Rivers bring with them the elixir of life for mankind and animals alike, and flow majestically along their merry way, usually from a lake or glacier, following the path of least resistance, and flowing into either another lake, an ocean or a sea. When that river starts to meander, the area that the river takes up starts to become larger and larger, and it also opens the river up to being blocked much more easier, as floating debris and tree branches get stuck when going around a rather sharp curve. Once a curve in a meandering river is partially blocked, that blockage stops more debris and a partial, if not complete natural dam is produced, and massive flooding can result.
Rivers are straightened in order to both reclaim land for either commercial or residential purposes, or to prevent flooding. The Chicago river in the United States was straightened in the 1920's for just this reason, with too many errant logs piling up in the sharp curves of the river, causing major flooding in the river's watershed. Since the process of river straightening worked, as far as lessening the threats of flooding, the process continued, and it worked as it was designed to do. The same was done to the Red river in Alberta, Canada for similar reasons, but it was ice chunks in the Spring that were the culprits of flooding here. The only problem was the animals, fish, birds and insects that thrived in the ecosystems that the meandering rivers had provided them was now different, with faster currents and less shade.
Of course, if left to their own designs, meandering rivers will eventually straighten themselves out, to some degree. However, that takes a lot of time, time that farmers, flood plains, land engineers and insurance companies do not have the patience for. That is where river engineering comes in. Rivers are straightened through engineering and lots of labour, in order to reclaim land that is taken up by the twisting shoreline's expanding land usage.
Through land reclamation, navigational improvements, protection from recurring floods, or simply putting the river to a different use, as in a logging canal or for recreational uses to generate tourism dollars, these straightened rivers are all forgoing the curves that the currents had forged through centuries to give mankind a helping hand. The local ecosystem is the loser when land is reclaimed using river straightening, as the current, sediment levels and river bottom will all change, as well as the vegetation and shade from trees on the shoreline. The long, faster pools could become the center of the river, the main current running straight through and depositing it's sediments on the pool's bottoms. Animals that lived on the pool's bottom are then relocated down current to areas that may not have the foods and environment that they need to sustain themselves.
In many ecosystems around the world, where rivers meander ever so beautifully through mountain valleys and long, expansive meadows, there are rare and nearly extinct animals, fish, birds, plants and insects that rely on the exact speed, current and temperature of the water in that river. When that snake-like river is straightened like a ruler, the current will quicken, more sediment will be in the water, making it darker, and thus warmer, as the water retains more heat while holding more dirt particles. Currents are changed as well, as the curves no longer cause currents to move across the river. Instead, that current will go straight down the river after it is straightened, making for faster and warmer river water.
No matter where on the planet a change is made to the natural flow of a river, the river's ecosystem will be effected. Once straightened, the river will immediately start to erode it's new banks, add phosphorous, nitrogen and other minerals and chemicals, natural and man made, and change the temperature and speed of the river. The loss of nearby wetlands is also a negative impact of river straightening, which is also bad for the water itself, as the wetlands act as a natural water purification system.
However, the impact on mankind when a river is straightened is immediate, especially when done for flood protection or land reclamation, whether for more farm land, a new housing subdivision, or recreational facilities. When floods can be stopped from happening by straightening a river, the financial savings are immense, especially for insurance companies and uninsured/underinsured homeowners and business owners. However, much of the flooded lands are farmlands that normally go through droughts, and a flood or two makes their land viable for farming without investing millions of dollars on irrigation. There are always winners and losers when anything is changed by mankind, and it is usually nature that loses.
With the ecology being the big loser when rivers are straightened, civic engineers who knowingly build on flood plains should be the ones to lose, not the life forms that were once supported by that beautiful, meandering river. It may be much harder to move a city than to straighten a river, but with the use of canals and other means of redistributing some of the river's water and debris, river straightening may just be a thing of the past.