..

Also fish are used to rising levels meaning an increase in the amount of food on offer. This is not a thought process but something ingrained. Go to Tasmania in spring or fish our non dammed rivers in September/October when they break their banks. The fish come out foraging on the newly inundated land as quickly as the water rises. The converse is also true. Somewhere deep in a trout’s DNA is the ‘survival gene’ that clearly equates to dropping water levels equals danger and time to move back to the main water courses. 

Moving on to dropping levels we can now join the dots and say that this sort of movement in river flow has much more of a negative effect on the fish. They are physically forced to relocate, often along with the bulk of the trout population of the river. Massive, short sharp drops in height really knock the fish about and mean poorer fishing. Competition for prime lies and shelter becomes more of a problem at lower levels and the fish become very wary to movement on the bank and around them.

So when does the reverse apply? When does a large drop in water level equate to better fishing and a significant rise in level result in worse fishing? From my experience this is how it goes. A big drop in river height on the Goulburn results in great fishing in autumn. After an extended period (several months) of high flows the river is shut off. This causes a bit of a local event. First of all we generally see a dropping in water temperatures as overnight air temperatures have more impact on the lower, slower flows. We also see more light reaching the river bed which has an effect on insect hatches. But the most important factor is that the huge drop after such a long period results in mass migration of sub aquatic critters that must move or die. Many of these insects get caught in the current and washed down river only to end up as trout food. The fish are aware of this sudden opportunity, not through memory but simply observation, as the water is practically alive.

We are talking about April/May here and the series of significant drops the river takes to get down to minimum flow.

What about a sudden rise resulting in poorer fishing? This occurs in early summer when water temperatures are increasing and slowing aquatic insect hatches. Sudden dramatic increases in levels really restrict the hatches, in light of the ever increasing water temperatures. I believe it also has a lot to do with light intensity on the bottom of the river and something to do with water pressure. Even though small by comparison with deep ocean waters and the extremes encountered there, nevertheless significant for tiny bugs living on the bottom of freshwater streams.

BREAKING THE RULES

So when is the best time to come up for a fish? Well like Geoff always says, it’s whenever you get the time! I used to hear him say that over and over to people on the phone and would just dismiss it out of hand, me being a skeptical Bob Dylan circa 1964 fan! But as I am spending more time fishing in different places for different species I am coming to realise this fact. Some of the best bass fishing I have had has been on a crashing barometer. The best bream fishing I had a few weeks ago was on the bottom half of a falling tide. The rules are just guidelines; not laws that are set in stone.

To further illustrate this point I should briefly describe my last three winters including the most recent month and a half. Each June/July I am spending about six weeks fishing on the mid north coast of NSW chasing bream. I pretty much fish every day, all day. As the days are short at this time of year, if you wait for the perfect tide you won’t be fishing much and the big tides are all night tides anyway.

While I did have some stellar sessions on the typical half run in to top of the tide, all stages worked well on most occasions with my best two sessions occurring on low tides. This is not how the books describe it! Trout fishing is much the same. Living here on the river since the summer of 96-97 and being able to walk out my back gate at any time of day, any day of the week, I have fished in all conditions at all river heights. I have had equally brilliant sessions in sunny, cloudy and rainy weather. Found brilliant fishing on low and high barometers and dropping and rising ones as well. Same goes for river heights, air temperatures, lightning storms.

The more I fish the more I realise that it’s about time spent on the water and reading/interpreting what is going on. If we here at GVFFC only went fishing when the conditions were ‘perfect’ we would be doing a hell of a lot of gardening and our diaries would show only a fraction of the fish captures each season.

PHOTOS

Please use the photo below to get an idea of the differing river heights. Photos were taken off Gilmore’s Bridge located between Alexandra and Thornton, looking downstream from the bridge. Clicking on the image will give you a larger version that is formatted width wise to fit on an A4 but it will use three sheets as there a number of pics to compare.

Goulburn River water levels comparison photo

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