For nearly 24 hours, according to the US Geological Survey, Isaac's storm surge drove upriver at a pace nearly 50 percent faster than the downstream flow. This backflow produced a crest some 10 feet above the river's prestorm height at Belle Chasse, La., in flood-beleaguered Plaquemines Parish southeast of New Orleans. The surge added eight feet to the river's height at Baton Rouge, father north.
Isaac had help. A scorching, rain-starved summer in the middle of the country sent river levels to lows not seen since a similar drought struck the region in 1998, easing the Mississippi's flow.
Still, reversing the Mississippi is no mean feat, and it points to the importance of understanding more about the effects of tropical cyclones beyond wind speeds. While many people still tend to focus on hurricane-intensity categories, scientists are working to provide accurate and easily available daily forecasts of other effects, such as storm surges and inland flooding, in regions subject to tropical cyclones.