August 31st 2012

The storm surge ahead of Hurricane Isaac made the Mississippi River run backwards for 24 hours.

US Geological Survey (USGS) instruments at Belle Chasse in Louisiana recorded the flow of the river, finding it running in reverse on Tuesday.

The flow reached nearly 5,200 cubic metres per second (182,000 cubic feet per second) upriver, with a height of nearly 3m (10ft) above average.

Normal flow is about 3,540 cubic metres per second in the opposite direction. But in rare cases, strong winds and the waves that they create can drive water up the mouths of coastal rivers.

The reversal of flow on the Mississippi - the world's fourth-longest river - has been seen before, notably during Hurricane Katrina in 2005, when the river reached a height of 4m (13ft) above normal.

But it is not just extreme weather that can cause the effect; an earthquake near the New Madrid fault in Missouri in 1812 also reversed the river's flow for several hours.

[link to www.bbc.co.uk]