But we can assume that an early flush will send smaller fry etc into the ocean and these are likely to have a lower survival rate than larger fry in a later flood. More time in the nursery means a larger fry on departure.

We can also assume that a flood does not need to be a major flood to perform its essential ecological function and we can also assume that most of the populations that need to be flushed will go on their journey in either a medium size flood or the initial stages of a large flood.

We can also assume that large floods also play an important role in clearing out the sediments that build up in the milder seasons. But we know that it does not follow that more flooding equals better ecological services.

What we don’t know is how intense, and for what duration, a flood must be to perform all of its functions properly. We don’t know the mix of volume, duration and timing that is needed to produce the optimum flood, or rather optimum range of floods, for maintaining the normal range of variation in populations of relevant species.

We don’t know the extent of the margins that nature has built into the life cycles of riverine species. And until we know these margins then any assumptions on the required volume of so-called “environmental flows” will be pure guess work.

When these core attributes of river flow are known for each system then we can get down to working out how a major storage capacity can be used to ensure that all these needs are met, and then allocate the surpluses to other users.

What do know for certain is that man is fully capable of exercising judgement and skill, and investing capital, in a way that can achieve substantial efficiencies on the very wide set of redundancies that nature normally works on. A large dam can, for example, fully capture an early flood that would flush out fry before they are fully ready, and hold that water, and even accumulate a number of small flow events, to be fully or partially released, in the best volume, over the best duration, to produce the optimum set of ecological services.

And having discharged this core obligation, capturing the remaining flows for distribution to users without guilt.

Our problem is that our decision makers and research community are so lacking in this vision that they don’t even want to know. They cannot even begin to gain the knowledge needed to make it happen. The best these turkeys can do is compare base percentages and make ignorant guesses as to the appropriateness of them and boorishly rant about more equalling better and no allocations being the best allocations. They are, as Christopher Isherwood put it, “those small men with frowns, beheading daisys with a stick, that one finds loitering without vision, in the foothills of great ideas”.

The only Australian research that had been done was on Tasmanian dams, which found emissions were around 30 per cent of a natural gas plant – a much higher reading than US dam emissions, Mr McCully said. Those readings would be higher in hotter parts of Australia, especially northern Queensland, he said.

I guess you can either or dismiss the assertions. However the dam builders are now well aware that storages are more than likely a major methane source.

However back on dams. Whether the Clarence needs the full flood flow to maintain ecological function is a moot point. Certainly the water that has just gone past would have considerable economic value for cotton irrigation in the Gwydir system. Does it add up – dunno. Cotton – horror of horrors – but strangely greenies love wearing it. Most of us do.

So there may be a compelling economic argument in a water constrained MDB.

However all dams are barriers to movement – so I’m not sure what the micro-satellite DNA and tracking electronics would tell us about species dispersal and genetic structure of the fish fauna. Certainly research in the upper MDB and Fitzroy is surprising and not what Mottsa may think – lots of movement on the first small flush and then little after in one study. We probably don’t know half enough.

The Clarence is not entirely unregulated; the Nymboida Weir draws water for modest hydro power use,and now to supply extra off-river domestic water storage of 30GL at Shannon Creek for distribution to Grafton and Coffs Harbour. Of course ,this has no flood mitigation value.. Proposals to divert basin waters over the divide were ill-costed and crudely researched, mainly of value to populists and pollies. The latest proposal involved damming the Clarence above Tabulam and piping water north over the Macpherson Range to the Logan catchment,suggested at the height of the SE Qld drought. This went down very well with the locals,who pointed out that you could step across the river at the dam site at the time,and this was platypus and bass heaven. While that drought has receded, Queenslanders also discovered that urban water conservation wasn’t as painful as they feared. This proposal has only marginal flood mitigation value,as its catchment is only about 10% of the total Clarence catchment,and is one of the less well-watered 10%s,to boot. However,it can’t be ruled out over the next half century.

Development on the flood-plain below Grafton is governed by management plans which have a pretty good handle on flood heights and recurrence.

The ” wide Clarence is always brimming ” because it is indeed tidal to upstream of Grafton. A dam on the river to intercept sediment is a vacuous idea; an expensive way to slowly destroy the ‘delta’ while increasing evaporation losses and interfering with the movement of river life,all processes which are irrationally discounted by our economic system. If one wants to reduce loss of interstate freight time, flood-free engineering of the Summerland Way avoids as much of the Clarence flood-plain as possible.

There is no overwhelming reason for major dam works on the major tributaries as yet.

clarencegirl, have you considered the Gold Coast without the Hinze Dam?
Instead of a: large open space ecozone that links up with other ecozones, provider of flood mitigation, agriculture, rec area and lots of low cost, quality fresh water, there would be another 10 suburbs and none of the above.
The development will surely come, don’t kid yourself, and the outcome will be more and more desal plants because they’re politically digestible but they’re a bigger disaster than dams ever will be.

You have no idea what a dam will do on the Clarence, none whatsoever. Neither do I. But, I suspect that the floods are extremely important. I do know that there are fish in The Great Sandy Strait that only spawn when the water is fresh ie in a major flood. I do know that flood mitigation works near Grafton have produced water that is very acidic. I certainly know there is a lot more to a river than meets the eye, and there are unimaginable connections between it and life that depends on it. That is, connections that are unimaginable until we stuff it up and say, “What the…”

With over 35 years as a civil engineer and 5000km observing rivers and landuse at walking pace I feel qualified enough to say that our arrogance has caused us major problems in the past and it still does. It is time to be humble, to work with nature, not to think we can control it.

If we control the floods how do we replenish the land? Currently almost every farm, unless certified organic, operates on what they call “inputs”. These come from oil and natural gas, energy stored for millennia and now being released. But from about now they will go into decline. How then do we eat unless we work with nature and replenish land the way it was always done until we turned up?

We have a very, very long way to go in our understanding of what a river really is and what it means to us.

An extra dam would play a major role in reducing sediment flows in the lower reaches. And there is no reason why the capture of the destructive half of major flood events should lead to any adverse impact on fisheries and habitat in the lower reaches. The benefit provided by a flood surge is delivered on the first day of the flush, not the third or fourth.

Luke, this is the “4%” you’re talking about and it’s highly assumptive and fixable as well.