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.
One of the longest rivers in the world; disputed to be between 2,320 – 2,550 miles long
Ranges from 20-30 feet at Lake Itasca to over four miles at Lake Onalaska
Less than 3 feet deep at the source, but more than 200 feet deep at one point in New Orleans
Drains between 1.2 – 1.8 million square miles (41% of North America) – a total of 31 states and 2 Canadian provinces
Over 1,475 feet above sea level at Lake Itasca, dropping to 0 feet at its mouth
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 flood
In 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.
The 1993 Floods
* Flooded areas of 500 miles along the channel, 200 miles wide, covering 27,000 miles square (land where 930,000 people lived) of nine states.
* About 3 billion meters cubed of water overflowed the river channel onto the floodplain.
* Destroyed crops on 12,000 miles square of farmland (some unusable afterwards because of the thick layers of sand deposited), totaling $6.4 billion in costs
* At least 75 towns were completely submerged under water
* Floods lasted almost 200 days in some places (e.g. 144 days at St Louis, Missouri)
* Total damage of $15 to $20 billion.
* 52 people died in the floods.
* 74,000 forced to leave their homes.
* 330,000 had to be rescued from roofs, trees or high ground.
* 50,000 houses damaged or destroyed
* Roads closed down; $500 million lost due to damage
* Barge traffic stopped for 2 months.
* All railroad traffic halted in the Midwest.
* Ten airports flooded and consequently closed.
* 1,000 of 1,300 failed to hold back flood waters
* Sewage and water treatment plants were destroyed. Water pipes were contaminated, carrying sewage and agricultural chemicals)
Causes of the Floods
1. The Mississippi River and its tributaries are in fact draining an extremely large proportion of the country.
2. Tributaries brought extra water to the Mississippi (in particular, Ohio River)
3. Precipitation was much higher than normal in 1993; in some places, rain was doubled.
4. Snowmelt from winter
5. Ground was saturated
6. High pressure system led to short, heavy individual storms instead of prolonged, steady rainfall.
1. Straightening the river course, by cutting through the meanders to speed up discharge, could lead to ‘flashier’ floods
2. Levees, although built to stop flooding, when they do break often cause worse floods by cutting off natural buffers (i.e. wetlands, floodplain). They also inhibit water outside the river channel from entering the river.
3. If a levee collapses, not only may it cause flooding, but will pile on top of the bed. Over time, the river bed level could build up so that it is above the level of the floodplain. Also, the material which would normally be deposited over floodplain during floods (i.e. silt) have no where to be deposited; may get caught in debris in the bed, thus raising the level of the bed further.
4. Deforestation to create space for urban growth reduces transpiration.
5. Farmland (with plants growing in even rows) is less likely to hold water than rough growth (roots growing in random directions in different types of rock and soil), thus throughflow to river increases.
6. Asphalt on roads of urban development means that runoff reaches rivers more quickly.
How each cause increases the chances of flooding
> Physical Causes
1. As you can see from the picture above, the Mississippi River and all its tributaries spread over a large portion of America. Its drainage basin seems to cover about half of the country. This means that wherever rain falls over the USA, much of it will eventually reach the Mississippi river. This increases the chances of flooding because if more water is reaching the river, the river level is more likely to raise and overflow its embankments.
2. Reason #2 is similar to #1 in that with more water, the chances of flooding are increased. In this case, the main point is the tributaries and not the drainage basin. How large the tributaries are and where they come from is also of importance. An engineer from the US Army Corps says, “We can handle three times the water coming down [the Missouri].” This is because the Missouri River comes from the dry Mid-West. The rain which falls there is often evapotranspirated (especially during the high temperatures of summer). The Ohio is from the Appalachians, which receives heavy cyclonic rainfall. Warm, moist air arrives from the coast and may condense in relief rainfall. This rain combined with snowmelt from the mountains, greatly increases the amount of water in the river.
3. As explained above in the “weather conditions before the 1993 floods” section, precipitation was much heavier this year than normal. Twice as heavy in fact; this very rarely occurs (once every 70+ years). Increases in rainfall will lead to increased volume in the river and thus increased risk of flooding.
4. <Explained in #2: snowmelt from Appalachian Mountains>
5. When it first begins to rain, water will infiltrate the ground. After a period, the water table rises and the ground becomes saturated. No more water can enter the ground, thus is forced to flow overland as runoff. This reaches rivers much faster than throughflow and will vastly increase the volume.
6. Short, heavy storms will have higher runoff (as rain has little time to infiltrate the ground). This leads to a sudden increase in the river’s volume and the high discharge may lead to flooding. Prolonged, steady rain will allow the ground to store water, as well as vegetation. Water is released into rivers at adequate amounts, so the risk of flooding is reduced.
> Human Causes
1. Engineers have blasted through the neck of meanders. As water always takes the easier course, the river results in being straightened. This straightening increases discharge as water is generally flowing in one direction, whereas in a meander the curving water means that inside is slower than the other. This means that the river is able to carry away water more quickly. The disadvantage of this is that it increases the chance of flash floods. A sudden downpour in a small area can lead to river levels and velocity rising incredibly high and becoming a flash flood.
2. Levees are built so that the embankments are raised and strengthened. The idea is to keep the water within the river even during annual floods. However, by building one flood prevention, it cuts off some natural ones (i.e. wetlands). In extreme floods (as in the 1993 flood), wetlands would be a better flood prevention then the levees. Evidence can be used to prove this (1000 of 1300 levees failed). Also, levees make it difficult for water outside the channel to be rid of. This water would have to be pumped back into the river of evaporated.
3. When a levee collapses, the flooding that follows can be quite damaging. This is because the there is a sudden release in the strain and pressure found within the channel. A lot of the water will take this ‘easier’ route and spread out into the floodplain. A levee may also increase the flooding risk as bits of it fall into the river channel. This builds up the river bed, increasing its height. If not dredged, the river may eventually be flowing above the floodplain. This would be extremely dangerous for people living on the floodplain if the river did break its embankments.
4. Forests may be cut down to provide space for urban growth. However, the trees are very effective in increasing the lag time in a hydrograph (i.e. delaying water from reaching the river). It transpires a lot of the water. The roots of plants can also secure sand and soil so that it is not so easily eroded by water. In effect, forests make a good storage body and without it, flooding chances are increased.
5. If land was used for farmland, instead of natural shrubbery, the risk of flooding is increased. This is because in farms, plants are grown mostly in rows. The area between rows are vulnerable to erosion, thus will not be effective in storing water. And agricultural farms need irrigation channels to distribute water to their crops evenly. However, these increase the rate of throughflow, so that water reaches the channel faster than if left alone to grow naturally.
6. The last reason as to why humans can cause an increase in flooding risk is because of urban development. The impermeable asphalt surface of the roads does not allow water to infiltrate the ground. Instead it is forced to flow as surface runoff into drains. The drains may release the water directly into the rivers. In which case, there would be very little delay from when it rains to when the water reaches the river. This means the discharge can increase suddenly and rapidly during storms in urban areas, thus flooding is more likely to occur.
The Mississippi was thought to have been tamed as there had been no serious flooding events in decades. Much time, research and investments have gone into making the area around the river safe for development. The people living on the floodplains were confident that they were safe (‘levee effect’).
The ironic combination of physical and human factors led to the completely unexpected flooding disaster in American history. There were huge amounts of money lost and much property was damaged and in need of repair, adding to the costs of the flood.
Some may argue that it was mostly humans to blame for the flood. The flood may have been unavoidable, but ultimately, humans made a flood much worse than it could have been.
Others argue that the benefits of taming the Mississippi River outweigh the losses. Floodplains are flat and easy to build upon, farms have fertile soils to grow crop, people have a stable supply of water and boats are now able to navigate the river. Their defense is that the levees performed to the extent to which they were designed for (not an extreme flood like the 1993).
Either way, the Mississippi continues to be a river that refuses to be tamed.