Kolks are tornadoes of water, formed when fast currents pass submerged obstacles to create strong shearing forces (i.e. braking and accelerating forces applied in opposition). On a small scale localized kolks can spin loose material that can, over time drill holes in the riverbed even into solid rock, these holes are known as rock-cut basins (this is the initial stage in the formation of tolmens). On larger scales kolks have been known to lift huge boulders, weighing many tons from riverbeds and transport them considerable distances. This phenomenon was first observed in the Netherlands where kolks were found to be dismantling dykes by plucking away the stones laid at their bases.
Etymology
Modern Dutch kolk ‘swirl’ also ‘abyss’, ‘chasm’; kolken ‘to swirl’