It is shown experimentally that the reverse pressure gradient leads to periodic deceleration of the boundary layer of a plane-parallel flow in an open channel. In the process of braking, due to the action of the inverse gradient of pressure and friction force, an instantaneous vertical velocity profile is formed with a local minimum located above the viscous layer between two points of inflection. It is shown that in a thin layer containing the upper inflection point, the plane-parallel flow is unstable, which is confirmed experimentally - the formation of a cylindrical vortex is recorded in this layer, at the critical value of the inverse pressure gradient at the end of the deceleration cycle
$^1$Department of Marine and Inland Water Physics, Faculty of physics, Lomonosov Moscow State University, chair of Physics of Sea and Inland Water.
$^2$Department of Marine and Inland Water Physics, Faculty of physics, Lomonosov Moscow State University