The backward Clarence River

I’ve always wondered about a couple of the major rivers in our region and why the flow the direction they do. In particular I have been interested in the Richmond River and Wilsons River systems and also the Clarence Systems. I recently did a blog post on the Wilsons River and how along a large part of its length it seems to have flowed away from the sea for quite some time, maybe millions of years. In the comments on that post, Mark asked me why a tributary of the Clarence River seems to join that river in a peculiar way and that reminded me to do a post on the topic. So here it is.

From Ollier and Haworth (1995) The effect of uplift on a denritic drainage

If you have a look at a map of the Clarence River system and its tributaries you will observe that the shape of the catchment is unusual. The major tributaries of the Clarence such as the Mann River, Orara River, Timbarra River, Boonoo Boonoo River and Cataract River flow north into the south flowing Clarence River. Usually the dendritic shape of river tributaries means that tributaries join with the main river at an acute angle. The major tributaries above, however, join at an obtuse angle. They almost look like they are flowing backwards.

How did this come to be? Ollier and Haworth (1994) came up with a surprising solution. Essentially, they thought that the angles the tributaries were joining the Clarence would make sense if the Clarence River once flowed the opposite direction. Today this seems like a far fetched idea, I mean, water can’t run up hill, can it? But Ollier and Haworth (1994) thought that prior to the Cenozoic volcanoes that make up the Main Range, Focal Peak and Tweed Volcanoes the land surface would have been much flatter. Indeed the sediments closer to these volcanic centres has been uplifted by hundreds of metres. If there were no mountain ranges along the Queensland border it would be quite conceivable for a river to flow from the New England highlands northward into the Condamine River in Queensland and then into the Murray-Darling River system.

The current trace of the Clarence River is a little bit strange. In many places it crosses between hard Palaeozoic basement rock of the New England into the softer rock of the Clarence Morton Basin and then back again. Rivers usually cut river channels preferentially into softer rock and will rarely flow from gentle valleys in softer rock into steep hard valleys as the Clarence River does along its southward path. Combining this with the knowledge that parts of the Clarence Morton Basin have been shown through various seismic exploration techniques that is has been warped in various directions adds further to the argument.

I for one, am convinced. I think that the Clarence River once flowed north before the Macpherson Range came into existence and the River would probably have joined with an earlier Condamine River. Need to check for yourself? Have a look at the rivers of the region on a map, follow the route of the Clarence River from the Pacific ocean and observe the rivers that join in. Surprisingly, you might find it makes sense!

References/Bibliography:

*Ollier, C., Haworth, R.J. (1994) Geomorphology of the Clarence-Moreton Basin. In Wells, A.T. & O’Brian, P.E. Geology and Petroleum Potential of the Clarence-Morton Basin. Australian Geological Survey Organisation, Bulletin 241.