Drainage basins are areas in which water drains from higher land to lower land and eventually meets the sea or a lake. Water that flows on top of land along a certain path is said to be in a channel. The argument in this essay is whether drainage basins and river channels should be managed or left to flow naturally. If people try to control or alter the natural flow of water in a drainage basin in some way it is said to be managed. If a drainage basin is left alone and has not been changed or altered in any way it is said to be natural. People change drainage basins and river channels for many reasons; one of the earliest examples of drainage basin management was the irrigation of crops around the Nile in ancient Egypt. People change the landscape around them to make it easier for them to live in a particular area, this was thought of as a good thing by many when the UK was newly developing. However, before this and indeed to some extent now, it has been suggested that managing and altering the landscape to suit our needs may not be such a good thing.
During the course of this essay, I will be looking at 3 case studies; water resource management in Bangladesh, Colorado and along the Mississippi.
The management of rivers and drainage basins is only needed when people live near them. River floods are not a problem if people do not live on the flood plain; the natural movement of watercourses is also only a problem when people chose to live by them.
The Colorado River is one of the most used and managed rivers in the world. It has been used as a source of water for over 2000 years by Native Americans but in the last 100 years has been extensively managed. As the population around the river grew so, management was needed.
The Colorado river is now managed to provide flood control, Hydro-electricity, irrigation, urban water supply and recreation. Without all of this management, there would be fewer settlements around the river, as there would be no means of transporting water to them. All of the cities that use the Colorado’s 120million kW of Hydroelectricity would be powerless and without the irrigation of 800,000 hectares of farmland, the area around would be barren and agriculturally poor.
Another reason the Colorado had to be managed is the fact that it runs through so many different states (7 in all as well as through Mexico). There had to be some kind of management to keep order over who was using what water. In the 1920s an agreement between the 7 states was developed, it allowed any state to build a dam and use the water as long as it didn’t take water away from a dam lower downstream.
Management of drainage basins is a good thing as it enables more people to be supported by one. Dam constructors and power companies benefit from drainage basin management, there are now 10 major dams along the course of The Colorado and its tributaries. These form lakes like Lake Powell on which boating and water sports are popular. Many opportunities have been created in water processing, construction, maintenance and agriculture by the management of the Colorado. The environment also benefits from the management of The Colorado drainage basin, without the clean hydroelectric power, many hundreds of tonnes of Carbon dioxide would be released by hydrocarbon power plants needed to meet the energy demand.
On average, the 7 states that the Colorado runs through have increased by 24% between 1981 and 1991. This increase would not have been possible if the Colorado drainage basin had not been managed.
However, drainage basin management has not always been so successful. In 1993, the Mississippi flooded even after a variety of flood prevention schemes had been installed. The US army corps, in charge of flood prevention, had constructed levï¿½es, some of which were over 15 m high. They cut through meanders shortening the river (for over 1094 miles it flows in artificial channels). Large floodwater over-spills were built to take the excess water in times of flood. The Mississippi’s three major tributaries (The Missuori, Ohio and Tennessee) had been controlled by dams.
Despite all of all of these flood defences, the Mississippi still flooded. There were two main views on the cause of the flood; conservationists thought that the flood defences had exaggerated the flood and those in favour of the flood defences blamed it on exceptionally high rainfall.
I think that the flood was caused by prolonged rainfall and that the flood defences turned what would have been a long minor flood into an extreme flash flood. This happened because the dams stored up the excess water until they were forced to release some, because of this the river level rose and caused some leves to be breached. Another fault of the US army corps engineers was the straightening of the river, this allowed the water to flow faster and gather more energy making it more likely to burst through the levï¿½es.
However, I am not totally against flood defences, although some of the levï¿½es failed, many towns like St. Louis were saved by them.
Different societies view river management differently, some people believe that rivers are far to unpredictable and powerful to be controlled and so simply accept flooding as a way of life. This natural approach to drainage basin management will only work where people are prepared to accept the risks of flooding, or in places where people do not live.
An example of drainage basin that has been left to flow almost completely naturally is the confluence of three major rivers in Bangladesh. The River Ganges, Brahmaputra, and the River Meghna, these three rivers form a huge delta flowing into the Bay of Bengal.
The delta floods every year covering around 25% of the Bangladesh in 1 metre of water. The people of Bangladesh do not view these floods, which to many countries would be devastating as a problem. They have developed ways of using the floods to their advantage, although they are cramped when the waters are high, for fishermen it is a time of plenty, and people know that the sediments improve their land. When the waters start to recede, farmers plant their rice in the fertile sediments that make the ground rich.
This is a much cheaper and easier was of managing rivers and is environmentally friendly too. The constantly changing delta coupled with the rise and fall of the tide in Bengali Bay create an expanse of mud flats that are home to much wildlife.
The natural solution to river management does have its drawbacks though, it means that if there is a major flood like a one in 10-50 year flood (e.g. 1988) the results are catastrophic. In the 1988 flood 2379 people lost their lives, 50% of Bangladesh was submerged while 7,179,000 homes were damaged or destroyed. Although Bangladesh is an extreme case (only 17% of Bangladesh is above normal flood levels) it illustrates the problems of the natural approach to flooding.
Each approach has its good and bad points but I think that the natural approach is better than trying to control a river. I think that if people wish to settle on the fertile soils that have built up on the flood plain, they should accept flooding as it is their decision to live there. Trying to control rivers can only ever be temporary, there will always be a larger flood that will breach levï¿½es, or overwhelm dams. Attempting to control the course of large rivers can also be a waste of money for example The Mississippi’s shortened sections gave the river more velocity, allowing it to erode more and therefore create more meanders.
When a river is left to reign free, floodwaters spread more slowly therefore they have less energy and deposit more silt. When levï¿½es are built to “protect” crops and farmland no silt is deposited (unless levï¿½es are unsuccessful) and more artificial fertilisers need to be added to enrich the soil.
Another reason why drainage basin management is not a good idea is the huge capital needed (although some can be retrieved through Hydroelectricity). Dams and alterations to rivers cost millions and can take years to construct. Even when they are in operation, they need constant maintenance and major repairs to dams are very difficult. It is very difficult to predict what a river is going to do and how it is going to react to changes made to it. This means that there is always a possibility that alterations made in an attempt to manage a drainage basin will not work at all or even make the problem worse.
The management of drainage basins also has detrimental effects on the environment, Dams flood valleys and dry up wetland downstream e.g. in 1780 there were 41.9 million acres of wetland around the Mississippi compared to 18.1 million acres today
Although I can see the need to protect settlements from flooding and the advantages of hydroelectric power, for me it is the true unpredictability and complex nature of drainage basins that makes management of them a less preferred option.