This picture of theNowitna River in Alaska shows two oxbow lakes – a short one at the bottom of the picture and a longer, more curved one at the middle-right. The picture also shows that a third oxbow lake is probably in the making: the isthmus or bank in the centre of the most prominent meander is very narrow – much narrower than the width of the river; eventually, the two sections of river on either side of an isthmus like that tend to break through and create a new, straighter course; a new river bank then starts to accumulate, sealing off the meander and leaving another oxbow lake.
Anoxbow lake is a U-shapedlake orpool that forms when a widemeander of ariver iscut off, creating a free-standing body of water. The word "oxbow" can also refer to a U-shaped bend in a river or stream, whether or not it is cut off from the main stream.[1][2] It takes its name from anoxbow which is part of a harness for oxen to pull a plough or cart.
An oxbow lake forms when a meandering river erodes through the neck of one of itsmeanders. This takes place because meanders tend to grow and become more curved over time. The river then follows a shorter course that bypasses the meander.[3] The entrances to the abandoned meander eventually silt up, forming an oxbow lake.[4] Oxbow lakes are stillwater lakes, with no current flowing through them, which causes the lake bed to gradually accumulate silt, becoming a bog or swamp and then evaporating completely.[3][5]
When a river reaches a low-lying plain, often in its final course to thesea or alake, it meanders widely. In the vicinity of ariver bend,deposition occurs on theconvex bank (the bank with the smaller radius). In contrast, both lateralerosion and undercutting occur on thecut bank orconcave bank (the bank with the greater radius). Continuous deposition on the convex bank and erosion of the concave bank of a meandering river cause the formation of a very pronouncedmeander with two concave banks getting closer. The narrow neck of land between the two neighboring concave banks is finally cut through, either by lateral erosion of the two concave banks or by the strong currents of aflood. When this happens a new, straighter riverchannel develops—and an abandoned meander loop, called acutoff, forms. When deposition finally seals off the cutoff from the river channel, an oxbow lake forms. This process can occur over a time from a few years to several decades, and may sometimes become essentially static.[4]
The gathering of erosion products near the concave bank and the transporting of them to the convex bank is the work of thesecondary flow across the floor of the river in the vicinity of a river bend. The process of silt deposition, sand and gravel on the convex bank is clearly illustrated inpoint bars.[6]
The effect of thesecondary flow can be demonstrated using a circular bowl. Partly fill the bowl with water and sprinkle dense particles such as sand or rice into the bowl. Set the water into circular motion with one hand or a spoon. The dense particles quickly sweep into a neat pile in the center of the bowl. This is the mechanism that leads to theformation of point bars and contributes to the formation of oxbow lakes. The primary flow of water in the bowl is circular and the streamlines are concentric with the side of the bowl. However, thesecondary flow of theboundary layer across the floor of the bowl is inward toward the center. The primary flow might be expected to fling the dense particles to the perimeter of the bowl, but instead the secondary flow sweeps the particles toward the center.[7]
The curved path of a river around a bend makes the water's surface slightly higher on the outside of the bend than on the inside. As a result, at any elevation within the river, water pressure is slightly greater near the outside of the bend than on the inside. A pressure gradient toward the convex bank provides thecentripetal force necessary for each parcel of water to follow its curved path.
Theboundary layer that flows along the river floor does not move fast enough to balance the pressure gradient laterally across the river. It responds to this pressure gradient, and its velocity is partly downstream and partly across the river toward the convex bank.[6][8] As it flows along the floor of the river, it sweeps loose material toward the convex bank. This flow of the boundary layer is significantly different from the speed and direction of the primary flow of the river, and is part of the river'ssecondary flow.
A Horseshoe or oxbow lake nearHughes,Arkansas, on the border between Arkansas andMississippi. The bulges in the border reflect changes in the course of the river; when the river shifted its course and cut off the former channel, the border remained unchanged.
River flood plains that contain rivers with a highly sinuous platform are populated by longer oxbow lakes than those with lowsinuosity. This is because rivers with high sinuosity have larger meanders, and greater opportunity for longer lakes to form. Rivers with lower sinuosity are characterized by fewer cutoffs and shorter oxbow lakes due to the shorter distance of their meanders.[9]
Oxbow lakes serve as important wetland ecosystems. In the United States, oxbow lakes serve as the primary habitat forwater tupelo and the iconicbald cypress.[10] The numerous oxbow lakes of theAmazon River are a favorable habitat for thegiant river otter.[3] Oxbow lakes may also be suitable locations foraquaculture.[11]
Oxbow lakes contribute to the health of a river ecosystem by trapping sediments and agricultural runoff, thereby removing them from the main river flow. However, this is destructive of the oxbow lake ecosystem itself.[12] Oxbow lakes are also vulnerable toheavy metal contamination from industrial sources.[13]
Oxbow lakes may be formed when a river channel is straightened artificially to improve navigation or for flood alleviation. This occurred notably on the upperRhine in Germany in the nineteenth century.[14]
An example of an entirely artificial waterway with oxbows is theOxford Canal in England. When originally constructed, it had a very meandering course, following the contours of the land, but the northern part of the canal was straightened out between 1829 and 1834, reducing its length from approximately 146 to 125 km (91 to77+1⁄2 mi) and creating a number of oxbow-shaped sections isolated from the new course.[15]
Carter Lake, United States was created after severe flooding in 1877 led to the Missouri River shifting approximatelytwo kilometres (1+1⁄4 miles) to the southeast.
The upper reaches of New Zealand'sTaieri River has also cut a multitude of oxbow lakes in its upper course near the town ofPaerau. Some of this area has been converted intowater meadows.
^abcRutledge, Kim; Ramroop, Tara; Boudreau, Diane; McDaniel, Melissa; Teng, Santani; Sprout, Erin; Costa, Hilary; Hall, Hilary; Hunt, Jeff (10 June 2011)."Oxbow lake". National Geographic. Archived fromthe original on 19 May 2022. Retrieved26 September 2021.
^abConstantine, José Antonio; Dunne, Thomas; Piégay, Hervé; Mathias Kondolf, G. (February 2010). "Controls on the alluviation of oxbow lakes by bed-material load along the Sacramento River, California".Sedimentology.57 (2):389–407.Bibcode:2010Sedim..57..389C.doi:10.1111/j.1365-3091.2009.01084.x.S2CID129166672.
^Kirschner, Alexander K. T.; Riegl, Bernhard; Velimirov, Branko (2001). "Degradation of Emergent and Submerged Macrophytesin an Oxbow Lake of an Embanked Backwater System: Implications for the Terrestrialization Process".International Review of Hydrobiology.86 (4–5):555–571.doi:10.1002/1522-2632(200107)86:4/5<555::AID-IROH555>3.0.CO;2-9.
^abHickin, Edward J (2002). "Meandering Channels". In Middleton, Gerard V. (ed.).Encyclopedia of Sediments and Sedimentary Rocks. New York: Springer. p. 432.ISBN1-4020-0872-4.
^Chant, R. J. (2002). "Secondary circulation in a region of flow curvature: Relationship with tidal forcing and river discharge".Journal of Geophysical Research.107 (C9):14-1 –14-11.Bibcode:2002JGRC..107.3131C.doi:10.1029/2001JC001082.
^Shankman, David; Kortright, Rosa Maria (May 1994). "Hydrogeomorphic conditions limiting the distribution of baldcypress in the southeastern United States".Physical Geography.15 (3):282–295.Bibcode:1994PhGeo..15..282S.doi:10.1080/02723646.1994.10642517.
^Ciazela, Jakub; Siepak, Marcin; Wojtowicz, Piotr (March 2018). "Tracking heavy metal contamination in a complex river-oxbow lake system: Middle Odra Valley, Germany/Poland".Science of the Total Environment.616–617:996–1006.Bibcode:2018ScTEn.616..996C.doi:10.1016/j.scitotenv.2017.10.219.PMID29103644.
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