<P> Secondary flow: A force balance exists between pressure forces pointing to the inside bend of the river and centrifugal forces pointing to the outside bend of the river . In the context of meandering rivers, a boundary layer exists within the thin layer of fluid that interacts with the river bed . Inside that layer and following standard boundary - layer theory, the velocity of the fluid is effectively zero . Centrifugal force, which depends on velocity, is also therefore effectively zero . Pressure force, however, remains unaffected by the boundary layer . Therefore, within the boundary layer, pressure force dominates and fluid moves along the bottom of the river from the outside bend to the inside bend . This initiates helicoidal flow: Along the river bed, fluid roughly follows the curve of the channel but is also forced toward the inside bend; away from the river bed, fluid also roughly follows the curve of the channel but is forced, to some extent, from the inside to the outside bend . Ultimately, the downstream velocity of the fluid is convectively transported to the outside bend, resulting in higher velocities at the outside bend . This secondary flow effect dominates over that of irrotational flow: In real meandering rivers, we observe higher downstream fluid velocities at the outside bends . </P> <P> The higher (lower) velocities at the outside (inside) bend result in higher (lower) shear stresses and therefore results in erosion (deposition). Thus meander bends erode at the outside bend, causing the river to becoming increasingly sinuous (until cutoff events occur). Deposition at the inside bend occur such that for most natural meandering rivers, the river width remains nearly constant, even as the river evolves . </P> <P> The technical description of a meandering watercourse is termed meander geometry or meander planform geometry . It is characterized as an irregular waveform . Ideal waveforms, such as a sine wave, are one line thick, but in the case of a stream the width must be taken into consideration . The bankfull width is the distance across the bed at an average cross-section at the full - stream level, typically estimated by the line of lowest vegetation . </P> <P> As a waveform the meandering stream follows the down - valley axis, a straight line fitted to the curve such that the sum of all the amplitudes measured from it is zero . This axis represents the overall direction of the stream . </P>

Explain how rivers can produce a meandering channel