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Whenever there’s rain in the forecast for the desert town of Mparntwe, otherwise known as Alice Springs, the same question usually surfaces: “Do you think the Todd will flow?”
This week, the answer was yes. The normally dry Todd River swelled with water on the back of a week’s worth of rain that dropped 184 mm (about 7.2 inches) from the skies after months of unrelenting heat, according to the Bureau of Meteorology. The rain picked up piles of debris, washed out roads and turned the usually pastel red-yellow landscape of the area in central Australia into a rich tapestry of brown-green.
Local officials issued severe flood warnings, but in a region usually defined by arid heat, people were excited — even more so when the water started to rise. People swarmed the Todd’s banks, reveling in the majestic sight of a river in motion.
It’s something people in this part of the world get to see only a few times a year. Although the land is never completely dry — a rich groundwater table makes itself known through the rows of mature river red gum trees that line the banks — aboveground flows of any significance are contingent on big rain.
John Wischusen, a hydrogeologist based in Alice Springs, said that the usual formula for determining whether the Todd would flow was 40 millimeters of rain at a rate of 50 millimeters per hour. That’s a heavy downpour typical of a summer thunderstorm, but he added that consecutive days of wet weather at a lower intensity also would (and this week, did) tip the scales.
“It’s like a roof and a water tank,” said Mr. Wischusen, explaining how the high concentration of Precambrian rocks (including gneiss and granite) in and around Alice Springs encouraged water runoff. “But the ground underneath has to be wet up to a certain level before it can flow past. That’s why you need a certain amount of rain and or intensity to get water to flow over the ground and into the river channels and through town.”
