I READ with interest an article in The Fremantle Herald newspaper in which global warming was blamed for rising sea levels, which in turn were said to be submerging the mud flats in the Swan River estuary and thus destroying the habitat of migratory birds.
Reading it, I could not but reflect on my own observations of water levels and on the accretion and erosion of mud banks in the Swan River over my life-time. I have known the river intimately since the early 1940s when as a toddler I first paddled in the waters of Freshwater Bay. Over the years I have swum and fished in, and canoed, rowed and sailed on the river. I have cycled around the riverside paths, explored the river’s shores and bushland, walked my dogs at the river’s edge, and enjoyed the wildlife – some of which (like the river cobbler) seems to have disappeared, while other species (like the black swans) appear to be flourishing. I have known the river from Preston Point at East Fremantle to the Perth Causeway and beyond for over 60 years, and since 1980 I have swum regularly at the old Bicton Baths.
Over all this time I have seen the river rise and fall with the ocean tides, respond to flood waters coming down from the Avon, and fill to its brim with the run-off from heavy rain storms. But I have also seen the Point Walter sand spit so far above the water that it has grown a small-vegetated island. And only last summer there were occasions when the water was so low at Bicton Baths that the bottoms of the swimmers’ ladders were exposed and there were acres of temporarily exposed mudflats west of Alfred Cove and along the Como foreshore.
I have also observed the wave-cut benches in the limestone cliffs along Blackwall Reach. These indicate river levels at least 2 metres higher at some time in the geological past, while the current benchmarks do not appear to have changed for decades, perhaps a century.
The literature on sea level rise suggests that global average sea levels are rising at a rate of about 2 mm per year, a trend going back for 150-odd years to the Little Ice Age. But the picture is confusing because in some places sea levels are falling, while in others there is only an apparent rise because the adjoining land is subsiding. In other places still, the rate of rise is much slower. For example, according to the Swan River Trust’s website, the sea level at Fremantle Harbour has risen at a rate of only 1.5 mm/year over the last 100 years (or about six inches in total) and the Trust does not report any sudden recent rise in levels, or in the rate of increase.
Within a tidal estuary connected to a vast inland catchment (as is the case with the Swan River) the picture is particularly complex because of the way mud flats and sand spits are eroded by tides, floods and boat wash, or built up by sedimentation. And if anyone wonders about sedimentation, they need only look at the colour of the river after winter rains inland – the water is stained brown from the topsoil carried down from wheatbelt farms. Much of this goes out into the ocean, but tonnes are also deposited in the estuary.
I do not dispute the data on sea level rises measured over the last 150 years. However the evidence of my own eyes is that there has been no sudden, recent rise in water levels in the Swan River as reported in The Fremantle Herald. It seems to me that the average river level today is pretty much where it was when I first knew it, and that the variation around that average is the same as it has been over the last half-century, a response to predictable and well-known factors such as ocean tides and rainfall in the catchment.
It is possible, of course, to speculate about the future, even to design future scenarios using computer models, and I am well aware of the belief some people hold that sea levels will rise many metres (some say 100 metres) due to the phenomenon of ‘global warming’. I don’t wish to discuss this here, but I would appreciate the presentation of actual data on river levels to help me to understand the true picture. This will enable me to either validate or deny my own experience and observations over more than 60 years, which appear to be different to (or at least less dramatic than) those reported in The Fremantle Herald.
In closing, it is interesting to compare two photographs taken of a well-known feature of the Swan River – the Crawley boatshed.
The first was published in George Seddon’s book Swan River Landscapes and is a photo taken by Professor Seddon in the late 1950s. The second, of the same scene, was taken fifty-three years later by me.
Apart from the colour scheme, and the absence of the elegant old boats in the Seddon photograph, the river water levels half a century later appear to be virtually the same.
There is an amusing postscript to this story. It was submitted to The Fremantle Herald as a follow-up to the concern that migratory bird habitat was being submerged by rising water levels in the Swan River. It was never published. When I asked why, I received a classic response from Steve Grant, the paper’s Chief of Staff: he queried my qualifications as a climate scientist.
Subsequently The Fremantle Herald has published a story about the impending demise of the Little Penguin due to “a marine heatwave” in the Indian Ocean. Neither the story about rising river levels or the one about endangered penguins drew on the expertise of a climate scientist, nor were any references given to climate science data… but then these were stories that supported the Herald’s position on climate change, and mine did not.
March 10, 2012