Rivers have played an important role in human development throughout history. They have served as trade routes, food sources, irrigation for our drops, defensive barriers, and national boundaries, since the beginning of organized human settlements. They have also been the inspiration for a lot of our art, literature, and music.
Humankind has depended upon, been in love with, and been at war with rivers for ages. Rivers, great and small, water our fields, slake our thirsts, and move our commerce. Sometimes, as happened with the writer Mark Twain (nee Samuel Clemens), they captivate us. At other times, as during the great Johnstown, Pennsylvania floods, they terrify us.
The deposits of silt, carried along by rivers, renew agricultural soil.
On occasion, man tampers with the rivers by straightening their wandering courses for short-term benefit. This can facilitate transport of goods, as the straighter courses are easier to navigate. It also opens up new land for human habitation and use, and facilitates community planning.
But, there can be negative consequences from messing with Mother Nature that can far outweigh the short-term gains. When a river is straightened, the water flow increases, raising the danger of downstream flooding. It can also cause flooding to be more severe. Changing a river's natural course alters the biosphere. Thousands of plant and animal species that have adapted to the environment of a natural flood plain have their habitats destroyed, and many are driven to extinction.
The alteration of ground cover, caused by changing a river's natural course, particularly when replaced by manmade structures, can alter weather patterns which can, in turn, negatively affect agricultural production.
Even levees and dikes, which provide a measure of flood protection in the area where they are constructed, can have a negative impact. By constricting the flow of the current, back pressure is created that increases the danger of upstream flooding. This is not an argument against levees, but it does call for awareness of the unintended consequences of our actions when we tamper with nature.
Left alone, rivers generally flow from higher to lower elevations, naturally meandering around most obstacles; continuing to carry our commerce and enrich our lives; on occasion overflowing their banks and inundating the surrounding terrain. Taking prudent measures to protect communities against the hazards of flooding is thus perfectly acceptable and understandable. But, extensive reengineering of a river's course for short-term commercial gain is something that should be carefully considered; and in most cases, avoided.