Rivers are straightened to make them more predictable. Straightening a river is part of the process of enclosing, confining, and managing it. An unmanaged river rises in spring, develops sandbars and snags, and is likely to meander. In flood, a river can take out bridges, wash out roads, and destroy crops and homes. A wild and scenic river can be dangerous to those who live beside it. It can kill.
Yet a riverside is a good place for a town. The river provides rich alluvial soil and water transportation. The flat land beside it is suitable for development. Trade comes up and down the river. Rivers are straightened partly to fit them into the regular framework of the riverside cities that naturally grow up beside them.
To fix rivers in place in populated areas, engineers use concrete culverts, sandbags, riprap, and stone. In the country, rivers are managed with earth moving machinery and dredging to clear channels.
Straightened rivers move faster, and their bottoms stay cleaner. They are easier to navigate, offering fewer surprises for boats and barges that carry industrial and agricultural products on the river. Therefore, engineers straighten rivers to improve their scouring action. The faster a river flows, the more sediment it can carry away. When rivers slow, they drop sediment, which forms impediments to navigation.
A natural river does not stay straight, especially in its lower reaches. Current washes away soil, undercutting the banks, carving them away underwater. As the shore is carved away, a bend begins to form.
Each bend widens, because the water must go faster to swing around the bend, so it hits the outside bank with greater force. It strips away more of the riverside. On the inside of the bend, water flows more slowly, and drops sediment in the elbow of the stream, forming a point bar. Bends widen this way.
Eventually, the river can create a huge loop, a meander. Inside the loop of a meander is a peninsula that is nearly an island. These meanders are easy to see on a large-scale map, for example on the lower Mississippi.
The peninsula inside a meander often has a narrow neck. If the neck is cut across by a cave-in or by floodwaters, the river shortens its path to the sea. If this happens, the meander loop is cut off from the main stream. It becomes a semi-circular ox-bow lake. Through time, the lake dries, to become an oxbow swamp. A mature river continually shifts its path this way, as it evades its channel and widens its floodplain.
So a river that bends refuses to stay in place. It changes its channel in a process as old as physics, as time. It pays no heed to property lines, or to property. Yet societies must protect the property of their citizens. Rivers that run wild do damage. They can kill.
That is why rivers are straightened, are managed. River straightening has consequences too, though. Areas downstream of a straightened area are more vulnerable to flooding. Floodwater hits them harder, carrying more sediment. River straightening increases the force of flowing water, in ways that planners may not have anticipated.
Sometimes the effect of river straightening on the environment is to trade the minor displacements of wet floodplains and moved landmarks for the enormous catastrophes of ungovernable floods, ruined homes, and lost towns.
Better options than river straightening include restricting development in floodplains and requiring higher foundations for buildings in flood areas. Already, cities and states are restoring many rivers. Gradually, people are beginning to understand that it is important for our societies to bend a little, to tolerate some disorder, in order to live in harmony with nature.
United States Geological Survey: Point Bars and Cut Banks