Kolk (vortex)

High-velocity gradients produce a high-shear rotating column of water, similar to a tornado.

Kolks also leave downstream deposits of gravel-supported blocks that show percussion but no rounding.

[1] The Larrelt kolk near Emden appeared during the 1717 Christmas flood which broke through a long section of the dyke.

In spite of the repair to the dyke, another breach occurred in 1721, which produced more kolks between 15 and 18 m deep.

Kolks are credited with creating the pothole-like features in the highly jointed basalts in the channeled scablands of the Columbia Basin region in Eastern Washington.