General River and Flood Conditions and Forecasts
National Weather Service – very detailed information and forcasts
USGS – US Geological Survey – includes a lot of historic data, e.g. including Hydrolocig Unit Runoff Maps
NASA Flood Visualisation, based on the 1993 flood
And, if you like, watch this amazing, historic silent film produced by the Signal Corps of the Mississippi, about the devastating flood of 1927.