A kolk (colc) is an underwater

vortex In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in ...

created when rapidly rushing water passes an underwater obstacle in boundary areas of high shear. High-velocity gradients produce a violently rotating column of water, similar to a

tornado A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, alt ...

. Kolks can pluck multiple-ton blocks of rock and transport them in suspension for thousands of metres. Kolks leave clear evidence in the form of plucked-bedrock pits, called

rock-cut basin A rock-cut basin is a natural cylindrical depression cut into stream or river beds, often filled with water. Such plucked-bedrock pits are created by kolks; powerful vortices within the water currents which spin small boulders around, eroding out ...

s or kolk lakes and downstream deposits of gravel-supported blocks that show percussion but no rounding.

Examples

Kolks were first identified by the Dutch, who observed kolks hoisting several-ton blocks of

riprap Riprap (in North American English), also known as rip rap, rip-rap, shot rock, rock armour (in British English) or rubble, is human-placed rock or other material used to protect shoreline structures against scour and water, wave, or ice erosion. ...

from dikes and transporting them away, suspended above the bottom. The Larrelt kolk near

Emden Emden () is an independent city and seaport in Lower Saxony in the northwest of Germany, on the river Ems. It is the main city of the region of East Frisia and, in 2011, had a total population of 51,528. History The exact founding date of ...

appeared during the 1717 Christmas flood which broke through a long section of the dyke. The newly formed body of water measured roughly 500 × 100 m and was 25 m deep. In spite of the repair to the dyke, another breach occurred in 1721, which produced more kolks between 15 and 18 m deep. In 1825 during the February flood near Emden, a kolk of 31 m depth was created. The soil was saturated from here for a further 5 km inland. Kolks are credited with creating the pothole-like features in the highly jointed basalts in the

channeled scablands The Channeled Scablands are a relatively barren and soil-free region of interconnected relict and dry flood channels, coulees and cataracts eroded into Palouse loess and the typically flat-lying basalt flows that remain after cataclysmic floods ...

of the Columbia Basin region in

Eastern Washington Eastern Washington is the region of the U.S. state of Washington located east of the Cascade Range. It contains the city of Spokane (the second largest city in the state), the Tri-Cities, the Columbia River and the Grand Coulee Dam, the Hanf ...

. Depressions were scoured out within the scablands that resemble virtually circular steep-sided potholes. Examples from the

Missoula flood The Missoula floods (also known as the Spokane floods or the Bretz floods or Bretz's floods) were cataclysmic glacial lake outburst floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ...

s in this area include: * The region below

Dry Falls Dry Falls is a scalloped precipice with four major alcoves, in central Washington scablands. This cataract complex is on the opposite side of the Upper Grand Coulee from the Columbia River, and at the head of the Lower Grand Coulee, northern ...

includes a number of lakes scoured out by kolks. * Sprague Lake is a kolk-formed basin created by a flow estimated to be wide and deep. * The Alberton Narrows on the

Clark Fork River The Clark Fork, or the Clark Fork of the Columbia River, is a river in the U.S. states of Montana and Idaho, approximately long. The largest river by volume in Montana, it drains an extensive region of the Rocky Mountains in western Montana and ...

show evidence that kolks plucked boulders from the canyon and deposited them in a rock and gravel bar immediately downstream of the canyon. * The south wall of Hellgate Canyon in

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columb ...

shows the rough-plucked surface characteristic of kolk-eroded rock. * Both the walls of the

Wallula Gap Wallula Gap () is a large water gap of the Columbia River in the northwest United States in southeast Washington. It cuts through the Horse Heaven Hills basalt anticlines in the Columbia River Basin, just south of the confluence of the Wa ...

and the

Columbia River Gorge The Columbia River Gorge is a canyon of the Columbia River in the Pacific Northwest of the United States. Up to deep, the canyon stretches for over as the river winds westward through the Cascade Range, forming the boundary between the st ...

also show the rough-plucked surfaces characteristic of kolk-eroded rock. * Oswego Lake, in the middle of Lake Oswego, Oregon (a Portland suburb), was an abandoned channel of the

Tualatin River The Tualatin River is a tributary of the Willamette River in Oregon in the United States. The river is about long, and it drains a fertile farming region called the Tualatin Valley southwest and west of Portland at the northwest corner of the W ...

that was scoured by a kolk.

References

External links

Video of kolks and rock-cut basins
{{Authority control Oceanography Geomorphology

Examples

See also

References

External links