This causes a return flow at depth, initiating a spanwise circulation cell. The reason for the sign of the circulation is explained by simple examination of the important terms in the equation of motion. Near the interface and at the wall, the fluid is fast-moving and therefore convective accelerations are large. However, at the bed, convective acceleration is small, in particular compared to the tranverse pressure gradient. As a result, the pressure gradient takes over near the bed and drives a return flow at the bottom.
Considerable work continues to understand the mechanism behind meander formation.
It is particularly well-suited for stability analysis. In other words, the St. Venant equations are solved with a perturbation in the shape of the channel. The spiral motion accentuates with time, as it builds up a meander bend. However, extension to field cases will always have the difficulty of characterizing bank erodibility, a key parameter in meandering stability. Alluvial streams are particularly hard to characterize because of the cohesion associated with vegetation.