Strong winds and a storm surge created by Isaac's landfall caused the Mississippi river to stream backwards for almost 24 hours on Thursday, the USGS says.
"This reversal of flow of the mighty Mississippi is but one measure of the extreme force of Isaac," said USGS Director Marcia McNutt in a statement. "While such events are ephemeral, they are yet another reminder of why we need to respect hurricane warnings."
During the height of the storm, USGS also observed Lousiana's coastal rivers losing and regaining water levels in quick succession.
This phenomenon is a common by-product of hurricanes, caused by the storm's rotating nature.
A reverse-flow is far less frequent, due to the strong winds and massive waves required to alter the stream.