It is very uncommon to see naturally straight rivers because water flows through a path of least resistance. Thus rivers meander around obstacles until they empty into another river, lake or ocean. For many reasons, human beings have devised means of straightening water courses. Straightening, in simple terms involves the moving of mud and sand along a river course until the bends of the river are filled and its main channel is excavated sufficiently enough to contain the volume of water.
Navigation and building of dams are major among the reasons river courses are straightened and dredged. However, the economic benefits of these projects must not divert attention from the ecological and socio-cultural effects of the projects on the environment.
Straight rivers provide an easier route for movement of transport vessels from one point to the other. The obvious advantages of straightening of rivers to navigation are the following:
- There is a swifter flow of water
- There is more volume of water flowing past a point at a given time.
- Vessels can travel in straight courses saving time, fuel and human effort
- Dredging of the channels, which often accompanies straightening makes the riverbed deeper and stops the river from overflowing its banks.
- The excavation of silt and topsoil makes the riverbed more solid to support other necessary structures like bridges
- There is Longer sight distance for pilots of ships
- There is shorter navigation distances between departure point and destinations (shortest distance between two points is a straight line)
- Human effort is also reduced by the use of the auto-pilot facilities.
Water transportation is a more environmentally friendly mode of transport when compared to road and rails because it introduces less carbon dioxide into the environment.
Dams serve many different functions like provision for drinking water, irrigation, power generation, flood control, and recreation. For good volume of water at very swift speeds, it is customary to make a dam at the downstream of a straightened river.
Impact on the environment
The above reasons obviously impact positively on the economy of the country and also have some positive impact on the environment. However, there are also negative impacts which must be weighed carefully before such projects are commissioned. Some of them are:
Ground water recharge is a major contribution of total volume of water in a river among others. When straightening occurs, the river is diverted away from its natural course thereby cutting off the contribution of ground water recharge. This causes an overall shortage in the total volume of water. The effect of this may not be noticeable in the rainy season but during dry seasons, it may.
Where rivers flow too swiftly, smaller tributaries may not be able to join. So they back-up and cause flooding in the environment destroying farms, weak aquatic animals, amphibians and vegetation. Apart from this, the straightened river is also robbed of additional supply of water.
During river straightening to enhance navigation, certain areas of rapids must be removed but this also destroys the aesthetic and recreational uses of the river.
The movement of vessels creates strong waves which affect the movement of young fish with low swimming capacity. Thus there is lower animal diversity along the river basin.
Natural habitats of water creatures are altered by human activities. This affects the reproduction and growth of aquatic animals, and this in turn affects the human economy especially in places where fishing is their major cash cow.
Some species of fish migrate upstream to spawn and then return downstream to develop; such migration will be hindered by dams, vessel movements and human activities like pumping and channeling.
Nutrients available to water animals are reduced or destroyed by human activities like dredging and pumping of fresh water.
Oil spillage, ship collision and sinking can cause pollution of the water and thereby pose great danger to the health of fish and other animals.
Rivers that carry large amounts of sediment may develop large deltas at their mouths, if conditions permit but when excavation has taken place upstream during straightening, the resources to create deltas are greatly hampered denying man the possible aesthetic, and economic use of the delta.
Natural flooding of rivers, an important natural cycle of erosion of channels and deposit of sediments on floodplains becomes upset by human activities like straightening. This is detrimental to the life span of the river.
A critical look at the pros and cons
Because rivers are very long and may span many countries between source and terminal end, care must be taken that all stake holders cooperate to ensure proper management and protection so as not to jeopardize the good uses of such rivers. The pros and cons of straightening rivers must be carefully examined by all governments of the countries that can lay claim on the river, from source to end.