How are flood plains formed?

Hint: A floodplain or flood-plain is an expanse of land nearby to a watercourse or river that extends from the banks of its channel to the base of the encircling gorge walls, and that faces submerging throughout phases of high expulsion.

Complete step-by-step answer:
Most floodplains are shaped by accumulation on the inward of river roams and by the overbank current. Anywhere the river wanders, the flowing water corrodes the river bank on the exterior of the meander, while residues are instantaneously dropped in a point bar on the inside of the meander. This is defined as lateral accretion, as the accumulation fabricates the point bar sideways into the river channel. Corrosion on the exterior of the meander typically carefully poises accumulation on the interior of the meander, so that the channel moves in the course of the meander without altering notably in breadth. The point bar is made up to a level very adjacent to that of the river banks. Noteworthy net corrosion of deposits happens only when the meander cuts into higher land. The general result is that, as the river wanders, it makes a level flood plain composed mostly of point bar sediments. The rate at which the channel changes differs significantly, with testified rates fluctuating from too slow to measure to as much as 2,400 feet per year for the Kosi River of India.

Note: Flood plain frequently forms good farming property, not just because water is present, but also because those floodwaters transfer deposits that augment the soil. The waters also occasionally transfer gravels and rocks, but that doesn't overshadow the rich deposits, so to speak. The wreckage that is left by running water is termed alluvium.