The Mississippi River, located in North America, begins in Lake Itasca, Minnesota and flows south, ending at the Gulf of Mexico, Louisiana. Its watershed boundary is marked by the Rocky Mountains in the West, Appalachian Mountains in the East, and lakes in the north-east.Length:One of the longest rivers in the world; disputed to be between 2,320 – 2,550 miles longWidth:Depth:Less than 3 feet deep at the source, but more than 200 feet deep at one point in New OrleansElevation:Drains between 1.2 – 1.8 million square miles (41% of North America) – a total of 31 states and 2 Canadian provincesWatershed Area:Over 1,475 feet above sea level at Lake Itasca, dropping to 0 feet at its mouthTributaries:Main ones are the R. Missouri (dry mid-west) to R. Ohio (wet highlands)Frequency of floods along the Mississippi> Minor Flooding (minimal, or no property damage; possibly with some public inconvenience)The Mississippi River, in its natural state would normally surpass its bankfull discharge and spill onto its floodplain annually.This would usually take place in the late spring with snowmelt increasing the volume in the river and heavy rainfall in the Appalachian Mountains. However, the engineers that worked on the river wanted to stop even these small annual floods, thus the flood prevention methods they used put an end to minor flooding incidents.> Major Flooding (extensive inundation and property damage; closure of main roads; evacuation of people and livestock)Major floods were predicted to occur every five to ten years.These floods are caused by exceptionally heavy rainfall (having a frequency of once in every ten years). Seasonal rainfall leads to the ground being already quite saturated, and when combined with heavier than usual rain, will cause serious flooding.> Extreme Flooding (large-scale inundation, requiring aid and resources from outside local communities; record flooding incidents)These are predicted to occur about once every 40 years. It may be caused by an unnaturally intense period of heavy rainfall. The Mississippi’s flood prevention techniques (levees, dams, straightening channel, spillways, etc.) were thought to be sufficiently strong enough to hold up against floods occurring once in a 100 years. However, the 1993 flood was calculated to be a 400-500 year flood, one where probably no one will witness ever again. The Mississippi was flooded at one point for over 144 days, from 1st April to 30th September 1993.Weather conditions before the 1993 floodIn many locations, rain fell approximately 200-250% more than normal (over 30 inches of rain) and sometimes for more than 20 days, instead of the average 8 to 9 days. Heavy rains persisted through June, July and August. In June, the rains and snowmelt led to soils being quite saturated and streams nearing their capacity. This meant that resulting runoff could not infiltrate the ground, but instead flowed straight into streams and river channels. The summer rains of 1993 were extremely rare, with frequencies of 75 to 300 year intervals.The uncharacteristic heavy rains were a result of warm, moist air streams from the Gulf of Mexico meeting cold, dry air from the Midwest and Canada. The colder air cools the warm air. This causes condensation, and the moisture in the warm air falls as precipitation.Although this usually occurs annually (therefore flooding should not have been on so large a scale), there was a high pressure system over the south east. The high pressure blocked the flow of air stream from distributing its precipitation more evenly over the northeaster states. Instead, precipitation was concentrated mainly over the Midwest, leading to high than normal rainfall levels in those particular areas.