Kolk
Encyclopedia
A kolk is an underwater vortex
that is 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
. Kolks are capable of plucking multi-ton blocks of rock and transporting them in suspension for some thousands of metres.
Kolks leave clear evidence in the form of plucked-bedrock pits, called rock-cut basin
s or kolk lakes and downstream deposits of gravel-supported blocks that show percussion but no rounding.
from dikes and transport them away suspended above the bottom.
Kolks are credited with creating the pothole-type geographical features in the highly jointed basalts in the channeled scablands
of the Columbia Basin
region in eastern Washington. Depressions were scoured out within the scablands that resemble virtually circular steep-sided potholes. Examples from the Missoula Floods in this area include:
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...
that is 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
Tornado
A tornado is a violent, dangerous, 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. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
. Kolks are capable of plucking multi-ton blocks of rock and transporting them in suspension for some thousands of metres.
Kolks leave clear evidence in the form of plucked-bedrock pits, called rock-cut basin
Rock-cut basin
A rock-cut basin, in this usage of the term, is a natural phenomenon. They are cylindrical depressions 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 hoist several ton blocks of riprapRiprap
Riprap — also known as rip rap, rubble, shot rock or rock armour or "Rip-rap" — is rock or other material used to armor shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour, water or ice erosion.It is made from a variety of rock types, commonly granite or...
from dikes and transport them away suspended above the bottom.
Kolks are credited with creating the pothole-type geographical features in the highly jointed basalts in the channeled scablands
Channeled scablands
The Channeled Scablands are a unique geological erosion feature in the U.S. state of Washington. They were created by the cataclysmic Missoula Floods that swept periodically across eastern Washington and down the Columbia River Plateau during the Pleistocene epoch. Geologist J Harlen Bretz coined...
of the Columbia Basin
Columbia Basin
The Columbia Basin, the drainage basin of the Columbia River, occupies a large area–about —of the Pacific Northwest region of North America. In common usage, the term often refers to a smaller area, generally the portion of the drainage basin that lies within eastern Washington.Usage of the term...
region in eastern Washington. Depressions were scoured out within the scablands that resemble virtually circular steep-sided potholes. Examples from the Missoula Floods in this area include:
- The region below Dry FallsDry FallsDry Falls is a 3.5 mile long scalloped precipice in central Washington, on the opposite side of the Upper Grand Coulee from the Columbia River, and at the head of the Lower Grand Coulee. Ten times the size of Niagara, Dry Falls is thought to be the greatest known waterfall that ever existed...
includes a number of lakes scoured out by kolks. - Sprague Lake is a kolk-formed basin created by a flow estimated to be 8 miles wide and 200 feet deep.
- The Alberton Narrows on the Clark Fork River shows 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 MontanaMontanaMontana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
shows the rough plucked surface characteristic of kolk-eroded rock. - Both the walls of the Wallula GapWallula GapWallula Gap is a large water gap of the Columbia River through basalt anticlines in the Columbia River Basin in the U.S. state of Washington, just south of the confluence of the Walla Walla and Columbia rivers...
and the Columbia River GorgeColumbia River GorgeThe 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 State of Washington to the north and Oregon to the south...
also show the rough plucked surfaces characteristic of kolk-eroded rock. - Oswego LakeOswego LakeOswego Lake is a privately owned lake in Clackamas County, Oregon that is completely surrounded by the city of Lake Oswego. Though the lake is naturally occurring , it is considered a reservoir because of the concrete dam that has increased its size to .-Geologic history:The lake is a former...
, in the middle of Lake Oswego, OregonLake Oswego, OregonLake Oswego is a city located primarily in Clackamas County in the U.S. state of Oregon. Small portions of the city are also located in neighboring Multnomah and Washington counties. Located south of Portland surrounding the Oswego Lake, the town was founded in 1847 and incorporated as Oswego in...
(a PortlandPortland, OregonPortland is a city located in the Pacific Northwest, near the confluence of the Willamette and Columbia rivers in the U.S. state of Oregon. As of the 2010 Census, it had a population of 583,776, making it the 29th most populous city in the United States...
suburb), was an abandoned channel of the Tualatin RiverTualatin RiverThe Tualatin River is a tributary of the Willamette River located in Oregon in the United States. The river is approximately long, and it drains a fertile farming region called the Tualatin Valley southwest and west of Portland at the northwest corner of the Willamette Valley...
that was scoured by a kolk - The Larrelt Kolk near EmdenEmdenEmden is a city and seaport in the northwest of Germany, on the river Ems. It is the main city of the region of East Frisia; in 2006, the city had a total population of 51,692.-History:...
appeared during the 1717 Christmas flood which broke through a long section of the dyke. The newly formed waterbody measured roughly 500 × 100 m and was 25 m deep. In spite of the repair to the dyke, there was another breach in 1721, which produced more kolks between 15 and 18 m deep. In 1825 during the February floodFebruary flood of 1825The February flood of 1825, also known in Germany as the Great Hallig Flood , was a flood disaster that occurred from 3 to 5 February 1825 on the North Sea coast, in which about 800 people were drowned....
near Emden a kolk of 31 m depth was created. The soil was saturated from here for a further 5 km inland.