There are three basic channel patterns for a river: straight, meandering, or braided.
Depending on such factors as terrain, soil, and water volume, different stretches of a river may show different patterns, and channels may shift over time.
There is no fixed rule for the evolution of a river’s channel, but one typical progression is from meandering to even more convoluted to straight.
What happens in such a case is that the flowing water, which reaches its maximum velocity at the outside of a bend, causes the greatest amount of soil erosion there. The eroded material is deposited as what is called a point bar on the inside of the bend.
The position of the strongest current shifts from side to side, and the meanders migrate and accentuate over time. The migration can be very rapid: Some Mississippi River meanders, for example, move as much as 65 feet a year.
As the looping meanders become more pronounced and migrate, getting closer and closer together, they are vulnerable to being cut off during a flood.
Eventually the river bypasses a loop, and an oxbow lake is formed as the main course of the river takes the straighter path, past the cutoff lake.