When you put your boat in at Cowan Creek, you know you’re paddling in the sea.  Yes, there are rocky slopes on every side, and eucalypts and banksias lean over the shoreline.  The rows of hot pink bells of epacris longifolia dangle amazingly close to the waves.

But look over the side of your boat, especially in this big dry, and your gaze falls metres deep into into the crystal clear green water.  It might be called a “creek” but there’s no murky river water here.  Further downstream, where Cowan Creek meets the Hawkesbury proper at Broken Bay, the Pacific meets the horizon and you really know you are in the ocean.

But far above the official mouth of the river – the limit of “flat water sailing” drawn between Juno Head and Flint and Steel Bay – the water is briny.  The tide rolls up the Hawkesbury as far as the Grose River Valley, 138 kilometres from the sea, taking a wedge of salty water upstream.  If Broken Bay, where these pictures were taken, is essentially a marine environment, Cowan Creek is not too much different.

Because of course, the Hawkesbury is a drowned river valley.   The river channel that once wound its way twenty five kilometres to the east, across the continent shelf, has long ago disappeared two hundred metres or more beneath the ocean.  The late Quaternary Marine Transgression that drowned the Hawkesbury started 18,000 years ago and went on for 10,000 years, with the water level peaking just a metre or two above current sea levels.

Paul Boon, in his fascinating history of the Hawkesbury, reports that during that time, sea level rose at perhaps 8-10 metres every millennium, at some times as 40 metres in a thousand years (Boon, 2017, Chapter 3). It‘s hard to imagine what that must have been like for the first people here, the Guringai and Dharug people, moving again and again ahead of the rising tide, away from the coastal flat lands to the hills of the Hornsby plateau.

Even more mindboggling to think, as Boon quietly points out, that over the last few decades sea level has been creeping up by around three millimetres a year. If the CSIRO scientists are right, and I for one don’t doubt they are, sea levels are changing as worryingly fast, if not as fast as the speediest rises in the Flandrian Transgression. On the bright side if I live long enough, there’ll be far more of the upper reaches of Cowan Creek to explore, in an admittedly sweaty and decrepit manner, in my kayak

Last weekend’s paddle reminded me of what a special place the Hawkesbury is, a drowned river valley where, after rain, waterfalls tumble off the sandstone straight into the sea.  I was coming back from a jaunt to Smith’s Creek when, heading into a bay to avoid a plague of water skiiers, I heard a lyrebird going for broke.

According to Birdlife Australia, superb lyrebirds are inhabitants of moist forests. I’m sure the one I heard scratching around in the undergrowth by the shore, would soon be heading back to the gullies and the treetops.  But in its distant melange of calls – of satin bowerbirds, kookaburras, currawongs, red wattlebirds and other things I just can’t recognise – was a sound I’d never heard a lyrebird make before.  I’m sure I heard an impression of that paradigmatic bird of the beach – the silver gull.

And that’s the soundtrack of the Hawkesbury estuary, right there,

 

What are your bird call spotting skills like, dear reader?  Can you recognise any other shore birds in this estuarine lyrebird’s song?  If you can spot any extra calls, please let me know!

Other stories from Cowan Creek and thereabouts

A glimpse of a peregrine falcon hunting: death and good fortune in Cowan Creek

Old hands: how Smith’s Creek was nearly Canberra

Stingray Bay – lost and found

The Hawkesbury in winter: the shortest days and how to use them

Broken Bay at low ebb: the troubled history of Hawkesbury oysters