The ten yearly push by inland vested interests to divert Clarence River water inland across, or through, the Great Dividing Range, is on again.
In 2007, a Federal Government proposal to dam either the Mann or Upper Clarence to divert 100,000 megalitres (ML) a year to SE Queensland, led to the Bourke and Cobar Councils requesting another 100,000 ML be pumped their way. Not to be left out, SA’s Alexandrina Council proposed 1.3 million ML be diverted to keep the Murray River flowing into their State. Now it's Griffith that wants the water.
Historically, around 1.3 million ML has been a common figure. Supposedly 20% of the Clarence River's average flow, this is based on the myth of a 5 million ML average. However, the Lilydale gauge, just above the tidal pool, shows average flows over 53 years are 3 million ML, not 5. So 1.3 million is actually closer to 40% of the flow.
The Upper Clarence's Tabulam gauge, showing average annual flows of only 670,000 ML, puts that site out of the question. The Mann attracts most attention because it carries the most water. On average the Mann River carries 1.5 million ML annually, but taking 1.3 million ML from there, equates to an unacceptable 80% of its flow.
Dam-filling floods can come decades apart, so a minimum 5 million ML dam is required, such as that proposed for the Mann downstream of Jackadgery in 1957. If built it would have inundated over 200 km of the Mann, Boyd and Nymboida wild rivers, and required another dam at Nymboida to stop water spilling over the range. Over 150 km of the Armidale, Gwydir and Old Glen Innes Roads would have required rerouting, and evaporation would have exceeded inflows during droughts, on average once in every six years.
Clearly proponents put little thought into these proposals, something borne out by the Griffith NSW Farmers' submission to a current NSW Upper House Inquiry, asking Government to “explore the possibility of diverting the Clarence River inland to mitigate flooding in Lismore”.
Obviously these schemes simply don't hold water!
- John Edwards