The Murchison rises on the southern slopes of the Robinson Ranges, about 75 kilometres north of Meekatharra, and its catchment sprawls across some 82,000 square kilometres of the Yilgarn craton - an ancient, almost flat slab of the oldest crust on Earth. In the dry eastern reaches, the river is really a chain of salt lakes that drain only after rain, their channels merging into a single course that turns west toward the distant Indian Ocean. Because the rains arrive mostly as summer cyclones in the upper basin, the river spends most of its life waiting. The water that does fall sinks, pools, and disappears, leaving a sandy bed that looks more like a road than a watercourse.

People have lived hard along the Murchison. Murchison House Station, established by Charles Von Bibra near the river's western end in 1858, ranks among the oldest pastoral stations in the state - a foothold in country that punishes overconfidence. Up and down the catchment, the pastoral story has always been a negotiation with water: too little for years, then far too much. The waterholes that sustained sheep and cattle also drew the diggers, and the riverlands are dotted with the ghosts of gold towns - Galena, Nannine, Reedy, Peak Hill - places that flared up around a strike and faded when the water or the gold ran short.

And then it rains. The Murchison's floods are the stuff of legend, and the records read like a catalogue of disaster and astonishment. In 1900, after a fortnight of downpours, the river ran eight miles wide near Cue, submerging roads under ten feet of water; further downstream it was reported fifteen miles across and seventy feet deep. In 1884, a flood swept away the Moorarie Station homestead along with some 3,000 ewes and lambs. In 1945, the old Galena Bridge was carried off, cutting Carnarvon's road link entirely - and the most urgent cargo ferried across by fishing boat was bananas, bound for market. After Cyclone Emma in 2006, the river reached more than twenty kilometres wide in places, and Kalbarri had to be sandbagged against it.

Here is the paradox of the Murchison: the same floods that drowned mines and homesteads were also the land's salvation. After the catastrophic flood of 1900, with damage everywhere, the pastoralists were not despairing but jubilant - because they could already see how fast the grass was rising from the soaked ground. In this country, a flood is a disaster and a windfall at once, death and abundance arriving on the same brown tide. The river finally reaches the sea at Kalbarri, the only town on its banks, where its mouth must be dredged each year so that rock-lobster boats can slip through the shifting sandbar to the Indian Ocean.