Giant current ripples (GCRs), also known as giant gravel bars or giant gravel dunes, are a form of subaqueous dune. They are active channel

topographic Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary scienc ...

forms up to 20 m high, which occur within near-

thalweg In geography, hydrography, and fluvial geomorphology, a thalweg or talweg () is the line or curve of lowest elevation within a valley or watercourse. Normally only the horizontal position of the curve is considered (as viewed on a map); the c ...

areas of the main outflow routes created by glacial lake outburst floods. Giant current

ripple marks In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water ( current or waves) or directly by wind. Defining ripple cross-laminae and asymmetric ripples * ''Current ripple ...

are large scale analogues of small current ripples formed by

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

in streams. Giant current ripple marks are important features associated with scablands. As a landscape component, they are found in several areas that were previously in the vicinity of large glacial lakes.

Setting

Giant current ripples are a geomorphological phenomenon associated with

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 ...

. Scablands form when lakes dammed off by glaciers suddenly burst through their dams and empty their contents in giant flooding events. They are found in the

Altai Mountains The Altai Mountains (), also spelled Altay Mountains, are a mountain range in Central Asia, Central and East Asia, where Russia, China, Mongolia, and Kazakhstan converge, and where the rivers Irtysh and Ob River, Ob have their headwaters. The ...

of Russia, as well as in the

Columbia Plateau The Columbia Plateau is an important geology, geologic and geography, geographic region that lies across parts of the U.S. states of Washington (state), Washington, Oregon, and Idaho. It is a wide flood basalt plateau between the Cascade Range a ...

of the

Pacific Northwest The Pacific Northwest (PNW; ) is a geographic region in Western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though no official boundary exists, the most common ...

and

Coast Mountains The Coast Mountains () are a major mountain range in the Pacific Coast Ranges of western North America, extending from southwestern Yukon through the Alaska Panhandle and virtually all of the British Columbia Coast, Coast of British Columbia sout ...

of British Columbia and Alaska.

Properties

Giant current ripples can reach a maximum height of and reach a maximum length of . They occur in ripple fields that can cover an area several kilometers across. As a result of the high energy environment in which they were deposited they consist mainly of pebbles and small boulders, with a small fraction of coarse sand. Giant current ripples are whale-back shaped, with large poorly-rounded boulders found near the peak. They are oriented with the long axis perpendicular to the flow direction and with the steep flank oriented against the direction of flow. The sediment is loose and dry, without

loam Loam (in geology and soil science) is soil composed mostly of sand (particle size > ), silt (particle size > ), and a smaller amount of clay (particle size < ). By weight, its mineral composition is about 40–40–20% concentration of sand–si ...

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

. The larger exposed segments are

cross-bedded In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The original ...

. While they are often called ripples, their topography responds to hydraulic flow as well as

boundary conditions In the study of differential equations, a boundary-value problem is a differential equation subjected to constraints called boundary conditions. A solution to a boundary value problem is a solution to the differential equation which also satis ...

, which means they are more accurately described as a type of

dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, flat ...

. The physical parameters of the ripples, such as height and width, can be used to estimate the hydraulics of the flood (e.g., depth and speed).

Notable localities

North America

Giant current ripples are an important feature of 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 ...

in Washington state, U.S., which formed during the

Last Glacial Maximum The Last Glacial Maximum (LGM), also referred to as the Last Glacial Coldest Period, was the most recent time during the Last Glacial Period where ice sheets were at their greatest extent between 26,000 and 20,000 years ago. Ice sheets covered m ...

as a result of at least 39 glacial lake bursts, called the Missoula floods, which originated from

glacial lake A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier. Formation Near the end of the last glacial period, roughly 10,0 ...

s Columbia in Washington and

Missoula Missoula ( ) is a city in and the county seat of Missoula County, Montana, Missoula County, Montana, United States. It is located along the Clark Fork River near its confluence with the Bitterroot River, Bitterroot and Blackfoot River (Montana), ...

in Montana. Giant current ripples have also been identified in the Yukon Territory, Canada, where they resulted from a outburst from Neoglacial Lake Alsek.

Altai Mountains, Russia

Another area where Giant current ripples are an important landscape feature is in the Altai Mountains, Russia At least seven major floods are believed to have happened in this area over a span of 150,000 years during Marine Isotope Stage 2, or the latter part of the last glaciation. The lakes that fed these events reached up to in capacity. Floodwater depths during the largest of these floods are estimated to have been up to and the water would have been moving at a speed of 60 meters per second. This resulted in giant current ripple deposits of up to a hundred meters thick.

Mars

American and British geologists and planetologists have discovered giant current ripple reliefs and other scabland features on Mars surrounding the Cerberus Fossae, indicating that megafloods have occurred there at some point in the recent past. While they have roughly the same dimensions as on earth when they occur, Giant current ripples appear to be much less common on Mars relative to other scabland features. This is likely a result of differences in the availability of certain types of sediment.

References

{{DEFAULTSORT:Giant Current Ripples Geomorphology Megafloods Sedimentary structures