Water flows downhill under the influence of gravity and uphill under the influence of money. However, there was recently a case of the Mississippi River flowing backwards because of Hurricane Isaac on 28 August 2012.
The force of the Hurricane’s wind and storm surge piled up water at the mouth of the Mississippi. The force was so strong that it reversed the river’s flow, pushing water as far north as Baton Rouge. According to a US Geological Survey report, the storm made the river go backwards at 182,000 cubic feet per second (5,153 m^3/s), which is about as fast as it normally is going downstream.
Something similar happened during Hurricane Katrina. The river also did an about-face for a few hours during a major earthquake in 1812.